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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Mar. Sci.</journal-id>
<journal-title>Frontiers in Marine Science</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Mar. Sci.</abbrev-journal-title>
<issn pub-type="epub">2296-7745</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="doi">10.3389/fmars.2023.1015323</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Marine Science</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>Effect of DIN and DON sources on the nitrogen uptake of the seagrass <italic>Zostera japonica</italic> and the macroalgae <italic>Ulva pertusa</italic> previously grown in different light levels</article-title>
</title-group>
<contrib-group>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Han</surname>
<given-names>Qiuying</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="author-notes" rid="fn001">
<sup>*</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/854397"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Qiu</surname>
<given-names>Chongyu</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Zeng</surname>
<given-names>Wenxuan</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Chen</surname>
<given-names>Yu</given-names>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Zhao</surname>
<given-names>Muqiu</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Shi</surname>
<given-names>Yunfeng</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Zheng</surname>
<given-names>Fengying</given-names>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
</contrib-group>
<aff id="aff1">
<sup>1</sup>
<institution>Key Laboratory for Coastal Marine Eco-Environment Process and Carbon Sink of Hainan province/Yazhou Bay Innovation Institute/Key Laboratory of Utilization and Conservation for Tropical Marine Bioresources of Ministry of Education/Modern Marine Ranching Engineering Research Center of Hainan, Hainan Tropical Ocean University</institution>, <addr-line>Sanya</addr-line>, <country>China</country>
</aff>
<aff id="aff2">
<sup>2</sup>
<institution>Marine College, Shandong University</institution>, <addr-line>Weihai, Shandong</addr-line>, <country>China</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>Edited by: Zhan Hu, Sun Yat-sen University, Zhuhai Campus, China</p>
</fn>
<fn fn-type="edited-by">
<p>Reviewed by: Peidong Zhang, Ocean University of China, China; Dilip Kumar Jha, National Institute of Ocean Technology, India</p>
</fn>
<fn fn-type="corresp" id="fn001"><p>*Correspondence: Qiuying Han, <email xlink:href="mailto:hanqiuying0312@sina.com">hanqiuying0312@sina.com</email>
</p>
</fn>
<fn fn-type="other" id="fn002">
<p>This article was submitted to Coastal Ocean Processes, a section of the journal Frontiers in Marine Science</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>03</day>
<month>02</month>
<year>2023</year>
</pub-date>
<pub-date pub-type="collection">
<year>2023</year>
</pub-date>
<volume>10</volume>
<elocation-id>1015323</elocation-id>
<history>
<date date-type="received">
<day>09</day>
<month>08</month>
<year>2022</year>
</date>
<date date-type="accepted">
<day>17</day>
<month>01</month>
<year>2023</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2023 Han, Qiu, Zeng, Chen, Zhao, Shi and Zheng</copyright-statement>
<copyright-year>2023</copyright-year>
<copyright-holder>Han, Qiu, Zeng, Chen, Zhao, Shi and Zheng</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.</p>
</license>
</permissions>
<abstract>
<p>This study quantified the absorption ability of the seagrass <italic>Zostera japonica</italic> and the macroalgae <italic>Ulva pertusa</italic> for dissolved inorganic nitrogen (DIN) (ammonium and nitrate) and dissolved organic nitrogen (DON) (urea and glycine) under different light conditions. The plants were cultured in filtered seawater (31&#x2030;, 25&#xb0;C) for 2&#xa0;weeks under three light levels. Macroalgae and the above- and belowground parts of seagrasses were separately placed into four different manmade seawater solutions with DIN (ammonium and nitrate) and DON (urea and glycine) stable isotopes for 1&#xa0;h. The results showed that macroalgae had higher absorption rates for ammonium and nitrate after higher light (14.67&#xa0;&#xb1;&#xa0;2.50 and 1.29&#xa0;&#xb1;&#xa0;0.16&#xa0;mg<sup>&#x2212;1</sup> dry weight (DW)&#xa0;h<sup>&#x2212;1</sup>) than after lower light (4.52&#xa0;&#xb1;&#xa0;0.95 and 0.18&#xa0;&#xb1;&#xa0;0.12&#xa0;mg<sup>&#x2212;1</sup>&#xa0;DW&#xa0;h<sup>&#x2212;1</sup>) treatments. Compared to the belowground seagrass portions that had previously been grown in high and low light conditions, the seagrass leaves assimilated ammonium more quickly. <italic>Z. japonica</italic> preferred glycine to nitrate and urea after the high- and low-light treatments; that is, the absorption rates of the belowground seagrass parts for glycine were 14.71&#xa0;&#xb1;&#xa0;1.85 and 6.38&#xa0;&#xb1;&#xa0;0.52&#xa0;mg<sup>&#x2212;1</sup>&#xa0;DW&#xa0;h<sup>&#x2212;1</sup> after the high- and low-light treatments, respectively, which were higher than those of ammonium, nitrate, and urea. The absorption rates of algae were lower than those for ammonium previously grown under medium- and low-light treatments. These results indicate that light reduction can impact the assimilation of DIN by <italic>Z. japonica</italic> and <italic>U. pertusa</italic>, and both have the ability to directly assimilate DON. This study provides information that could help reduce the negative effects of eutrophication on macroalgae and seagrasses in order to protect seagrass meadows.</p>
</abstract>
<kwd-group>
<kwd>species competition</kwd>
<kwd>light reduction</kwd>
<kwd>organic nitrogen</kwd>
<kwd>inorganic nitrogen</kwd>
<kwd>macroalgal blooms</kwd>
<kwd>seagrasses</kwd>
</kwd-group>
<counts>
<fig-count count="3"/>
<table-count count="4"/>
<equation-count count="4"/>
<ref-count count="106"/>
<page-count count="11"/>
<word-count count="6330"/>
</counts>
</article-meta>
</front>
<body>
<sec id="s1" sec-type="intro">
<label>1</label>
<title>Introduction</title>
<p>Seagrasses provide important ecological services, including altering nutrient cycling; producing organic carbon; enhancing biodiversity and food sources; supporting critical habitats for economic animals such as nereids, sipunculids, shellfish, shrimps, crabs, and fish; and stabilizing sediments in coastal areas (<xref ref-type="bibr" rid="B18">Costanza et&#xa0;al., 1997</xref>; <xref ref-type="bibr" rid="B38">Hemminga and Duarte, 2000</xref>; <xref ref-type="bibr" rid="B3">Beck et&#xa0;al., 2001</xref>; <xref ref-type="bibr" rid="B63">Orth et&#xa0;al., 2006</xref>; <xref ref-type="bibr" rid="B43">Huang et&#xa0;al., 2017</xref>). In recent years, land-derived nitrogen loading, largely from anthropogenic origin, has increased the eutrophication in coastal areas, resulting in a decline in seagrass meadows (<xref ref-type="bibr" rid="B89">Valiela et&#xa0;al., 1992</xref>; <xref ref-type="bibr" rid="B24">Duarte, 1995</xref>; <xref ref-type="bibr" rid="B63">Orth et&#xa0;al., 2006</xref>). More than half of the seagrass areas are reduced when land-derived nitrogen loads exceed 100&#xa0;kg&#xa0;N&#xa0;ha<sup>&#x2212;1</sup>&#xa0;year<sup>&#x2212;1</sup> (<xref ref-type="bibr" rid="B88">Valiela and Cole, 2002</xref>). Seagrasses serve as one of the sensitive indicators of nutrient pollution (<xref ref-type="bibr" rid="B6">Bricker et&#xa0;al., 2003</xref>). The nutrient concentrations in the environment may alter the competitive advantage of seagrasses and macroalgae. Seagrasses can absorb nutrients from seawater and sediments in oligotrophic environments, although they have lower nutrient absorption rates than macroalgae (<xref ref-type="bibr" rid="B34">Harvens et&#xa0;al., 2001</xref>; <xref ref-type="bibr" rid="B31">Han and Liu, 2014</xref>). Eutrophication may result in fast-growing macroalgal blooms (<xref ref-type="bibr" rid="B24">Duarte, 1995</xref>; <xref ref-type="bibr" rid="B34">Harvens et&#xa0;al., 2001</xref>), which often aggravate the decline of seagrasses in temperate estuaries (<xref ref-type="bibr" rid="B90">Valiela et&#xa0;al., 1997</xref>; <xref ref-type="bibr" rid="B36">Hauxwell et&#xa0;al., 2001</xref>; <xref ref-type="bibr" rid="B58">McGlathery, 2001</xref>; <xref ref-type="bibr" rid="B12">Burkholder et&#xa0;al., 2007</xref>) by promoting light reduction and increasing nutrient turnover in ecosystems, thereby altering the absorption ability of seagrasses for nitrogen (<xref ref-type="bibr" rid="B31">Han and Liu, 2014</xref> and references therein; <xref ref-type="bibr" rid="B32">Han et&#xa0;al., 2016</xref>).</p>
<p>Nutrients are some of the major environmental factors that control the primary production of seagrasses (<xref ref-type="bibr" rid="B34">Harvens et&#xa0;al., 2001</xref>; <xref ref-type="bibr" rid="B52">Leoni et&#xa0;al., 2008</xref>). Sea nitrogen is composed of dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON) (<xref ref-type="bibr" rid="B30">Gruber, 2004</xref>; <xref ref-type="bibr" rid="B103">Zhang et&#xa0;al., 2021</xref>). Wastewater from aquaculture releases some inorganic and organic nutrients from uneaten feed and from feces of farmed animals (<xref ref-type="bibr" rid="B40">Holmer et&#xa0;al., 2008</xref>; <xref ref-type="bibr" rid="B39">Herbeck et&#xa0;al., 2013</xref>). DIN, such as ammonium and nitrate, is considered an important nitrogen source for seagrass and macroalgal assimilation (<xref ref-type="bibr" rid="B90">Valiela et&#xa0;al., 1997</xref>; <xref ref-type="bibr" rid="B50">Lee and Dunton, 1999</xref>; <xref ref-type="bibr" rid="B91">van Alstyne, 2008</xref>; <xref ref-type="bibr" rid="B26">Fan et&#xa0;al., 2014</xref>; <xref ref-type="bibr" rid="B1">Alexandre et&#xa0;al., 2015</xref>). Some seagrass species, such as <italic>Zostera noltii</italic>, can absorb more nitrate when ammonium is absent (<xref ref-type="bibr" rid="B2">Alexandre et&#xa0;al., 2011</xref>). Ammonium can be toxic to seagrasses because its accumulation can increase protein breakdown (<xref ref-type="bibr" rid="B93">van Katwijk et&#xa0;al., 1997</xref>). DON constitutes a large part of the total dissolved nitrogen pool in coastal areas (<xref ref-type="bibr" rid="B86">Tyler et&#xa0;al., 2005</xref>; <xref ref-type="bibr" rid="B98">Wang, 2015</xref> and references therein). Urea and dissolved free amino acids are important nitrogen sources for autotrophic organisms (<xref ref-type="bibr" rid="B7">Bronk, 2002</xref>). The hydrolysis of seagrass leaf litter drives the rapid release of DON during the early phase of decomposition (<xref ref-type="bibr" rid="B99">Wang et&#xa0;al., 2014</xref>; <xref ref-type="bibr" rid="B20">Delgado et&#xa0;al., 2017</xref>; <xref ref-type="bibr" rid="B65">Prasad et&#xa0;al., 2019</xref>). To date, there have been few studies on the DON absorption of seagrasses (<xref ref-type="bibr" rid="B96">Vonk et&#xa0;al., 2008</xref>; <xref ref-type="bibr" rid="B92">van England et&#xa0;al., 2011</xref>; <xref ref-type="bibr" rid="B1">Alexandre et&#xa0;al., 2015</xref>). Some seagrasses (e.g., <italic>Cymodocea nodosa</italic> and <italic>Z. noltii</italic>) can absorb nitrogen from small organic substrates (<xref ref-type="bibr" rid="B92">van England et&#xa0;al., 2011</xref>). <xref ref-type="bibr" rid="B1">Alexandre et&#xa0;al. (2015)</xref> found that DON was a complementary nitrogen source to DIN, although <italic>Zostera marina</italic> preferred DON to nitrate. The macroalgal uptake of DIN in coastal waters is also well known (<xref ref-type="bibr" rid="B90">Valiela et&#xa0;al., 1997</xref>; <xref ref-type="bibr" rid="B91">van Alstyne, 2008</xref>; <xref ref-type="bibr" rid="B26">Fan et&#xa0;al., 2014</xref>). The macroalgal absorption of DON compounds is still not as well understood as that of DIN (<xref ref-type="bibr" rid="B86">Tyler et&#xa0;al., 2005</xref>; <xref ref-type="bibr" rid="B101">Xu, 2020</xref>; <xref ref-type="bibr" rid="B103">Zhang et&#xa0;al., 2021b</xref>). Phytoplankton and detritus supported by DIN are defined as PON (<xref ref-type="bibr" rid="B103">Zhang et&#xa0;al., 2021a</xref>), which could be difficult for seagrasses and macroalgae to directly absorb.</p>
<p>Light reduction from eutrophication and extreme climatic events such as hurricanes and tsunamis, resulting in pulsed turbidity, has been considered as the primary factor that leads to seagrass decline (<xref ref-type="bibr" rid="B13">Cambridge et&#xa0;al., 1986</xref>; <xref ref-type="bibr" rid="B97">Walker and McComb, 1992</xref>; <xref ref-type="bibr" rid="B66">Preen and Marsh, 1995</xref>). Light reduction may increase the mortality of seagrasses and decrease their growth and coverage (<xref ref-type="bibr" rid="B66">Preen and Marsh, 1995</xref>; <xref ref-type="bibr" rid="B15">Collier et&#xa0;al., 2007</xref>; <xref ref-type="bibr" rid="B17">Collier et&#xa0;al., 2011</xref>) by contracting respiratory and growth requirements with a combination of photosynthetic carbon fixation and reallocation of reserves (<xref ref-type="bibr" rid="B68">Ralph et&#xa0;al., 2007</xref>). Thin macroalgae (e.g., <italic>Ulva</italic> sp.) have lower light requirements than seagrasses; therefore, macroalgae can use more incidental light compared to seagrasses (<xref ref-type="bibr" rid="B24">Duarte, 1995</xref>). The growth superiority of the two communities may depend on irradiance (<xref ref-type="bibr" rid="B60">Moore and Wetezl, 2000</xref>), nutrient quantity (<xref ref-type="bibr" rid="B49">Lapointe et&#xa0;al., 1994</xref>) and type.</p>
<p>Limited light may have negative effects on the nitrogen absorption of seagrasses (<xref ref-type="bibr" rid="B83">Touchette et&#xa0;al., 2003</xref>). Because plants may have a relatively fixed cell quota for proteins, lipids, and carbohydrates (<xref ref-type="bibr" rid="B37">Hedges et&#xa0;al., 2002</xref>), they tend to absorb different elements with relatively fixed ratios (<xref ref-type="bibr" rid="B30">Gruber, 2004</xref>). Energy is consumed when plants assimilate nitrogen (<xref ref-type="bibr" rid="B83">Touchette et&#xa0;al., 2003</xref>). The metabolism of nitrogen in plants is strongly linked to the photosynthetic fixation of carbon (<xref ref-type="bibr" rid="B84">Turpin, 1991</xref> and references therein) because nitrogen and carbon are needed to build living organic tissues (<xref ref-type="bibr" rid="B29">Griffiths et&#xa0;al., 2020</xref>). Nitrogen assimilation requires carbon in the respiratory pathway (<xref ref-type="bibr" rid="B52">Leoni et&#xa0;al., 2008</xref>). The synthesis of amino acids by seagrasses requires more carbon in comparison with their carbon fixation capacity under conditions of nitrogen enrichment (<xref ref-type="bibr" rid="B84">Turpin, 1991</xref> and references therein; <xref ref-type="bibr" rid="B8">Brun et&#xa0;al., 2002</xref>), which will result in a lower carbohydrate content, leading to an internal carbon limit (<xref ref-type="bibr" rid="B83">Touchette et&#xa0;al., 2003</xref>; <xref ref-type="bibr" rid="B44">Inverse et&#xa0;al., 2004</xref>). Macroalgal blooms resulting from eutrophication may lead to light reduction and may impact the nitrogen cycle in seagrass ecosystems (<xref ref-type="bibr" rid="B31">Han and Liu, 2014</xref>; <xref ref-type="bibr" rid="B61">Moreira-Saporiti et&#xa0;al., 2021</xref>), thus altering the ability of seagrasses to absorb inorganic and organic forms of nitrogen.</p>
<p>
<italic>Zostera japonica</italic>, one of the dominant seagrass species in the Shandong coast of China, such as Swan Lake, has shown a decreasing trend, although it invaded and expanded along the Pacific Northwest coast (<xref ref-type="bibr" rid="B70">Ruesink et&#xa0;al., 2010</xref>; <xref ref-type="bibr" rid="B33">Han et&#xa0;al., 2017</xref>). The high nutrient and organic matter loads originating from benthos aquaculture and wastewater discharge could have contributed to the decline of <italic>Z. japonica</italic> in Shandong coast (<xref ref-type="bibr" rid="B105">Zhang et&#xa0;al., 2014</xref>; <xref ref-type="bibr" rid="B33">Han et&#xa0;al., 2017</xref>). In some areas along the Shandong coast, macroalgal mats (e. g., <italic>Ulva pertusa</italic>) have replaced the seagrass meadows (<xref ref-type="bibr" rid="B105">Zhang et&#xa0;al., 2014</xref>). Moreover, macroalgal mats cover the seagrass meadows during extended algal blooms every summer from June to July in Swan Lake, Shandong (<xref ref-type="bibr" rid="B32">Han et&#xa0;al., 2016</xref>). Until now, there have only been a few studies on how macroalgal blooms induce light attenuation and how they impact the capability of seagrasses and macroalgae to absorb DIN and DON.</p>
<p>The overall aim of this study was to compare the ability of seagrasses and green algae to absorb DIN and DON after light stress. Our hypotheses were as follows: firstly, plants require different amounts of energy for the two nitrogen forms (DIN and DON), and the nitrogen absorption ability of plants is lower because of the small amount of energy left for assimilation at lower light levels. Secondly, the light availability and the amount of energy required for nutrient uptake differ between seagrass and macroalgal species. Our results could provide information for environment managers regarding the control of nutrient inputs by intensive human activities and the reduction of the negative effects of macroalgae when macroalgal blooms result in more DIN and DON released into coastal eutrophication ecosystems in early low-light stress.</p>
</sec>
<sec id="s2" sec-type="materials|methods">
<label>2</label>
<title>Materials and methods</title>
<sec id="s2_1">
<label>2.1</label>
<title>Experimental design</title>
<p>
<italic>Z. japonica</italic> and <italic>U. pertusa</italic> were collected from Swan Lake, whichis located in Rongcheng City, Shandong Province, China (37.3382&#xb0; N&#x2013;37.3588&#xb0; N, 122.5551&#xb0; E&#x2013;122.5793&#xb0; E), and were exposed for 2&#xa0;weeks to three light levels: high light (HL 160&#xa0;mol&#xa0;photons&#xa0;m<sup>&#x2212;2</sup>&#xa0;s<sup>-1</sup>), medium light (ML, 40% HL), and low light (LL, 10% HL). The seagrasses and macroalgae were collected in June when the daylight is longer than the dark, so light was set up with a photoperiod of 18-h light and 6-h dark. Light intensity was measured using an underwater fluorometer (DIVING-PAM, WALZ company, Germany). Round macroalgae (wet weight, 2&#xa0;g) were placed in PVC cylinders with muddy sand (diameter, 11&#xa0;cm; height, 10&#xa0;cm). On top of the cylinders were nets with large holes to maintain sufficient light and to keep the plants inside. <italic>Z. japonica</italic> propagules (one apical shoot plus the first lateral shoot, and respective internodes; wet biomass, 11.35&#xa0;&#xb1;&#xa0;0.52) were buried in the same PVC cylinders as mentioned above. The cylinders were filled with muddy sand. The depth of the sediment cover of the propagules was 1.5&#xa0;cm. A total of 75 macroalgae and seagrasses were cultivated. There were 25 macroalgal replicates and 25 seagrass replicates for each light treatment in three different tanks. The PVC cylinders were submerged in aerated and filtered seawater (31&#x2030;). The seawater flowed slowly and was changed once every 2&#xa0;days. The temperature was maintained at 25&#xb0; using constant temperature system in the climate laboratory. The temperature and salinity of the seawater were measured using a YSI 30 portable meter (YSI, Yellow Springs, OH, USA).</p>
</sec>
<sec id="s2_2">
<label>2.2</label>
<title>Stable nitrogen isotope treatments and plant measurements</title>
<p>After 2&#xa0;weeks, the seagrasses and algae were separately divided into five groups for each light treatment, with five replicates for each group. In one group, the dry biomass of the macroalgae was weighed. The leaf length, width, rhizome node diameter, node length, and root length of seagrasses were measured, and the above- and belowground dry biomass was weighed. The total dry biomass was also calculated. Finally, the N content and &#x3b4;<sup>15</sup>N&#x2030; of the macroalgae and the aboveground (leaves) and belowground (roots with rhizomes) parts of seagrasses were measured using IRMS (MAT253; Thermo Fisher, Waltham, MA, USA) (<italic>n</italic>&#xa0;=&#xa0;5).</p>
<p>Four different artificial seawater solutions with DIN and DON stable isotopes (Sigma-Aldrich, St. Louis, MO, USA) were prepared. The DIN solution contained ammonium (ammonium chloride, 10&#xa0;&#x3bc;mol/L) and nitrate (sodium nitrate, 10&#xa0;&#x3bc;mol/L), while the DON solution contained urea (10&#xa0;&#x3bc;mol/L) and glycine (1&#xa0;&#x3bc;mol/L). The glycine concentration is lower than that of DIN and urea in natural environments; therefore, the concentration of the glycine solution was lower than that of ammonium, nitrate, and urea. In the other four plant groups, the macroalgae and the above- and belowground parts of seagrasses were separately placed in the above-mentioned solutions. Each plant tissue was cultured in a container with 1&#xa0;L stable nitrogen isotope solution, for a total of 180 treatments [three light treatments&#xa0;&#xd7;&#xa0;four nitrogen solutions&#xa0;&#xd7;&#xa0;three plant tissues (macroalgae and aboveground and belowground parts of seagrasses)&#xa0;&#xd7;&#xa0;five replicates]. The seagrasses and macroalgae were incubated for 2&#x2013;3&#xa0;h in stable nitrogen isotope solutions in order to study their absorption ability (<xref ref-type="bibr" rid="B92">van England et&#xa0;al., 2011</xref>). Our preliminary experiments indicated that 1&#xa0;h was sufficient for the absorption of DIN and DON stable isotopes by <italic>Z. japonica</italic> and <italic>U. pertusa</italic>. Therefore, in this study, 1&#xa0;h was taken as the absorption time for plant tissues (macroalgae, aboveground parts and belowground parts of seagrasses) to absorb stable nitrogen isotopes. The temperature was maintained at 25&#xb0;C. Lastly, the &#x3b4;<sup>15</sup>N&#x2030; values of macroalgae and the above- and belowground parts of seagrasses were measured using IRMS (MAT253; Thermo Fisher) (<italic>n</italic>&#xa0;=&#xa0;5).</p>
</sec>
<sec id="s2_3">
<label>2.3</label>
<title>Data calculation</title>
<p>&#x3b4;<sup>15</sup>N&#x2030;, which can be used to compare the amount of nitrogen absorbed by plants or plant tissues, was calculated as follows:</p>
<disp-formula>
<label>(1)</label>
<mml:math display="block" id="M1">
<mml:mrow>
<mml:msup>
<mml:mi>&#x3b4;</mml:mi>
<mml:mrow>
<mml:mn>15</mml:mn>
</mml:mrow>
</mml:msup>
<mml:mi>N</mml:mi>
<mml:mo>&#x2030;</mml:mo>
<mml:mo>&#xa0;</mml:mo>
<mml:mo>=</mml:mo>
<mml:mo>&#xa0;</mml:mo>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:msub>
<mml:mi>R</mml:mi>
<mml:mrow>
<mml:mi>s</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>m</mml:mi>
<mml:mi>p</mml:mi>
<mml:mi>l</mml:mi>
<mml:mi>e</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mo>&#xa0;</mml:mo>
<mml:mo>&#x2212;</mml:mo>
<mml:mo>&#xa0;</mml:mo>
<mml:msub>
<mml:mi>R</mml:mi>
<mml:mrow>
<mml:mi>s</mml:mi>
<mml:mi>t</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>n</mml:mi>
<mml:mi>d</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>r</mml:mi>
<mml:mi>d</mml:mi>
</mml:mrow>
</mml:msub>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
<mml:mo stretchy="false">/</mml:mo>
<mml:msub>
<mml:mi>R</mml:mi>
<mml:mrow>
<mml:mi>s</mml:mi>
<mml:mi>t</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>n</mml:mi>
<mml:mi>d</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>r</mml:mi>
<mml:mi>d</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mo>&#xa0;</mml:mo>
<mml:mo>*</mml:mo>
<mml:mo>&#xa0;</mml:mo>
<mml:mn>1</mml:mn>
<mml:mo>,</mml:mo>
<mml:mn>000</mml:mn>
</mml:mrow>
</mml:math>
</disp-formula>
<p>In Equation 1, <italic>R</italic>
<sub>standard</sub> and <italic>R</italic>
<sub>sample</sub> denote the <sup>15</sup>N/<sup>14</sup>N of the standard and the sample, respectively. Nitrogen in air was used as the analysis standard for stable nitrogen isotopes.</p>
<p>Isotope excess (<italic>E</italic>
<sub>sample</sub>) was calculated as the difference between the isotope fraction in the sample (<italic>F</italic>
<sub>sample</sub>) and the natural abundance (i.e., initial isotope fraction, <italic>F</italic>
<sub>nat</sub>) (<xref ref-type="bibr" rid="B92">van England et&#xa0;al., 2011</xref>), as follows:</p>
<disp-formula>
<label>(2)</label>
<mml:math display="block" id="M2">
<mml:mrow>
<mml:msub>
<mml:mi>E</mml:mi>
<mml:mrow>
<mml:mi>s</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>m</mml:mi>
<mml:mi>p</mml:mi>
<mml:mi>l</mml:mi>
<mml:mi>e</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mo>&#xa0;</mml:mo>
<mml:mo>=</mml:mo>
<mml:mo>&#xa0;</mml:mo>
<mml:msub>
<mml:mi>F</mml:mi>
<mml:mrow>
<mml:mi>s</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>m</mml:mi>
<mml:mi>p</mml:mi>
<mml:mi>l</mml:mi>
<mml:mi>e</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mo>&#xa0;</mml:mo>
<mml:mo>&#x2212;</mml:mo>
<mml:mo>&#xa0;</mml:mo>
<mml:msub>
<mml:mi>F</mml:mi>
<mml:mrow>
<mml:mi>n</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>t</mml:mi>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</disp-formula>
<p>The specific uptake rate of <sup>15</sup>N, <italic>V</italic>
<sub>sample</sub> [in micromoles <sup>15</sup>N per milligram dry weight (DW) per hour], was calculated using the following equation:</p>
<disp-formula>
<label>(3)</label>
<mml:math display="block" id="M3">
<mml:mrow>
<mml:msub>
<mml:mi>V</mml:mi>
<mml:mrow>
<mml:mi>s</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>m</mml:mi>
<mml:mi>p</mml:mi>
<mml:mi>l</mml:mi>
<mml:mi>e</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mo>&#xa0;</mml:mo>
<mml:mo>=</mml:mo>
<mml:mo>&#xa0;</mml:mo>
<mml:mi>P</mml:mi>
<mml:mi>O</mml:mi>
<mml:mi>M</mml:mi>
<mml:mo>&#xa0;</mml:mo>
<mml:mtext>x</mml:mtext>
<mml:mo>&#xa0;</mml:mo>
<mml:msub>
<mml:mi>E</mml:mi>
<mml:mrow>
<mml:mi>s</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>m</mml:mi>
<mml:mi>p</mml:mi>
<mml:mi>l</mml:mi>
<mml:mi>e</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mo stretchy="false">/</mml:mo>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:mi>t</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>m</mml:mi>
<mml:mi>e</mml:mi>
<mml:mo>&#xa0;</mml:mo>
<mml:mo>&#xd7;</mml:mo>
<mml:mo>&#xa0;</mml:mo>
<mml:mi>d</mml:mi>
<mml:mi>r</mml:mi>
<mml:mi>y</mml:mi>
<mml:mtext>&#xa0;</mml:mtext>
<mml:mi>w</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>g</mml:mi>
<mml:mi>h</mml:mi>
<mml:mi>t</mml:mi>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</disp-formula>
<p>where POM is the value of the stable nitrogen isotope in the plant tissue (in micromoles <sup>15</sup>N) (<xref ref-type="bibr" rid="B92">van England et&#xa0;al., 2011</xref>). Correction of varying substrate concentrations was accomplished by dividing <italic>V</italic>
<sub>sample</sub> by the substrate concentration (nitrogen added, <italic>N</italic>
<sub>added</sub>) and multiplying by 100 to convert to % (mg&#xa0;DW)<sup>&#x2212;1</sup>&#xa0;h<sup>&#x2212;1</sup>.</p>
<disp-formula>
<label>(4)</label>
<mml:math display="block" id="M4">
<mml:mrow>
<mml:mo>%</mml:mo>
<mml:msub>
<mml:mi>V</mml:mi>
<mml:mrow>
<mml:mi>s</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>m</mml:mi>
<mml:mi>p</mml:mi>
<mml:mi>l</mml:mi>
<mml:mi>e</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mo>&#xa0;</mml:mo>
<mml:mo>=</mml:mo>
<mml:mo>&#xa0;</mml:mo>
<mml:mn>100</mml:mn>
<mml:mo>&#xa0;</mml:mo>
<mml:mtext>x</mml:mtext>
<mml:mo>&#xa0;</mml:mo>
<mml:msub>
<mml:mi>V</mml:mi>
<mml:mrow>
<mml:mi>s</mml:mi>
<mml:mi>a</mml:mi>
<mml:mi>m</mml:mi>
<mml:mi>p</mml:mi>
<mml:mi>l</mml:mi>
<mml:mi>e</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mo stretchy="false">/</mml:mo>
<mml:msub>
<mml:mi>N</mml:mi>
<mml:mrow>
<mml:mi>a</mml:mi>
<mml:mi>d</mml:mi>
<mml:mi>d</mml:mi>
<mml:mi>e</mml:mi>
<mml:mi>d</mml:mi>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</disp-formula>
<p>%<italic>V</italic>
<sub>sample</sub> can be used to compare the nitrogen uptake rates of the different plants or plant tissues.</p>
</sec>
<sec id="s2_4">
<label>2.4</label>
<title>Statistical analysis</title>
<p>For the effects of light treatments on macroalgal dry biomass, seagrass leaf length, leaf width, rhizome node diameter, node and root lengths, total dry biomass, and above- and belowground dry biomass, the &#x3b4;<sup>15</sup>N values of plants were analyzed using one-way ANOVA and <italic>post-hoc</italic> tests. The light and nitrogen effects on the %<italic>V</italic>
<sub>sample</sub> of algae and the above- and belowground components of seagrasses were separately analyzed using two-way ANOVA and <italic>post-hoc</italic> tests. Differences in the %<italic>V</italic>
<sub>sample</sub> values among groups (light&#xa0;&#xd7;&#xa0;nitrogen&#xa0;&#xd7;&#xa0;plant tissues) were analyzed using three-way ANOVA and <italic>post-hoc</italic> tests. When ANOVA was significant (<italic>p</italic>&#xa0;&lt;&#xa0;0.05), Tukey&#x2019;s test was applied to determine which treatments were significantly different.</p>
</sec>
</sec>
<sec id="s3" sec-type="results">
<label>3</label>
<title>Results</title>
<sec id="s3_1">
<label>3.1</label>
<title>Physiological and morphological parameters of algae and seagrasses</title>
<p>The N% and &#x3b4;<sup>15</sup>N&#x2030; of algae after light treatment were not significantly different (<xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>). The dry biomass of <italic>U. pertusa</italic> after HL treatment was significantly higher than that after ML and LL treatments (<italic>p</italic>&#xa0;&lt;&#xa0;0.01) (<xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>). The highest (0.32&#xa0;&#xb1;&#xa0;0.05&#xa0;g) and the lowest (0.25&#xa0;&#xb1;&#xa0;0.05&#xa0;g) values were recorded for the HL and ML treatments, respectively (<xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>).</p>
<table-wrap id="T1" position="float">
<label>Table&#xa0;1</label>
<caption>
<p>Content of nitrogen, &#x3b4;<sup>15</sup>N, and dry biomass of <italic>Ulva pertusa</italic> after light treatments.</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="middle" align="left">&#xa0;</th>
<th valign="bottom" align="center">High light</th>
<th valign="bottom" align="center">Medium light</th>
<th valign="bottom" align="center">Low light</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="middle" align="left">N (%dry biomass)</td>
<td valign="middle" align="char" char="&#xb1;">4.81&#xa0;&#xb1;&#xa0;0.59</td>
<td valign="middle" align="char" char="&#xb1;">4.69&#xa0;&#xb1;&#xa0;0.49</td>
<td valign="middle" align="char" char="&#xb1;">5.37&#xa0;&#xb1;&#xa0;0.57</td>
</tr>
<tr>
<td valign="middle" align="left">&#x3b4;<sup>15</sup>N&#x2030;</td>
<td valign="middle" align="char" char="&#xb1;">6.49&#xa0;&#xb1;&#xa0;0.35</td>
<td valign="middle" align="char" char="&#xb1;">6.8&#xa0;&#xb1;&#xa0;0.36</td>
<td valign="middle" align="char" char="&#xb1;">7.05&#xa0;&#xb1;&#xa0;1.18</td>
</tr>
<tr>
<td valign="middle" align="left">Dry biomass (g)</td>
<td valign="middle" align="char" char="&#xb1;">0.32&#xa0;&#xb1;&#xa0;0.05a</td>
<td valign="middle" align="char" char="&#xb1;">0.25&#xa0;&#xb1;&#xa0;0.05b</td>
<td valign="middle" align="char" char="&#xb1;">0.27&#xa0;&#xb1;&#xa0;0.07b</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn>
<p>Lowercase letters indicate that there were differences between the two data points for each line after different light treatments. The same letters indicate no significant differences in the same line.</p>
</fn>
</table-wrap-foot>
</table-wrap>
<p>The total dry biomass of <italic>Z. japonica</italic> in the HL treatment was significantly higher than that in the lower light treatments (<italic>p</italic>&#xa0;&lt;&#xa0;0.05) (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>). The highest and the lowest total dry biomass values in the HL and LL treatments were 2.14&#xa0;&#xb1;&#xa0;0.39 and 1.19&#xa0;&#xb1;&#xa0;0.23&#xa0;mg, respectively (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>). The belowground dry biomass of <italic>Z. japonica</italic> was significantly different after the three light treatments (<italic>p</italic>&#xa0;&lt;&#xa0;0.05) (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>). The highest (1.04&#xa0;&#xb1;&#xa0;0.17&#xa0;mg) and the lowest (0.55&#xa0;&#xb1;&#xa0;0.13&#xa0;mg) belowground dry biomass values were found in the HL and LL treatments, respectively (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>).</p>
<table-wrap id="T2" position="float">
<label>Table&#xa0;2</label>
<caption>
<p>Biomass and morphological parameters of <italic>Zostera japonica</italic> after different light treatments.</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="middle" align="left"/>
<th valign="middle" align="center">High light</th>
<th valign="middle" align="center">Medium light</th>
<th valign="middle" align="center">Low light</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="middle" align="left">Total dry biomass (mg)</td>
<td valign="middle" align="char" char="&#xb1;">2.14&#xa0;&#xb1;&#xa0;0.39a</td>
<td valign="middle" align="char" char="&#xb1;">1.38&#xa0;&#xb1;&#xa0;0.24b</td>
<td valign="middle" align="char" char="&#xb1;">1.19&#xa0;&#xb1;&#xa0;0.23b</td>
</tr>
<tr>
<td valign="middle" align="left">Aboveground dry biomass (mg)</td>
<td valign="middle" align="char" char="&#xb1;">1.10&#xa0;&#xb1;&#xa0;0.29a</td>
<td valign="middle" align="char" char="&#xb1;">0.73&#xa0;&#xb1;&#xa0;0.17ab</td>
<td valign="middle" align="char" char="&#xb1;">0.64&#xa0;&#xb1;&#xa0;0.13b</td>
</tr>
<tr>
<td valign="middle" align="left">Belowground dry biomass (mg)</td>
<td valign="middle" align="char" char="&#xb1;">1.04&#xa0;&#xb1;&#xa0;0.17a</td>
<td valign="middle" align="char" char="&#xb1;">0.65&#xa0;&#xb1;&#xa0;0.15b</td>
<td valign="middle" align="char" char="&#xb1;">0.55&#xa0;&#xb1;&#xa0;0.13b</td>
</tr>
<tr>
<td valign="middle" align="left">N (% aboveground dry biomass)</td>
<td valign="middle" align="char" char="&#xb1;">2.17&#xa0;&#xb1;&#xa0;0.08</td>
<td valign="middle" align="char" char="&#xb1;">2.08&#xa0;&#xb1;&#xa0;0.09</td>
<td valign="middle" align="char" char="&#xb1;">2.11&#xa0;&#xb1;&#xa0;0.21</td>
</tr>
<tr>
<td valign="middle" align="left">N (% belowground dry biomass)</td>
<td valign="middle" align="char" char="&#xb1;">1.33&#xa0;&#xb1;&#xa0;0.11</td>
<td valign="middle" align="char" char="&#xb1;">1.32&#xa0;&#xb1;&#xa0;0.08</td>
<td valign="middle" align="char" char="&#xb1;">1.29&#xa0;&#xb1;&#xa0;0.07</td>
</tr>
<tr>
<td valign="middle" align="left">Leaf length (cm)</td>
<td valign="middle" align="char" char="&#xb1;">6.17&#xa0;&#xb1;&#xa0;0.81a</td>
<td valign="middle" align="char" char="&#xb1;">4.98&#xa0;&#xb1;&#xa0;0.59b</td>
<td valign="middle" align="char" char="&#xb1;">3.36&#xa0;&#xb1;&#xa0;0.69c</td>
</tr>
<tr>
<td valign="middle" align="left">Leaf width (mm)</td>
<td valign="middle" align="char" char="&#xb1;">0.96&#xa0;&#xb1;&#xa0;0.14a</td>
<td valign="middle" align="char" char="&#xb1;">0.77&#xa0;&#xb1;&#xa0;0.09b</td>
<td valign="middle" align="char" char="&#xb1;">0.79&#xa0;&#xb1;&#xa0;0.08b</td>
</tr>
<tr>
<td valign="middle" align="left">Rhizome diameter (mm)</td>
<td valign="middle" align="char" char="&#xb1;">1.07&#xa0;&#xb1;&#xa0;0.12a</td>
<td valign="middle" align="char" char="&#xb1;">1.00&#xa0;&#xb1;&#xa0;0.09b</td>
<td valign="middle" align="char" char="&#xb1;">0.96&#xa0;&#xb1;&#xa0;0.07b</td>
</tr>
<tr>
<td valign="middle" align="left">Node length (mm)</td>
<td valign="middle" align="char" char="&#xb1;">3.28&#xa0;&#xb1;&#xa0;0.94</td>
<td valign="middle" align="char" char="&#xb1;">3.69&#xa0;&#xb1;&#xa0;0.33</td>
<td valign="middle" align="char" char="&#xb1;">3.72&#xa0;&#xb1;&#xa0;0.29</td>
</tr>
<tr>
<td valign="middle" align="left">Root length (cm)</td>
<td valign="middle" align="char" char="&#xb1;">2.02&#xa0;&#xb1;&#xa0;0.24</td>
<td valign="middle" align="char" char="&#xb1;">1.96&#xa0;&#xb1;&#xa0;0.18</td>
<td valign="middle" align="char" char="&#xb1;">2.31&#xa0;&#xb1;&#xa0;0.40</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn>
<p>Lowercase letters indicate differences between the two data points for each line after different light treatments. The same letters indicate no significant differences in the same line.</p>
</fn>
</table-wrap-foot>
</table-wrap>
<p>The leaf length, width, and rhizome diameter of the seagrasses showed significant differences after the three light treatments (<italic>p</italic>&#xa0;&lt;&#xa0;0.05) (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>). The highest (6.17&#xa0;&#xb1;&#xa0;0.81&#xa0;mg) and the lowest (3.36&#xa0;&#xb1;&#xa0;0.69&#xa0;mg) values for seagrass leaf length were recorded in the HL and LL treatments, respectively (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>), while the highest (0.96&#xa0;&#xb1;&#xa0;0.14&#xa0;mm) and the lowest (0.77&#xa0;&#xb1;&#xa0;0.09&#xa0;mm) values for seagrass leaf width were found in the HL and ML treatments, respectively (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>). On the other hand, the highest (1.07&#xa0;&#xb1;&#xa0;0.12&#xa0;mm) and the lowest (0.96&#xa0;&#xb1;&#xa0;0.07&#xa0;mm) values for seagrass rhizome diameter were recorded in the HL and LL treatments, respectively (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>). The rhizome node and root lengths of seagrasses did not differ significantly among the three light treatments (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>).</p>
<p>The &#x3b4;<sup>15</sup>N values of algae after the absorption of ammonium by plant tissues were significantly higher than those of the other three nutrients and natural groups after each light treatment (<italic>p</italic>&#xa0;&lt;&#xa0;0.01) (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1</bold>
</xref>). The highest (97.26&#xa0;&#xb1;&#xa0;30.84&#x2030;) and the lowest (35.65&#xa0;&#xb1;&#xa0;9.15&#x2030;) &#x3b4;<sup>15</sup>N values of algae after ammonium absorption were found after HL and LL treatments, respectively (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1</bold>
</xref>).</p>
<fig id="f1" position="float">
<label>Figure&#xa0;1</label>
<caption>
<p>The <bold>&#x3b4;</bold><sup>15</sup>N value of <italic>U. pertusa.</italic> The different letters indicated significant differences in the same light treatment..</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1015323-g001.tif"/>
</fig>
<p>The &#x3b4;<sup>15</sup>N values of the aboveground seagrass tissues after the absorption of ammonium were significantly higher than those of the natural groups and other three nutrients previously grown under each light treatment (<italic>p</italic>&#xa0;&lt;&#xa0;0.01) (<xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2A</bold>
</xref>). The &#x3b4;<sup>15</sup>N values of the aboveground seagrass tissues after ammonium absorption were 62.99&#xa0;&#xb1;&#xa0;15.94&#x2030;, 91.95&#xa0;&#xb1;&#xa0;34.38&#x2030;, and 74.14&#xa0;&#xb1;&#xa0;22.49&#x2030; after the HL, ML, and LL treatments, respectively (<xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2A</bold>
</xref>). The &#x3b4;<sup>15</sup>N value of seagrass belowground tissues after nutrient absorption was significantly different after each light treatment (<xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2B</bold>
</xref>, p&lt;0.05). The highest &#x3b4;15N values of seagrass belowground tissues after ammonium absorption appeared after HL (67.90&#xb1;10.49&#x2030;), ML (128.37&#xb1;38.74&#x2030;) and LL (64.66&#xb1;22.06&#x2030;) treatments (<xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2B</bold>
</xref>).</p>
<fig id="f2" position="float">
<label>Figure&#xa0;2</label>
<caption>
<p>&#x3b4;<sup>15</sup>N values in the aboveground and belowground tissues of <italic>Zostera japonica</italic>. <bold>(A)</bold> Aboveground seagrasses. <bold>(B)</bold> Belowground seagrasses.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1015323-g002.tif"/>
</fig>
</sec>
<sec id="s3_2">
<label>3.2</label>
<title>%V<sub>sample</sub> value of alagae and seagrasses</title>
<p>Both nitrogen and light significantly impacted the %<italic>V</italic>
<sub>sample</sub> values of algae (<italic>p</italic>&#xa0;&lt;&#xa0;0.001) (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3A</bold>
</xref> and <xref ref-type="table" rid="T3">
<bold>Table&#xa0;3</bold>
</xref>). The highest absorption rates of algae for ammonium (14.67&#xa0;&#xb1;&#xa0;2.50&#xa0;mg<sup>&#x2212;1</sup>&#xa0;DW&#xa0;h<sup>&#x2212;1</sup>), nitrate (1.29&#xa0;&#xb1;&#xa0;0.16&#xa0;mg<sup>&#x2212;1</sup>&#xa0;DW&#xa0;h<sup>&#x2212;1</sup>), and urea (1.67&#xa0;&#xb1;&#xa0;0.65&#xa0;mg<sup>&#x2212;1</sup>&#xa0;DW&#xa0;h<sup>&#x2212;1</sup>) were found after HL treatment (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3A</bold>
</xref>). The lowest %<italic>V</italic>
<sub>sample</sub> values of algae for ammonium (4.52&#xa0;&#xb1;&#xa0;0.95&#xa0;mg<sup>&#x2212;1</sup>&#xa0;DW&#xa0;h<sup>&#x2212;1</sup>) and nitrate (0.18&#xa0;&#xb1;&#xa0;0.12&#xa0;mg<sup>&#x2212;1</sup>&#xa0;DW&#xa0;h<sup>&#x2212;1</sup>) occurred after LL treatment (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3A</bold>
</xref>). The lowest %<italic>V</italic>
<sub>sample</sub> value of algae for urea was 0.58&#xa0;&#xb1;&#xa0;0.41&#xa0;mg<sup>&#x2212;1</sup>&#xa0;DW&#xa0;h<sup>&#x2212;1</sup>, which was found after ML treatment (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3A</bold>
</xref>).</p>
<fig id="f3" position="float">
<label>Figure&#xa0;3</label>
<caption>
<p>The N absorption rates between algae and the aboveground and belowground parts of seagrasses for the different N sources afterlight treatments. <bold>(A)</bold> Algae. <bold>(B)</bold> Aboveground seagrasses. <bold>(C)</bold> Belowground seagrasses.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1015323-g003.tif"/>
</fig>
<table-wrap id="T3" position="float">
<label>Table&#xa0;3</label>
<caption>
<p>Analysis of variance of the effects of light and nitrogen on the %<italic>V</italic>
<sub>sample</sub> values of plant tissues.</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="middle" rowspan="2" align="left">&#xa0;</th>
<th valign="middle" colspan="3" align="center">Macroalgae</th>
<th valign="middle" colspan="3" align="center">Aboveground seagrass parts</th>
<th valign="middle" colspan="3" align="center">Belowground seagrass parts</th>
</tr>
<tr>
<th valign="middle" align="center">
<italic>df</italic>
</th>
<th valign="middle" align="center">
<italic>F</italic>
</th>
<th valign="middle" align="center">
<italic>p</italic>
</th>
<th valign="middle" align="center">
<italic>df</italic>
</th>
<th valign="middle" align="center">
<italic>F</italic>
</th>
<th valign="middle" align="center">
<italic>p</italic>
</th>
<th valign="middle" align="center">
<italic>df</italic>
</th>
<th valign="middle" align="center">
<italic>F</italic>
</th>
<th valign="middle" align="center">
<italic>p</italic>
</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="middle" align="left">Nitrogen</td>
<td valign="middle" align="center">3</td>
<td valign="middle" align="center">188.38</td>
<td valign="middle" align="center">&lt;0.001</td>
<td valign="middle" align="center">3</td>
<td valign="middle" align="center">91.06</td>
<td valign="middle" align="center">&lt;0.01</td>
<td valign="middle" align="center">3</td>
<td valign="middle" align="center">121.46</td>
<td valign="middle" align="center">&lt;0.001</td>
</tr>
<tr>
<td valign="middle" align="left">Light</td>
<td valign="middle" align="center">2</td>
<td valign="middle" align="center">12.57</td>
<td valign="middle" align="center">&lt;0.001</td>
<td valign="middle" align="center">2</td>
<td valign="middle" align="center">1.38</td>
<td valign="middle" align="center">n.s.</td>
<td valign="middle" align="center">2</td>
<td valign="middle" align="center">19.66</td>
<td valign="middle" align="center">&lt;0.001</td>
</tr>
<tr>
<td valign="middle" align="left">Nitrogen&#xa0;&#xd7;&#xa0;light</td>
<td valign="middle" align="center">6</td>
<td valign="middle" align="center">14.34</td>
<td valign="middle" align="center">&lt;0.001</td>
<td valign="middle" align="center">6</td>
<td valign="middle" align="center">0.77</td>
<td valign="middle" align="center">n.s.</td>
<td valign="middle" align="center">6</td>
<td valign="middle" align="center">57.45</td>
<td valign="middle" align="center">&lt;0.001</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn>
<p>n.s., no significant differences.</p>
</fn>
</table-wrap-foot>
</table-wrap>
<p>Nitrogen had significant effects on the %<italic>V</italic>
<sub>sample</sub> values of the aboveground seagrass parts (<italic>p</italic>&#xa0;&lt;&#xa0;0.001) (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3B</bold>
</xref> and <xref ref-type="table" rid="T3">
<bold>Table&#xa0;3</bold>
</xref>). The %<italic>V</italic>
<sub>sample</sub> values of the aboveground seagrass parts for ammonium were higher than those for glycine after all light treatments (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3B</bold>
</xref>). The highest ammonium absorption rates of the aboveground seagrasses were 8.27&#xa0;&#xb1;&#xa0;1.97, 9.93&#xa0;&#xb1;&#xa0;2.42, and 9.70&#xa0;&#xb1;&#xa0;2.12&#xa0;mg<sup>&#x2212;1</sup>&#xa0;DW&#xa0;h<sup>&#x2212;1</sup> after HL, ML, and LL treatments, respectively (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3B</bold>
</xref>). For glycine, the highest absorption rates of the aboveground seagrasses were 3.92&#xa0;&#xb1;&#xa0;1.11, 3.35&#xa0;&#xb1;&#xa0;1.99, and 5.32&#xa0;&#xb1;&#xa0;0.27&#xa0;mg<sup>&#x2212;1</sup>&#xa0;DW&#xa0;h<sup>&#x2212;1</sup> after HL, ML, and LL treatments, respectively (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3B</bold>
</xref>).</p>
<p>Light and nitrogen significantly impacted the %<italic>V</italic>
<sub>sample</sub> values of the belowground seagrass parts (<italic>p</italic>&#xa0;&lt;&#xa0;0.001) (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3C</bold>
</xref> and <xref ref-type="table" rid="T3">
<bold>Table&#xa0;3</bold>
</xref>). After HL, ML, and LL treatments, the %<italic>V</italic>
<sub>sample</sub> values of the belowground seagrass parts for ammonium were 4.56&#xa0;&#xb1;&#xa0;0.04, 11.97&#xa0;&#xb1;&#xa0;1.89, and 3.87&#xa0;&#xb1;&#xa0;1.16&#xa0;mg<sup>&#x2212;1</sup>&#xa0;DW&#xa0;h<sup>&#x2212;1</sup>, respectively, while those for glycine were 14.71&#xa0;&#xb1;&#xa0;1.85, 3.98&#xa0;&#xb1;&#xa0;0.20, and 6.38&#xa0;&#xb1;&#xa0;0.52&#xa0;mg<sup>&#x2212;1</sup>&#xa0;DW&#xa0;h<sup>&#x2212;1</sup>, respectively (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3C</bold>
</xref>).</p>
<p>Nitrogen also altered the %<italic>V</italic>
<sub>sample</sub> values of algae and the above- and belowground seagrass parts under different light pressures (<italic>p</italic>&#xa0;&lt;&#xa0;0.001) (<xref ref-type="table" rid="T4">
<bold>Table&#xa0;4</bold>
</xref>). The %<italic>V</italic>
<sub>sample</sub> values of algae for ammonium were higher than those of seagrass leaves after HL treatment, but were lower than those of seagrass leaves after LL treatments (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). For ammonium, the %<italic>V</italic>
<sub>sample</sub> values of the aboveground seagrass parts were higher than those of the belowground parts after HL and LL treatments (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). For nitrite, the %<italic>V</italic>
<sub>sample</sub> values of the belowground seagrass parts were higher than those of the aboveground seagrass parts and algae after all light treatments (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). The %<italic>V</italic>
<sub>sample</sub> values of algae for nitrate after LL treatments were lower than those of the aboveground seagrass parts, while these values in the belowground seagrass parts for urea after all treatments were higher than those in the aboveground seagrass parts (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). For glycine, the %<italic>V</italic>
<sub>sample</sub> values of algae were higher than those of seagrass leaves after all light treatments, while these values in the belowground seagrass parts were higher than those of the aboveground seagrass parts after HL treatment (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>).</p>
<table-wrap id="T4" position="float">
<label>Table&#xa0;4</label>
<caption>
<p>Analysis of variance of the three-factor model with the %<italic>V</italic>
<sub>sample</sub> of plants as the response variable and light, nitrogen, and plant tissues (algae and the aboveground and belowground seagrasses) and all possible interactions.</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="middle" align="left">Source</th>
<th valign="middle" align="center">
<italic>df</italic>
</th>
<th valign="middle" align="center">
<italic>F</italic>
</th>
<th valign="middle" align="center">
<italic>p</italic>
</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="middle" align="left">Light</td>
<td valign="middle" align="center">2</td>
<td valign="middle" align="center">1.44</td>
<td valign="middle" align="left">n.s.</td>
</tr>
<tr>
<td valign="middle" align="left">Nitrogen</td>
<td valign="middle" align="center">3</td>
<td valign="middle" align="center">18.70</td>
<td valign="middle" align="left">&lt;0.001</td>
</tr>
<tr>
<td valign="middle" align="left">Plant tissues</td>
<td valign="middle" align="center">2</td>
<td valign="middle" align="center">1.01</td>
<td valign="middle" align="left">n.s.</td>
</tr>
<tr>
<td valign="middle" align="left">Light&#xa0;&#xd7;&#xa0;nitrogen</td>
<td valign="middle" align="center">6</td>
<td valign="middle" align="center">2.02</td>
<td valign="middle" align="left">n.s.</td>
</tr>
<tr>
<td valign="middle" align="left">Light&#xa0;&#xd7;&#xa0;plant tissues</td>
<td valign="middle" align="center">4</td>
<td valign="middle" align="center">0.75</td>
<td valign="middle" align="left">n.s.</td>
</tr>
<tr>
<td valign="middle" align="left">Nitrogen&#xa0;&#xd7;&#xa0;plant tissues</td>
<td valign="middle" align="center">6</td>
<td valign="middle" align="center">0.26</td>
<td valign="middle" align="left">n.s.</td>
</tr>
<tr>
<td valign="middle" align="left">Light&#xa0;&#xd7;&#xa0;nitrogen&#xa0;&#xd7;&#xa0;plant tissues</td>
<td valign="middle" align="center">12</td>
<td valign="middle" align="center">1.41</td>
<td valign="middle" align="left">n.s.</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn>
<p>n.s., no significant differences.</p>
</fn>
</table-wrap-foot>
</table-wrap>
<p>The three-way ANOVA (light&#xa0;&#xd7;&#xa0;nitrogen&#xa0;&#xd7;&#xa0;plant tissues) illustrated the significant effects of nitrogen type on the ability of plant tissues (algae and the above- and belowground parts of seagrasses) to absorb nitrogen and showed that light and the nitrogen type altered the absorption ability of algae and seagrasses for nitrogen (<xref ref-type="table" rid="T4">
<bold>Table&#xa0;4</bold>
</xref>).</p>
</sec>
</sec>
<sec id="s4" sec-type="discussion">
<label>4</label>
<title>Discussion</title>
<p>This study showed that the dry weight of <italic>U. pertusa</italic> after HL treatment was significantly higher than that after lower light treatments (<xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>), which is similar to the results of <xref ref-type="bibr" rid="B91">van Alstyne (2008)</xref> and <xref ref-type="bibr" rid="B73">Schmid et&#xa0;al. (2021)</xref>, who found that macroalgae showed a higher growth rate under higher light than under lower light conditions. Macroalgae have evolved physiological responses to light reduction, driving photosynthesis (<xref ref-type="bibr" rid="B78">Takahashi and Murata, 2008</xref>; <xref ref-type="bibr" rid="B25">Esteban et&#xa0;al., 2015</xref>), thereby impacting algal biomass. The physiological responses of algae to stress can increase amino acids or reduce sugars at the cellular level (<xref ref-type="bibr" rid="B74">Sharma et&#xa0;al., 2016</xref>). High light intensity increases the concentrations of &#x3b2;-carotene and zeaxanthin in marine macroalgae (<xref ref-type="bibr" rid="B100">Xie et&#xa0;al., 2020</xref>), resulting in an increase in storage lipids in some algae (<xref ref-type="bibr" rid="B45">Khotimchenko and Yakovleva, 2005</xref>). However, some macroalgae showed lower chlorophyll a (Chl-a) and pigment composition under higher light conditions (see, for example, <xref ref-type="bibr" rid="B25">Esteban et&#xa0;al., 2015</xref>; <xref ref-type="bibr" rid="B73">Schmid et&#xa0;al., 2021</xref>). <italic>Ulva fenestra</italic> exhibited higher concentrations of lipids and higher proportions of polyunsaturated fatty acids in low-light habitats than in moderate-light conditions (<xref ref-type="bibr" rid="B42">Hotimchenko, 2002</xref>). <italic>Ulva lactuca</italic> grown under high-light conditions had lower nitrogen, carbon, pigment, and dimethylsulfoniopropionate concentrations relative to algae in low light (<xref ref-type="bibr" rid="B91">van Alstyne, 2008</xref>). This could be due to the physiological responses and resource allocation of macroalgae to light reduction being species-specific.</p>
<p>Light availability is the main factor limiting the growth and distribution of seagrasses compared with all other factors (<xref ref-type="bibr" rid="B21">Dennison et&#xa0;al., 1993</xref>; <xref ref-type="bibr" rid="B51">Lee et&#xa0;al., 2007</xref>). Seagrasses show physiological and morphological responses to light reduction depending on localized environmental conditions and timescales (<xref ref-type="bibr" rid="B4">Bertelli and Unsworth, 2018</xref>; <xref ref-type="bibr" rid="B46">Kim et&#xa0;al., 2019</xref>; <xref ref-type="bibr" rid="B104">Zhang et&#xa0;al., 2020</xref>). The responses of seagrasses to light reduction also include biomass decrease (<xref ref-type="bibr" rid="B41">Holmer and Laursen, 2002</xref>) and growth reduction (<xref ref-type="bibr" rid="B69">Rruiz and Romero, 2001</xref>). The total dry biomass and belowground biomass of <italic>Z. japonica</italic> in the HL treatment were significantly higher than those in the lower light treatments in this study (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>), which is in accordance with other reports on seagrass habitats (<italic>Z. marina</italic> and <italic>C. nodosa</italic>: <xref ref-type="bibr" rid="B76">Silva et&#xa0;al., 2013</xref>
<italic>;</italic> <xref ref-type="bibr" rid="B14">Collier et&#xa0;al., 2016</xref>
<italic>; Z. muelleri</italic>: <xref ref-type="bibr" rid="B29">Griffiths et&#xa0;al., 2020</xref>
<italic>).</italic>
</p>
<p>Exposure to low light may lead to carbonhydrate reduction in seagrasses (<xref ref-type="bibr" rid="B35">Hasler-Sheetal et&#xa0;al., 2016</xref>; <xref ref-type="bibr" rid="B48">Kumar et&#xa0;al., 2017</xref>). Smaller species are less resistant to light reduction (<xref ref-type="bibr" rid="B14">Collier et&#xa0;al., 2016</xref>), and the responses of seagrasses to light reduction are species-specific (<xref ref-type="bibr" rid="B15">Collier et&#xa0;al., 2007</xref>; <xref ref-type="bibr" rid="B57">Manassa et&#xa0;al., 2017</xref>; <xref ref-type="bibr" rid="B77">Statton et&#xa0;al., 2018</xref>). For example, the low light requirements of <italic>Z. noltii</italic>, <italic>Posidonia oceanica</italic>, <italic>Thalassia testudinum</italic>, <italic>Halodule pinifolia</italic>, and <italic>Syringodium filiforme</italic> for survival were 2%, 10%, 14%, 14%, and 24% of surface irradiance (SI), respectively (<xref ref-type="bibr" rid="B52">Leoni et&#xa0;al., 2008</xref> and references therein). Our results showed that the rhizome diameters of the seagrasses in the HL treatment were significantly higher than those in the lower treatments (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>). The lower rhizome diameters of seagrasses demonstrate that carbon can be transported from belowground to the aboveground parts of seagrasses under low-light conditions due to disruption of the carbon fixation and energy metabolism (<xref ref-type="bibr" rid="B29">Griffiths et&#xa0;al., 2020</xref>). In this study, the leaf length and width were longer and thicker, respectively, in the HL treatment than in the ML and LL treatments (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>). A smaller area of seagrass leaves further limits photosynthesis. Our experiments prevented seagrasses from receiving nutrients and carbon <italic>via</italic> adjacent plants, as a result of isolating plant cultivation. This may be different from natural conditions, which should be considered in the future.</p>
<p>Light reduction and nitrogen availability can affect the physiological or biochemical properties of seaweeds by altering their carbon and nitrogen contents (<xref ref-type="bibr" rid="B19">Cruz-Rivera and Hay, 2003</xref>; <xref ref-type="bibr" rid="B9">Buapet et&#xa0;al., 2008</xref>). Although <xref ref-type="bibr" rid="B91">van Alstyne (2008)</xref> found that light and nitrate always independently affect the algal response, the absorption of nutrients by some algae is related to light and carbon metabolism (<xref ref-type="bibr" rid="B23">Dortch, 1990</xref>). The nitrogen concentrations in void tissues are often correlated with the DIN concentrations of seawater (<xref ref-type="bibr" rid="B28">Fujita et&#xa0;al., 1989</xref>; <xref ref-type="bibr" rid="B5">Bj&#xf6;rns&#xe4;ter and Wheeler, 1990</xref>; <xref ref-type="bibr" rid="B80">Teichberg et&#xa0;al., 2010</xref>; <xref ref-type="bibr" rid="B26">Fan et&#xa0;al., 2014</xref>). In this study, it was found that the &#x3b4;<sup>15</sup>N values of the algal tissues after ammonium absorption were significantly higher than those of the other three nutrients and natural groups after each light treatment (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1</bold>
</xref>). The absorption rate of ammonium by algae is higher than that of nitrate because the uptake of ammonium into cells requires less energy than nitrate (<xref ref-type="bibr" rid="B27">Flynn, 1991</xref>).</p>
<p>During the macroalgal bloom and decay phases, <italic>Ulva</italic> sp. are an important source of DON in the aquatic environment (<xref ref-type="bibr" rid="B85">Tyler et l., 2001</xref>; <xref ref-type="bibr" rid="B103">Zhang et&#xa0;al., 2021</xref>). During the early period of macroalgal blooms, green macroalgae can take up DIN, thus lowering the DIN concentration in ambient seawater and effectively relieving eutrophication (<xref ref-type="bibr" rid="B102">Zhang and Wang, 2017</xref>). Subsequently, algae assimilate DIN into DON, continuously releasing DON into the ambient environment, or depositing it into sediments (<xref ref-type="bibr" rid="B55">Li et&#xa0;al., 2016</xref>). This explains the increase of the DON concentration in the culture environment, mainly because the DON produced by <italic>Ulva</italic> sp. was released into the ambient media (<xref ref-type="bibr" rid="B75">Sharp, 1977</xref>). Higher DON concentrations may increase the soluble sugar and protein contents of macroalgae (<xref ref-type="bibr" rid="B101">Xu, 2020</xref>). <xref ref-type="bibr" rid="B86">Tyler et&#xa0;al. (2005)</xref> found that <italic>Gracilaria vermiculophylla</italic> can assimilate urea and amino acids and that <italic>U. lactuca</italic> has higher uptake rates than <italic>G. vermiculophylla</italic>. The DON and ammonium from decomposed algae can be important nitrogen sources and take part in the nitrogen cycle in seagrass ecosystems, thereby further changing the biochemical cycle processes in the long term (<xref ref-type="bibr" rid="B103">Zhang et&#xa0;al., 2021b</xref>).</p>
<p>It is conceivable that the nitrogen uptake of seagrasses may change during or after macroalgal blooms. Macroalgal blooms lead to light limitations, which affect the functions of seagrass ecosystems, such as nutrient uptake. In this study, light reduction and the nitrogen type altered the absorption ability of seagrasses and algae for nitrogen (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). After light reduction, seagrass growth is limited, which may result in a reduction in the nutrient requirements for growth. The physiological responses of seagrasses to low irradiance include the increase in the nitrogen and amino acid contents in tissues (<xref ref-type="bibr" rid="B94">van Lent et&#xa0;al., 1995</xref>; <xref ref-type="bibr" rid="B56">Longstaff and Dennison, 1999</xref>; <xref ref-type="bibr" rid="B52">Leoni et&#xa0;al., 2008</xref>; <xref ref-type="bibr" rid="B59">McMahon et&#xa0;al., 2013</xref>). Nitrogen enrichment can promote the demand for energy and carbon skeletons from photosynthates to drive the assimilation of DIN (<xref ref-type="bibr" rid="B44">Inverse et&#xa0;al., 2004</xref>), affecting the productivity and survival of seagrasses, which may aggravate the deleterious effects of low light (<xref ref-type="bibr" rid="B95">Villaz&#xe1;n et&#xa0;al., 2013</xref>). Our results showed that the aboveground parts of seagrasses had higher absorption ability for ammonium than for nitrate, urea, and glycine after all light treatments (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). Some seagrass species have increased affinity for ammonium over nitrate (<xref ref-type="bibr" rid="B81">Terrados and Williams, 1997</xref>; <xref ref-type="bibr" rid="B50">Lee and Dunton, 1999</xref>). The preferential uptake of ammonium by seagrasses may be due to the physiological demands associated with nitrate uptake (<xref ref-type="bibr" rid="B62">Nayar et&#xa0;al., 2018</xref> and references therein). Furthermore, seagrass tissues require far less energy than nitrate to transform ammonium into organic nitrogen (<xref ref-type="bibr" rid="B62">Nayar et&#xa0;al., 2018</xref>). <xref ref-type="bibr" rid="B72">Sandoval-Gil et&#xa0;al. (2015)</xref> found that seagrass roots showed reduced capacity to absorb ammonium compared to leaves because of the very high availability of this nutrient in sediments. This is in agreement with our results, in which the absorption rates of ammonium by the aboveground parts of seagrasses were higher than those of the belowground parts after HL and LL treatments (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). <xref ref-type="bibr" rid="B50">Lee and Dunton (1999)</xref> found that ammonia is mostly absorbed by the rhizomes and roots of seagrasses. The belowground parts of seagrasses grow in sediments, where there is less oxygen concentration than in coastal waters; therefore, seagrass roots and rhizomes may adapt to environments with less oxygen through evolved physiological responses. The substratum type can influence the leaf <italic>versus</italic> root nutrient uptake, and fine-grained sediments generally support a higher absorption rate of seagrasses for ammonium (<xref ref-type="bibr" rid="B10">Bulthuis et&#xa0;al., 1992</xref>). We recommend further research involving the combined effects of sediment types on the absorption ability of seagrasses for organic and inorganic nitrogen nutrients to lower the negative effects of eutrophication on seagrass ecosystems through a long-term investigation.</p>
<p>
<xref ref-type="bibr" rid="B71">Sandoval-Gil et&#xa0;al. (2019)</xref> found that nitrate is mainly absorbed by seagrass leaves. This is contrary to our results, in which the absorption rates of nitrate by the belowground seagrass parts were significantly higher than those of seagrass leaves and macroalgae after all light treatments (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). Nitrate enrichment in sediments can stimulate the growth of seagrasses (<xref ref-type="bibr" rid="B64">Peralta et&#xa0;al., 2003</xref>), while water column nitrate enrichment may lead to the die-off of seagrasses (<xref ref-type="bibr" rid="B52">Leoni et&#xa0;al., 2008</xref>). <xref ref-type="bibr" rid="B106">Zimmerman et&#xa0;al. (1987)</xref> reported that most nitrogen assimilation occurs in the roots; however, limited light conditions interfere with the nitrogen assimilation in the roots because the root&#x2013;rhizome system is photosynthesis-dependent (<xref ref-type="bibr" rid="B67">Pregnall et&#xa0;al., 1987</xref>). The nitrogen uptake rates of seagrass roots depend on the belowground biomass of seagrasses (<xref ref-type="bibr" rid="B47">Kraemer et&#xa0;al., 1997</xref>; <xref ref-type="bibr" rid="B52">Leoni et&#xa0;al., 2008</xref>). Some seagrass species (e.g., <italic>Z. noltii</italic>) can absorb more nitrate in the absence of ammonium (<xref ref-type="bibr" rid="B2">Alexandre et&#xa0;al., 2011</xref>). The assimilation of seagrasses into nitrate requires more energy because it involves an active transport system (<xref ref-type="bibr" rid="B53">Lepoint et&#xa0;al., 2002</xref>), which may aggravate the potentially harmful effects of nitrate enrichment, such as toxicity and metabolic costs (<xref ref-type="bibr" rid="B11">Burholder et&#xa0;al., 1992</xref>; <xref ref-type="bibr" rid="B72">Sandoval-Gil et&#xa0;al., 2015</xref>). Our results demonstrate that limited light use did not alter the absorption ability of the above- and belowground parts of seagrasses for nitrogen, especially nitrate.</p>
<p>When the nitrogen requirement is lower than the uptake, the absorbed nutrients may be reserved in the plant tissues as amino acids or proteins (<xref ref-type="bibr" rid="B87">Udy et&#xa0;al., 1999</xref>; <xref ref-type="bibr" rid="B44">Invers et&#xa0;al., 2004</xref>). The carbon requirements for synthesizing amino acids can exceed the carbon fixation capacity under nitrogen enrichment conditions. The results of this study showed that the absorption ability of seagrasses for glycine was significantly higher than that for nitrate and urea after HL and LL treatments (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>), which is in agreement with <xref ref-type="bibr" rid="B1">Alexandre et&#xa0;al. (2015)</xref>, who found that <italic>Z. marina</italic> showed preferential uptake of DON over nitrate and that DON was a complementary nitrogen source compared to DIN. Seagrass tissues can limit the use of nitrogen once they reach substrate saturation (<xref ref-type="bibr" rid="B82">Touchette and Burkholder, 2000</xref>). The lower concentration of glycine than nitrate and urea may have enhanced the uptake ratio of seagrasses to glycine in this study. The belowground parts of seagrasses had higher absorption rates for urea than the aboveground parts after all light treatments (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). The urea in coastal waters resulting from agricultural fertilizers, coastal aquaculture, domestic sewage, and industrial waste can be deposited in sediments (<xref ref-type="bibr" rid="B54">Li et&#xa0;al., 2015</xref>), thereby providing an organic nitrogen source for the roots and rhizomes of seagrasses. Organic nitrogen (e. g., amino acid nitrogen) can be mineralized and directly assimilated by plants and microbes (<xref ref-type="bibr" rid="B22">Dong et&#xa0;al., 2021</xref>). For example, some dinoflagellates can absorb urea and nitrate (<xref ref-type="bibr" rid="B98">Wang, 2015</xref>). <xref ref-type="bibr" rid="B96">Vonk et&#xa0;al. (2008)</xref> found that seagrasses may prefer organic nitrogen at low ambient nitrogen concentrations; however, several studies have shown that seagrasses more preferred absorbing DIN (<xref ref-type="bibr" rid="B11">Burkholder et&#xa0;al., 1992</xref>; <xref ref-type="bibr" rid="B50">Lee and Dunton, 1999</xref>; <xref ref-type="bibr" rid="B72">Sandoval-Gil et&#xa0;al., 2015</xref>). The DON uptake of seagrasses may be more widespread than is traditionally recognized (<xref ref-type="bibr" rid="B92">van England et&#xa0;al., 2011</xref>). Microorganisms can facilitate the assimilation of DON by mineralizing amino acids in the seagrass <italic>Posidonia sinuosa</italic>, which may promote the growth and productivity of seagrasses (<xref ref-type="bibr" rid="B79">Tarquinio et&#xa0;al., 2018</xref>). However, the microbial catabolism of amino acids can lead to exuded ammonium, which is harmful to plant tissues at extremely high concentrations (<xref ref-type="bibr" rid="B93">van Katwijk et&#xa0;al., 1997</xref>). The mechanisms by which microorganisms affect the uptake of DON by seagrasses require further research.</p>
<p>Under lower light conditions, seagrasses may need more energy for the absorption of organic nitrogen in order to reduce the negative effects of light limitation (<xref ref-type="bibr" rid="B16">Collier et&#xa0;al., 2010</xref>). A reduced photosynthetic carbon fixation under low light conditions leads to less carbon being transferred from the leaves to seagrass roots and rhizomes. This may explain our results where the assimilation of glycine by <italic>Z. japonica</italic> roots and rhizomes after HL treatment was higher than that of the lower treatment (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3D</bold>
</xref>). Processes that minimize the utilization of reserved carbon by reducing the organic nitrogen assimilation within seagrass roots and rhizomes may contribute to shading tolerance. Our results indicate that <italic>Z. japonica</italic> quickly responded to short-term light reduction due to changes in organic nitrogen metabolism processes. The internal assimilation to organic nitrogen and the transfer mechanism to carbon of seagrasses should be considered in the future.</p>
</sec>
<sec id="s5" sec-type="conclusion">
<label>5</label>
<title>Conclusion</title>
<p>The eutrophication problem is complex because it may change the primary producers and environmental variables through ecosystem self-organization. Overall, this study provides evidence that shading alters the absorption ability of macroalgae and seagrasses for inorganic and organic nitrogen, although it is also necessary to deeply understand the difference between the limited, highly artificial study and the non-linear, highly interactive responses in the seagrass meadows. Light reduction may alter the competition status between seagrasses and macroalgae through a change in their inorganic and organic absorption strategies. This difference should be well considered in future coastal management and seagrass protection. Regular seagrass monitoring, coupled with long-term studies to assess the effects of macroalgal bloom progression and duration on seagrass ecosystems, is required. Additional tests of the thresholds of the different nitrogen sources and their effects under different environmental, social, and economic contexts are also required.</p>
</sec>
<sec id="s6" sec-type="data-availability">
<title>Data availability statement</title>
<p>The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.</p>
</sec>
<sec id="s7" sec-type="author-contributions">
<title>Author contributions</title>
<p>QH and CQ: Conceptualization. QH and YC: Methodology. QH, WZ, and YC: Formal analysis. QH, CQ, and FZ: Writing&#x2014;original draft preparation. QH, MZ, YS, and CQ: Writing&#x2014;review and editing. All authors contributed to the article and approved the submitted version.</p>
</sec>
</body>
<back>
<sec id="s8" sec-type="funding-information">
<title>Funding</title>
<p>This research was supported by the projects ofthe National Natural Science Foundation of China (41730529) andthe High-Level Talented Person Project of the Natural ScienceFoundation of Hainan Province (420RC657).</p>
</sec>
<ack>
<title>Acknowledgments</title>
<p>We thank Laura M. Soissons for her support during the experiment design. </p>
</ack>
<sec id="s9" sec-type="COI-statement">
<title>Conflict of interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
<sec id="s10" sec-type="disclaimer">
<title>Publisher&#x2019;s note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.</p>
</sec>
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