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<article article-type="brief-report" dtd-version="2.3" xml:lang="EN" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">
<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Environ. Sci.</journal-id>
<journal-title>Frontiers in Environmental Science</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Environ. Sci.</abbrev-journal-title>
<issn pub-type="epub">2296-665X</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="publisher-id">918570</article-id>
<article-id pub-id-type="doi">10.3389/fenvs.2022.918570</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Environmental Science</subject>
<subj-group>
<subject>Brief Research Report</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>Differential Comprehensive Effects of Food Quality and ZnO Nanoparticles on the Key Traits of Early Life History of <italic>Daphnia magna</italic>
</article-title>
<alt-title alt-title-type="left-running-head">Sun et al.</alt-title>
<alt-title alt-title-type="right-running-head">Synergies of <italic>Microcystis</italic> and ZnO-NPs</alt-title>
</title-group>
<contrib-group>
<contrib contrib-type="author">
<name>
<surname>Sun</surname>
<given-names>Yunfei</given-names>
</name>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Yu</surname>
<given-names>Bo</given-names>
</name>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Lei</surname>
<given-names>Yulu</given-names>
</name>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Qin</surname>
<given-names>Shanshan</given-names>
</name>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Lyu</surname>
<given-names>Kai</given-names>
</name>
<uri xlink:href="https://loop.frontiersin.org/people/1369228/overview"/>
</contrib>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Yang</surname>
<given-names>Zhou</given-names>
</name>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
<uri xlink:href="https://loop.frontiersin.org/people/28590/overview"/>
</contrib>
</contrib-group>
<aff>
<institution>Jiangsu Key Laboratory for Biodiversity and Biotechnology</institution>, <institution>School of Biological Sciences</institution>, <institution>Nanjing Normal University</institution>, <addr-line>Nanjing</addr-line>, <country>China</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>
<bold>Edited by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/1359009/overview">Chenglian Feng</ext-link>, Chinese Research Academy of Environmental Sciences, China</p>
</fn>
<fn fn-type="edited-by">
<p>
<bold>Reviewed by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/749356/overview">Andi Alijagic</ext-link>, &#xd6;rebro University, Sweden</p>
<p>
<ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/1786378/overview">Jianchao Liu</ext-link>, Hohai University, China</p>
</fn>
<corresp id="c001">&#x2a;Correspondence: Zhou Yang, <email>yangzhou@njnu.edu.cn</email>
</corresp>
<fn fn-type="other">
<p>This article was submitted to Toxicology, Pollution and the Environment, a section of the journal Frontiers in Environmental Science</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>08</day>
<month>06</month>
<year>2022</year>
</pub-date>
<pub-date pub-type="collection">
<year>2022</year>
</pub-date>
<volume>10</volume>
<elocation-id>918570</elocation-id>
<history>
<date date-type="received">
<day>12</day>
<month>04</month>
<year>2022</year>
</date>
<date date-type="accepted">
<day>06</day>
<month>05</month>
<year>2022</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2022 Sun, Yu, Lei, Qin, Lyu and Yang.</copyright-statement>
<copyright-year>2022</copyright-year>
<copyright-holder>Sun, Yu, Lei, Qin, Lyu and Yang</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>In aquatic ecosystems, cladocerans, an important part of zooplankton, are often exposed to new pollutant nanoparticles and poor quality food simultaneously. To evaluate the combined effects of poor quality food and nanoparticles on the development and early reproductive performance of cladocerans, we exposed <italic>Daphnia magna</italic>, a representative cladoceran, to different concentrations of ZnO nanoparticles under the food conditions containing different proportions of non-toxic <italic>Microcystis</italic>, recorded some key indicators of the early life history, and analyzed the possible differential dose effects. The results showed that non-toxic <italic>Microcystis</italic> and ZnO nanoparticles significantly delayed the times to maturation and reproduction and also significantly reduced the survival time, the body length at maturation, the number of offspring in the first brood, and the total offspring per female of <italic>D. magna</italic>. When the food contained non-toxic <italic>Microcystis</italic>, <italic>D. magna</italic> did not reach sexual maturity at 0.25&#xa0;mg&#xa0;L<sup>&#x2212;1</sup> ZnO nanoparticles and did not develop eggs and reproduce offspring at 0.20&#xa0;mg&#xa0;L<sup>&#x2212;1</sup> ZnO nanoparticles, especially non-toxic <italic>Microcystis</italic> and ZnO nanoparticles had a significant or nearly significant synergistic effect on the time to maturation, time to first brood, and the total offspring per female of <italic>D. magna</italic>, whereas for other life history indicators, non-toxic <italic>Microcystis</italic> and ZnO nanoparticles showed an additive effect, based on comparing the slopes of linear fitting of the relevant indicators with nanoparticle concentration under different food quality conditions. Such findings are helpful to realistically evaluate the comprehensive stress effect on zooplankton under the complex conditions of poor food quality and coexistence of pollutants.</p>
</abstract>
<kwd-group>
<kwd>
<italic>Daphnia magna</italic>
</kwd>
<kwd>development</kwd>
<kwd>life history</kwd>
<kwd>non-toxic <italic>Microcystis</italic>
</kwd>
<kwd>reproduction</kwd>
<kwd>ZnO nanoparticles</kwd>
</kwd-group>
<contract-sponsor id="cn001">National Natural Science Foundation of China<named-content content-type="fundref-id">10.13039/501100001809</named-content>
</contract-sponsor>
</article-meta>
</front>
<body>
<sec id="s1">
<title>Introduction</title>
<p>Zooplankton, the primary consumers in aquatic ecosystems, plays a key role in transferring the energy fixed by the primary producer algae to the higher trophic levels (<xref ref-type="bibr" rid="B21">Miner et al., 2012</xref>). Under natural conditions, in addition to the fluctuation of algal abundance, there are also changes in algal composition in the aquatic environment (<xref ref-type="bibr" rid="B25">Roshith et al., 2018</xref>), which will change the food quality of zooplankton, especially in eutrophic freshwaters, cyanobacteria blooms often break out, in which <italic>Microcystis</italic> is the dominant group (<xref ref-type="bibr" rid="B12">Humbert et al., 2013</xref>; <xref ref-type="bibr" rid="B29">Sun et al., 2013</xref>). Both toxic <italic>Microcystis</italic> and non-toxic <italic>Microcystis</italic> strains can be found in the same waters (<xref ref-type="bibr" rid="B33">Wilson et al., 2005</xref>), and their composition changes with seasonal succession (<xref ref-type="bibr" rid="B24">Rinta-Kanto et al., 2009</xref>). Toxic <italic>Microcystis</italic> not only lacks key nutritional composition but also produces microcystin, which brings fatal harm to zooplankton (<xref ref-type="bibr" rid="B34">Yang et al., 2011</xref>; <xref ref-type="bibr" rid="B17">Lyu et al., 2016a</xref>; <xref ref-type="bibr" rid="B10">Huang et al., 2020</xref>). Compared with green alga <italic>Chlorella</italic>, non-toxic <italic>Microcystis</italic> lacks essential fatty acids, sterols, and other nutrients necessary for zooplankton (<xref ref-type="bibr" rid="B31">Von Elert and Wolffrom, 2001</xref>; <xref ref-type="bibr" rid="B19">Martin-Creuzburg et al., 2008</xref>); thus, it is also a low-quality food for zooplankton. As a type of non-selective filter-feeding zooplankton, cladocerans inevitably ingest non-toxic <italic>Microcystis</italic>, which may lead to malnutrition of cladocerans living in eutrophic waters, resulting in a series of negative effects (<xref ref-type="bibr" rid="B15">L&#xfc;rling, 2003</xref>; <xref ref-type="bibr" rid="B10">Huang et al., 2020</xref>). <xref ref-type="bibr" rid="B18">Lyu et al. (2016b)</xref> demonstrated that long-term feeding of non-toxic <italic>Microcystis aeruginosa</italic> will hinder the development of cladocerans and reduce its reproductive output.</p>
<p>Metal oxide nanoparticles (NPs) are commonly used in industry and daily necessities, and the discharge of untreated sewage containing NPs leads to the increasing concentration of NPs in natural aquatic ecosystems, which brings increasingly serious harm to plankton. Many studies have shown that metal oxide NPs have significant negative effects on zooplankton (<xref ref-type="bibr" rid="B14">Lekamge et al., 2018</xref>; <xref ref-type="bibr" rid="B7">Geppert et al., 2021</xref>; <xref ref-type="bibr" rid="B23">Qi et al., 2022</xref>), which usually can last for several generations (<xref ref-type="bibr" rid="B20">Martins et al., 2020</xref>). In general, non-toxic <italic>Microcystis</italic> and NPs can co-exist simultaneously in an aquatic environment. Some studies reported that NPs can inhibit the growth of <italic>Microcystis</italic> (<xref ref-type="bibr" rid="B26">Sankar et al., 2014</xref>), whereas more studies showed that NPs have toxic effects on cladocerans (<xref ref-type="bibr" rid="B32">Wang et al., 2019</xref>; <xref ref-type="bibr" rid="B27">Silva et al., 2021</xref>; <xref ref-type="bibr" rid="B30">Sun et al., 2022</xref>). Therefore, when cladocerans are exposed to <italic>Microcystis</italic> and NPs simultaneously, it may lead to complex interactions. The combined effect of toxic <italic>Microcystis</italic> and NPs on cladocerans has been reported (<xref ref-type="bibr" rid="B32">Wang et al., 2019</xref>), but when the aquatic ecosystem is dominated by non-toxic <italic>Microcystis</italic>, the combined stress of NPs and non-toxic <italic>Microcystis</italic> may have different consequences, especially in the sensitive period of the early life history stage of cladocerans.</p>
<p>Based on the aforementioned related research status, we put forward the following scientific hypothesis: except that different proportions of non-toxic <italic>Microcystis</italic> and different concentrations of NPs will produce different degrees of stress on cladocerans, the harm of non-toxic <italic>Microcystis</italic> and NPs may have synergistic effects or at least additive effects on cladocerans. Therefore, in order to test the aforementioned scientific hypothesis and accurately evaluate the possible changes in the tolerance of cladocerans to NPs when eating different quality foods, we placed a representative cladoceran <italic>Daphnia magna</italic> to different concentrations of ZnO NPs, one of the most commonly used metal oxide NPs in industry and daily necessities, under the food conditions containing different proportions of non-toxic <italic>Microcystis</italic>, recorded some key indicators of the early life history, and analyzed the possible differential dose effects.</p>
</sec>
<sec sec-type="materials|methods" id="s2">
<title>Materials and Methods</title>
<sec id="s2-1">
<title>Maintenance of Test Organisms</title>
<p>
<italic>D. magna</italic> organisms were maintained in COMBO medium and fed with green alga <italic>Chlorella pyrenoidosa</italic> (1.5&#xa0;mgC&#xa0;L<sup>&#x2212;1</sup>) at 21&#xb0;C, 40&#xa0;&#xb5;mol photons m<sup>&#x2212;2</sup>s<sup>&#x2212;1</sup> light intensity, and 14&#xa0;h:10&#xa0;h light: dark cycle. <italic>C. pyrenoidosa</italic> and non-toxic <italic>Microcystis aeruginosa</italic> (FACHB-469) were cultured separately in BG-11 medium under the same conditions as described previously and harvested at the exponential growth phase. These harvested algae were used as food of <italic>D. magna</italic> in our following experiments.</p>
</sec>
<sec id="s2-2">
<title>Experimental Design</title>
<p>The fresh stock solution of ZnO NPs (0.10&#xa0;g&#xa0;L<sup>&#x2212;1</sup>) was prepared by distributing the ZnO NPs in deionized water and then sonicated for 30&#xa0;min at 40&#xa0;kHz before the experiment (<xref ref-type="bibr" rid="B32">Wang et al., 2019</xref>; <xref ref-type="bibr" rid="B30">Sun et al., 2022</xref>). ZnO NPs aggregated flocculation in different shapes (including round, rectangular, and anomalous) with sizes of dozens of nanometers in diameter (for detailed physicochemical characterization, see <xref ref-type="bibr" rid="B32">Wang et al. (2019</xref>)). The specific testing solutions with different concentrations of ZnO NPs (0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, and 0.40&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>) were obtained by diluting the stock solution of ZnO NPs in COMBO medium. The test concentrations of ZnO NPs, respectively, represented the environmentally relevant concentrations in some specific aquatic environments at present and the possible increased levels without effective control in the future. Three food combinations with different nutritional qualities but containing the same carbon biomass (1.5&#xa0;mg&#xa0;C&#xa0;L<sup>&#x2212;1</sup>) were used in the experiment: 1) 100% <italic>C. pyrenoidosa</italic> (100%<italic>Ch</italic>), 2) a mixture of 85% <italic>C. pyrenoidosa</italic>&#x2b;15% non-toxic <italic>M. aeruginosa</italic> (85%<italic>Ch</italic>&#x2b;15%<italic>MF</italic>), and 3) a mixture of 70% <italic>C. pyrenoidosa</italic>&#x2b;30% non-toxic <italic>M. aeruginosa</italic> (70%<italic>Ch</italic>&#x2b;30%<italic>MF</italic>). There were a total of 24 treatments based on a fully factorial design of eight ZnO NP concentrations and three food types, and 10 replicates were set for each treatment. The neonates (&#x3c;12&#xa0;h) from the third broods of 20 maternal <italic>D. magna</italic> that grow simultaneously were randomly selected for the experiment. One <italic>D. magna</italic> neonate was randomly placed in a 50-ml beaker containing 50&#xa0;ml COMBO medium as one replicate. The experiments were conducted under the same conditions as described previously and lasted for 16 days, which met our requirements for testing the early life history. The culture medium in each beaker was changed daily to maintain the relative stability of food abundance and ZnO NP levels.</p>
</sec>
<sec id="s2-3">
<title>Measurements of the Early Life History Traits</title>
<p>To analyze the early life history traits of <italic>D. magna</italic> under the aforementioned treatments, six key life history traits were recorded: survival time, time to maturation, time to first brood, body length at maturation, number of offspring in the first brood per female, and total offspring per female. Survival time was calculated according to the death of <italic>D. magna</italic>, which was determined as the time when the heart stopped beating under a dissecting microscope; time to maturation was determined when the first batch of eggs appeared in the brood pouch; time to first brood was determined when first brood offspring was released; body length at maturation was measured from the top of head to the base of the tail-spine under a stereo-microscope using Nikon software; the number of offspring in the first brood was counted and then removed; total offspring was the sum of neonates recorded daily.</p>
</sec>
<sec id="s2-4">
<title>Statistical Analysis</title>
<p>All data were presented in the form of mean values &#xb1; SE. Two-way ANOVA and Tukey test were used to assess the interaction of ZnO NPs concentrations and food types on <italic>D. magna</italic> life history traits, and tests statistics were deemed significant when <italic>p</italic>-values were less than 0.05. Since the life history traits of <italic>D. magna</italic> showed a linear change trend with the increase of ZnO NP concentrations, in order to accurately fit and obtain more meaningful parameters to compare the effects of food quality, all life history traits of <italic>D. magna</italic> changing with ZnO NPs concentrations were, respectively, fitted by a linear function, when the slope had a significant difference from zero (<italic>&#x3b1;</italic> &#x3d; 0.05); this indicated that the ZnO NP concentration significantly affects this trait. When ZnO NP concentrations were 0.30 and 0.40&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>, the reproductive indicator was 0 and not brought into the fitting. To determine which effect (synergistic, additive, or antagonistic effect) exists between food quality and ZnO NPs, the differences of slopes derived from the aforementioned linear functions under the three food types were compared with one-way ANOVA, followed by the Holm&#x2013;Sidak method. All statistical analyses were performed using SigmaPlot 14.0.</p>
</sec>
</sec>
<sec sec-type="results" id="s3">
<title>Results</title>
<sec id="s3-1">
<title>Survival Time</title>
<p>Two-way ANOVA showed that both non-toxic <italic>Microcystis</italic> and ZnO NPs significantly reduced the survival time of <italic>D. magna</italic>, but there was no significant interaction between them (<xref ref-type="table" rid="T1">Table 1</xref>; <xref ref-type="fig" rid="F1">Figure 1</xref>). The survival time decreased linearly with the increase of ZnO NP concentrations, and the slopes of the linear functions under the three food types were similar (<xref ref-type="fig" rid="F1">Figure1B</xref>), indicating that there was only an additive effect of non-toxic <italic>Microcystis</italic> on the harmful effects of ZnO NPs.</p>
<table-wrap id="T1" position="float">
<label>TABLE 1</label>
<caption>
<p>Two-way ANOVA on survival, development, and reproduction subjected to various ZnO NP levels and food types.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Trait</th>
<th align="left">DF</th>
<th align="center">SS</th>
<th align="center">MS</th>
<th align="center">
<italic>F</italic>
</th>
<th align="center">
<italic>P</italic>
</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td colspan="6" align="left">Survival time</td>
</tr>
<tr>
<td align="left">&#x2003;Food type (A)</td>
<td align="char" char=".">2</td>
<td align="char" char=".">39.8</td>
<td align="char" char=".">19.9</td>
<td align="char" char=".">4.844</td>
<td align="char" char=".">0.01</td>
</tr>
<tr>
<td align="left">&#x2003;ZnO NP concentration (B)</td>
<td align="char" char=".">7</td>
<td align="char" char=".">1064.9</td>
<td align="char" char=".">152.129</td>
<td align="char" char=".">37.029</td>
<td align="char" char=".">&#x3c;0.001</td>
</tr>
<tr>
<td align="left">&#x2003;A x B</td>
<td align="char" char=".">14</td>
<td align="char" char=".">26.6</td>
<td align="char" char=".">1.9</td>
<td align="char" char=".">0.462</td>
<td align="char" char=".">0.947</td>
</tr>
<tr>
<td colspan="6" align="left">Time to maturation</td>
</tr>
<tr>
<td align="left">&#x2003;Food type (A)</td>
<td align="char" char=".">2</td>
<td align="char" char=".">180.289</td>
<td align="char" char=".">90.144</td>
<td align="char" char=".">181.635</td>
<td align="char" char=".">&#x3c;0.001</td>
</tr>
<tr>
<td align="left">&#x2003;ZnO NP concentration (B)</td>
<td align="char" char=".">5</td>
<td align="char" char=".">213.665</td>
<td align="char" char=".">42.733</td>
<td align="char" char=".">86.104</td>
<td align="char" char=".">&#x3c;0.001</td>
</tr>
<tr>
<td align="left">&#x2003;A x B</td>
<td align="char" char=".">10</td>
<td align="char" char=".">12.29</td>
<td align="char" char=".">1.229</td>
<td align="char" char=".">2.476</td>
<td align="char" char=".">0.008</td>
</tr>
<tr>
<td colspan="6" align="left">Time to the first brood</td>
</tr>
<tr>
<td align="left">&#x2003;Food type (A)</td>
<td align="char" char=".">2</td>
<td align="char" char=".">29.742</td>
<td align="char" char=".">14.871</td>
<td align="char" char=".">26.95</td>
<td align="char" char=".">&#x3c;0.001</td>
</tr>
<tr>
<td align="left">&#x2003;ZnO NPs concentration (B)</td>
<td align="char" char=".">4</td>
<td align="char" char=".">38.418</td>
<td align="char" char=".">9.604</td>
<td align="char" char=".">17.406</td>
<td align="char" char=".">&#x3c;0.001</td>
</tr>
<tr>
<td align="left">&#x2003;A x B</td>
<td align="char" char=".">8</td>
<td align="char" char=".">5.783</td>
<td align="char" char=".">0.723</td>
<td align="char" char=".">1.31</td>
<td align="char" char=".">0.258</td>
</tr>
<tr>
<td colspan="6" align="left">Body length at maturation</td>
</tr>
<tr>
<td align="left">&#x2003;Food type (A)</td>
<td align="char" char=".">2</td>
<td align="char" char=".">1.009</td>
<td align="char" char=".">0.336</td>
<td align="char" char=".">29.697</td>
<td align="char" char=".">&#x3c;0.001</td>
</tr>
<tr>
<td align="left">&#x2003;ZnO NP concentration (B)</td>
<td align="char" char=".">5</td>
<td align="char" char=".">0.0641</td>
<td align="char" char=".">0.0307</td>
<td align="char" char=".">2.487</td>
<td align="char" char=".">0.09</td>
</tr>
<tr>
<td align="left">&#x2003;A x B</td>
<td align="char" char=".">10</td>
<td align="char" char=".">0.764</td>
<td align="char" char=".">0.127</td>
<td align="char" char=".">10.32</td>
<td align="char" char=".">0.956</td>
</tr>
<tr>
<td colspan="6" align="left">Offspring of the first brood</td>
</tr>
<tr>
<td align="left">&#x2003;Food type (A)</td>
<td align="char" char=".">2</td>
<td align="char" char=".">1599.652</td>
<td align="char" char=".">799.826</td>
<td align="char" char=".">149.378</td>
<td align="char" char=".">&#x3c;0.001</td>
</tr>
<tr>
<td align="left">&#x2003;ZnO NP concentration (B)</td>
<td align="char" char=".">4</td>
<td align="char" char=".">680.61</td>
<td align="char" char=".">170.152</td>
<td align="char" char=".">31.778</td>
<td align="char" char=".">&#x3c;0.001</td>
</tr>
<tr>
<td align="left">&#x2003;A x B</td>
<td align="char" char=".">8</td>
<td align="char" char=".">30.379</td>
<td align="char" char=".">3.797</td>
<td align="char" char=".">0.709</td>
<td align="char" char=".">0.682</td>
</tr>
<tr>
<td colspan="6" align="left">Total offspring per female</td>
</tr>
<tr>
<td align="left">&#x2003;Food type (A)</td>
<td align="char" char=".">2</td>
<td align="char" char=".">14741.86</td>
<td align="char" char=".">7370.931</td>
<td align="char" char=".">209.456</td>
<td align="char" char=".">&#x3c;0.001</td>
</tr>
<tr>
<td align="left">&#x2003;ZnO NP concentration (B)</td>
<td align="char" char=".">4</td>
<td align="char" char=".">16308.82</td>
<td align="char" char=".">4077.205</td>
<td align="char" char=".">115.86</td>
<td align="char" char=".">&#x3c;0.001</td>
</tr>
<tr>
<td align="left">&#x2003;A x B</td>
<td align="char" char=".">8</td>
<td align="char" char=".">794.709</td>
<td align="char" char=".">99.339</td>
<td align="char" char=".">2.823</td>
<td align="char" char=".">0.01</td>
</tr>
</tbody>
</table>
</table-wrap>
<fig id="F1" position="float">
<label>FIGURE 1</label>
<caption>
<p>Survival time <bold>(A)</bold> and slope <bold>(B)</bold> of the fitted liner function of <italic>D. magna</italic> exposed to different ZnO NP levels under the three food types. Some error bars were too short and covered by the data symbol.</p>
</caption>
<graphic xlink:href="fenvs-10-918570-g001.tif"/>
</fig>
</sec>
<sec id="s3-2">
<title>Development Traits</title>
<p>The non-toxic <italic>Microcystis</italic> and ZnO NPs significantly delayed the time to maturation and the time to the first brood of <italic>D. magna</italic> and reduced the body length at maturation, but two-way ANOVA showed that food types and ZnO NPs only had a significant interaction with the time to maturation of <italic>D. magna</italic> (<xref ref-type="fig" rid="F2">Figure 2</xref>; <xref ref-type="table" rid="T1">Table 1</xref>). These developmental traits changed linearly with the increase of ZnO NP concentrations. The slope derived from the linear fitting on the time to maturation changing with ZnO NP concentrations under 30% non-toxic <italic>Microcystis</italic> food type was significantly higher than that under only <italic>Chlorella</italic> food (<xref ref-type="fig" rid="F2">Figure 2B</xref>, <italic>p</italic> &#x3d; 0.0163), indicating that the addition of 30% non-toxic <italic>Microcystis</italic> in the food significantly exacerbated the delay of ZnO NPs on the time to maturation of <italic>D. magna</italic>, thus causing a synergistic effect. Additionally, to a certain extent (tending to be significant), the addition of 30% non-toxic <italic>Microcystis</italic> aggravated the delay of ZnO NPs on the time to the first brood of <italic>D. magna</italic> based on the slope difference (<italic>p</italic> &#x3d; 0.149). Both time to maturation and time to the first brood jointly showed that the addition of non-toxic <italic>Microcystis</italic> to food can significantly aggravate the harm of ZnO NPs to the development of <italic>D. magna</italic>. Additionally, linear fitting showed that there was no significant difference in the slopes of body length at maturation decreasing with the concentration of ZnO NPs under the three food types (<xref ref-type="fig" rid="F2">Figure 2F</xref>), indicating that there was only an additive effect on body length at maturation between non-toxic <italic>Microcystis</italic> and ZnO NPs. More seriously, under food types containing non-toxic <italic>Microcystis</italic>, <italic>D. magna</italic> could not reach sexual maturity at 0.25&#xa0;mg&#xa0;L<sup>&#x2212;1</sup> ZnO NPs and could not produce offspring at 0.20&#xa0;mg&#xa0;L<sup>&#x2212;1</sup> ZnO NPs, meaning that <italic>D. magna</italic> organisms were subjected to serious comprehensive stress under these conditions.</p>
<fig id="F2" position="float">
<label>FIGURE 2</label>
<caption>
<p>Development indicators and their slopes of <italic>D. magna</italic> under different conditions. Some error bars were too short and covered by the data symbol. Different lowercase letters in the figure represent significant differences.</p>
</caption>
<graphic xlink:href="fenvs-10-918570-g002.tif"/>
</fig>
</sec>
<sec id="s3-3">
<title>Reproductive Performances</title>
<p>Two-way ANOVA showed that non-toxic <italic>Microcystis</italic> and ZnO NPs significantly reduced the reproductive indicators of <italic>D. magna</italic> (neonates of the first brood and the total offspring per female), and there was an interaction between the two factors with the total offspring per female (<xref ref-type="table" rid="T1">Table 1</xref>). When ZnO NP concentration was 0.40&#xa0;mg&#xa0;L<sup>&#x2212;1</sup>, <italic>D. magna</italic> could not develop eggs and produce offspring even at 100% <italic>Chlorella</italic> of completely good food, while in food containing non-toxic <italic>Microcystis</italic>, <italic>D. magna</italic> could not reproduce offspring even when ZnO NP concentration was 0.25&#xa0;mg&#xa0;L<sup>&#x2212;1</sup> (<xref ref-type="fig" rid="F3">Figures 3A, C</xref>). These reproductive performances decreased linearly with the increase of ZnO NP concentrations. The slopes of neonates of the first brood under the three food types had no significant difference (<xref ref-type="fig" rid="F3">Figure 3B</xref>), indicating that there was only an additive effect on this indicator between non-toxic <italic>Microcystis</italic> and ZnO NPs. Moreover, the slope under the food type containing 15% non-toxic <italic>Microcystis</italic> was almost significantly lower than that under 100% <italic>Chlorella</italic> of completely good food (<italic>p</italic> &#x3d; 0.079), indicating that there was a tendency toward synergistic effect and non-toxic <italic>Microcystis</italic> exacerbated the reduction of total offspring production of <italic>D. magna</italic> by ZnO NPs to a certain extent.</p>
<fig id="F3" position="float">
<label>FIGURE 3</label>
<caption>
<p>Reproductive performances and their slopes of <italic>Daphnia</italic> in different treatments. Some error bars were too short and covered by the data symbol.</p>
</caption>
<graphic xlink:href="fenvs-10-918570-g003.tif"/>
</fig>
</sec>
</sec>
<sec sec-type="discussion" id="s4">
<title>Discussion</title>
<p>Our study showed that both non-toxic <italic>Microcystis</italic> and ZnO NPs negatively affected the survival time, development, and reproductive performances of <italic>D. magna</italic> significantly, and there was a significant interaction between non-toxic <italic>Microcystis</italic> and ZnO NPs only in the time to maturation and the total offspring per female, indicating that there were differential comprehensive effects of poor food quality and ZnO nanoparticles on the different key traits of the early life history of <italic>D. magna</italic>. Based on whether there was a significant difference between the slopes of each trait under different food types, we concluded that poor food quality and ZnO NPs had significant synergistic effects on time to maturation or nearly significant synergistic effects on time to the first brood and the total offspring per female, whereas for survival, body length at maturation, and neonates of the first brood, there was only an additive effect between non-toxic <italic>Microcystis</italic> and ZnO NPs. Overall, the increase of the proportion of non-toxic <italic>Microcystis</italic> in mixed food significantly or to some extent exacerbated the harmful effects of ZnO NPs on the early life history traits of <italic>D. magna</italic>, which basically confirmed our scientific hypothesis.</p>
<p>As cladocerans are key ecological players in the aquatic ecosystem (<xref ref-type="bibr" rid="B21">Miner et al., 2012</xref>), poor quality food aggravating the ecotoxicological effects of pollutants on cladocerans may affect the higher trophic-level organisms through the food chain. In this study, we not only used two different nutritional quality algae but also tested different combinations of the algae. Although some studies have demonstrated the toxicity of ZnO NPs to <italic>D. magna</italic> (<xref ref-type="bibr" rid="B1">Adam et al., 2014</xref>; <xref ref-type="bibr" rid="B2">2015</xref>), so far, our experiment was the first study about the comprehensive effects of non-toxic <italic>Microcystis</italic> and ZnO NPs on cladocerans, which provided a realistic evaluation about the comprehensive stress effect on zooplankton under the complex conditions of poor food quality and coexistence of pollutants.</p>
<p>In natural waters, cladocerans usually ingest green algae and <italic>Microcystis</italic> simultaneously (<xref ref-type="bibr" rid="B8">Hanazato and Yasuno, 1984</xref>). As non-toxic <italic>Microcystis</italic> lacks nutrients such as essential fatty acids and sterols (<xref ref-type="bibr" rid="B31">Von Elert and Wolffrom, 2001</xref>; <xref ref-type="bibr" rid="B19">Martin-Creuzburg et al., 2008</xref>), many studies have demonstrated that non-toxic <italic>Microcystis</italic> is a poor quality food for zooplankton (<xref ref-type="bibr" rid="B8">Hanazato and Yasuno, 1984</xref>; <xref ref-type="bibr" rid="B28">Smith, 1995</xref>). The nutritional deficiency of non-toxic <italic>Microcystis</italic> in mixed food in our experiment was compensated to a certain extent by the nutrition of a higher proportion of <italic>Chlorella</italic>, but it cannot be completely compensated, as reported in some previous studies that although cladocerans ingesting a good food with a small amount of non-toxic <italic>Microcystis</italic> can maintain their survival (<xref ref-type="bibr" rid="B10">Huang et al., 2020</xref>; <xref ref-type="bibr" rid="B3">Alva-Mart&#xed;nez et al., 2001</xref>), they also show a significant decline in development, reproduction, and growth (<xref ref-type="bibr" rid="B8">Hanazato and Yasuno, 1984</xref>; <xref ref-type="bibr" rid="B10">Huang et al., 2020</xref>), which was consistent with the results of our experiment using a certain proportion of non-toxic <italic>Microcystis</italic>.</p>
<p>
<italic>D. magna</italic> can ingest all the suitable sizes of nanoparticles in the aquatic environment (<xref ref-type="bibr" rid="B5">Danabas et al., 2020</xref>) and exhibit delayed maturation and decreased offspring production (<xref ref-type="bibr" rid="B22">Park et al., 2021</xref>), which was further demonstrated in our experiment. The harmful effects of various concentrations of ZnO NPs on some indicators related to the development and reproduction of <italic>D. magna</italic> were exacerbated by the consumption of mixed food containing non-toxic <italic>Microcystis</italic> compared with high-quality food in our study. Most studies have shown that the sensitivity of zooplankton to environmental stress is affected by food quality (<xref ref-type="bibr" rid="B9">Heugens et al., 2006</xref>; <xref ref-type="bibr" rid="B13">Ieromina et al., 2014</xref>; <xref ref-type="bibr" rid="B4">Betini et al., 2020</xref>). <italic>Daphnia</italic> are non-selective filter-feeding zooplankton, so non-toxic <italic>Microcystis</italic> does not affect its total food intake, but the nutrients obtained are different. When fed with a high proportion of poor-quality <italic>Microcystis</italic>, <italic>D. magna</italic> cannot obtain enough energy or essential nutrition to resist the nanoparticles (<xref ref-type="bibr" rid="B16">Lyu et al., 2013</xref>), and thus the development and reproduction are inhibited synergistically (<xref ref-type="bibr" rid="B6">Elert et al., 2003</xref>; <xref ref-type="bibr" rid="B19">Martin-Creuzburg et al., 2008</xref>). It should be emphasized that we got special findings in our experiments, that is, some life history traits were affected by poor food quality and ZnO NPs synergistically or nearly synergistically, whereas other traits were only affected by the additive effect of the two factors, indicating there were sensitivity differences among different traits. The synergistic effect of non-toxic <italic>Microcystis</italic> and ZnO NPs basically acts on the traits of the reproductive period of <italic>D. magna</italic>, that is, the traits related to reproduction are more sensitive and vulnerable to the synergistic effects of low-quality food and pollutants, which may be due to the narrow tolerance range of organisms to external ecological factors during the reproductive period, just as the reproductive indicators of <italic>D. magna</italic> are more sensitive to the increase of salinity (<xref ref-type="bibr" rid="B11">Huang et al., 2022</xref>). Therefore, it is of great significance to select specific indicators when evaluating the comprehensive effects of pollutants.</p>
</sec>
<sec sec-type="conclusion" id="s5">
<title>Conclusion</title>
<p>We supported the wide body of literature that recognizes ZnO NPs as hazardous substances and extended the results under complex food conditions. Non-toxic <italic>Microcystis</italic> and ZnO NPs significantly delayed the time to maturation and the time to reproduction and also significantly reduced the survival time, the body length at maturation, the number of offspring in the first brood, and the total offspring per female of <italic>D. magna</italic>. Although the <italic>Microcystis</italic> used in this experiment was non-toxic, its low nutritional quality still had a significant or nearly significant synergistic effect with ZnO NPs on some key life history traits of <italic>D. magna</italic>, such as time to maturation, time to the first brood, and the total offspring per female of <italic>D. magna</italic>, whereas for other life history indicators, non-toxic <italic>Microcystis</italic> and ZnO NPs only showed an additive effect. Such findings are helpful to realistically evaluate the comprehensive stress effect on zooplankton under the complex conditions of poor food quality and coexistence of pollutants. However, the robustness of these findings requires evaluation of other clones and species of cladocerans from pristine and non-toxic <italic>Microcystis</italic>-impacted waters.</p>
</sec>
</body>
<back>
<sec id="s6">
<title>Data Availability Statement</title>
<p>The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.</p>
</sec>
<sec id="s7">
<title>Author Contributions</title>
<p>YS and ZY conceived and designed the experiment. YS, BY, YL, SQ, and KL performed the experiments and analyzed the data. YS and ZY wrote the manuscript. All authors gave their final approval of the submitted and published versions.</p>
</sec>
<sec id="s8">
<title>Funding</title>
<p>The study was supported by the National Natural Science Foundation of China (31730105) and the Priority Academic Program Development of Jiangsu Higher Education Institutions of China.</p>
</sec>
<sec sec-type="COI-statement" id="s9">
<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 sec-type="disclaimer" id="s10">
<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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