Abstract
The vast holopelagic Sargassum assemblages within the Great Atlantic Sargassum belt (GASB) have been described as an important habitat for marine fauna. While previous studies have investigated the composition of the prokaryotic and eukaryotic communities associated with holopelagic Sargassum, the impact of these organisms on surrounding marine environments remains unclear. Sargassum aquafarming is a novel approach developed to provide companies with a stable biomass supply throughout the seasons. Due to potential impacts of aquafarming on surrounding marine ecosystems, an environmental impact assessment is necessary. Using state-of-the-art genomic methods, we investigated the prokaryotic and eukaryotic communities linked to holopelagic Sargassum mats, and compared the biodiversity of a Sargassum aquafarm off St. Vincent to an unimpacted adjacent bay. We found that water within wild Sargassum mats present a very unique community composition when compared to surrounding open ocean waters, underscoring its ecological importance as a pelagic habitat for a variety of species. We identified Rivularia spp., a nitrogen-fixing cyanobacterium, as a main component within the microbiome, potentially functioning as a nitrogen provider in oligotrophic waters, and a range of epiphytic eukaryotes, such as cnidarians and bryozoans, with E. bellula as one of the main epiphytes. When comparing the aquafarm bay with its adjacent bay, we found slight changes in community composition driven by the presence of Rivularia, but no significant impact on the alpha-biodiversity; this group disappeared in the surrounding marine environment after the trial, indicating its association with the cultivated Sargassum. These insights are relevant for the development of sustainable management practices for wild Sargassum influxes and its aquafarming as a sustainable source of biomass for the blue economy.
1 Introduction
Over the last decade, the two brown holopelagic seaweed species Sargassum natans (Sargassum natans var. natans and Sargassum natans var. wingeri) and Sargassum fluitans var. fluitans () have formed a new habitat in the North Atlantic: The Great Atlantic Sargassum Belt. When Sargassum mats reach the coast, accumulate and rot, they have profound environmental (; ; ; ) and economic implications (; ). Additionally, mass accumulations and stranding events can have indirect ecological impacts such as fish dying and altering population dynamics, especially for coastal reef fish (80% of recorded fish species), through environmental impacts such as oxygen depletion and excess nutrient content in the affected waters (; ).
Wild holopelagic Sargassum mats have been described as dynamic ecosystems that provide essential habitats for various marine organisms. The large aggregations offer structural complexity in an otherwise vast featureless oceanic environment (; ). Juvenile turtles (), fish () and a variety of marine invertebrates including 10 endemic species () inhabit this ecosystem, which provides shelter and food, especially important for early developmental stages. This ecological role is of significant importance in nutrient-poor waters of the tropical Atlantic, where the macroalga serves as a floating oasis supporting marine biodiversity. Additionally, the mats facilitate the transport of marine organisms across the North Atlantic and may contribute to gene flow and species dispersal in the open ocean (). The capacity of Sargassum to transport associated microbes is particularly relevant for pathogenic species that could be spread using Sargassum as a vector. , reported Sargassum-mediated transport of potentially pathogenic Vibrio depending on a complex interplay of environmental factors such as nutrients, wind speed, and zooplankton abundance. These findings emphasize the need to further understand the impact of Sargassum aggregations and their associated microbiome on the biodiversity of the surrounding oceanic waters.
While a variety of studies investigated the Sargassum-associated prokaryotic communities (; ; ), research on nutrient cycling and maintenance of the Sargassum ecosystem is scarce. Little is known about the nutrient sources that enable Sargassum to thrive in the oligotrophic regions of the GASB. Early studies have suggested Dichothrix spp. as a potential nitrogen-fixing microbe associated with the macroalgae (). Recent research has identified a variety of Cyanobacteria (mainly Dichotrix penicillata and Rivularia atra) on pelagic Sargassum () but also lead to the conclusion that the nitrogen fixation by heterotrophic diazotrophs plays a crucial role in Sargassum’s nutrient availability (). Investigating the Sargassum-bacteria associations is important for understanding the resilience of Sargassum in nutrient-poor conditions and predicting the ecological impacts of Sargassum aggregations and their responses to environmental changes.
Beyond its ecological role, holopelagic Sargassum has gained attention for its potential as a sustainable carbon source and for its potential as a carbon dioxide removal (CDR) strategy (; ). As a fast-growing macroalga, Sargassum captures atmospheric CO2 through photosynthesis and converts it into biomass. Oceanic and coastal cultivation of holopelagic Sargassum represents a novel approach to mitigate climate change through sinking and storage of the carbon in the deep sea or through the utilization of its carbon for durable materials. However, the efficiency of carbon uptake depends on the number of epiphytes growing on it () and the amount of dissolved organic carbon it produces (). These factors have to be considered to evaluate the true CDR potential of holopelagic Sargassum.
Macroalgae aquafarming is a rapidly progressing form of mariculture used as feedstock for food and diverse products or as a carbon dioxide removal strategy (CDR). Large-scale macroalgae cultivation can have significant implications for marine biodiversity. Several studies have shown that macroalgal aquafarms can enhance marine biodiversity by providing nursery grounds, shelter, and resources for a variety of marine organisms (; ; ). Initial studies on impact of macroalgae cultivation showed that Porphyra yezoensis and Sargassum fusiforme cultivation in China alter the community composition of native marine ecosystems (; ). Holopelagic Sargassum has not been aquafarmed at large scale to date and there are still open questions regarding the direction of its impact on marine biodiversity. Understanding these impacts or co-benefits is crucial for developing sustainable practices in the macroalgae cultivation industry. The floating nature of Sargassum calls for a unique design of aquafarm prototypes to contain the algae, while having minimal impact on the surrounding ecosystem. Large-scale aquafarming of holopelagic Sargassum has been delayed because it requires non-standard offshore infrastructure and its fragmentation-based reproduction complicates controlled seeding and farm management. These technical barriers, together with its long-standing perception of being a threat rather than a valuable resource, have slowed the industrial development.
Our study aims to investigate if the organisms associated with wild Sargassum aggregations leave an imprint in the surrounding waters and if Sargassum aquafarms impact local biodiversity during the first trial of a Sargassum aquafarm prototype deployed for one month in the Caribbean. By examining the biodiversity patterns and microbial associations using eDNA and metabarcoding of pro- and eukaryotes, we seek to enhance the understanding of Sargassum`s role in marine ecosystems and its broader environmental significance. The impact of Sargassum aquafarms on the biodiversity of surrounding waters was tested during one of the first trials of a Sargassum aquafarm prototype.
2 Material and methods
2.1 Study site
Sample collection took place between April 18 and May 20, 2023, on the west coast of the island St. Vincent, Saint Vincent and the Grenadines. To evaluate the impact of wild Sargassum mats on the biodiversity of the surrounding waters, environmental DNA (eDNA) samples were taken within a Sargassum mat, within immediate vicinity, and at distances of 16 km and 23 km (Figure 1). The sampled wild Sargassum mat was approximately 10 m² in size and part of a longer windraft of smaller Sargassum mats (Supplementary Figure 1). Bathymetry and geolocation data derived from GEBCO () and OpenStreetMap Coastlines () showed the locations of the Inside Mat and Next to Mat samples to be similar in depth and distance to shore, with 1276 m and 1289 m depth and 5609 m and 5824 m distance to shoreline respectively. The 16 km sample was found to be the most shallow sample location, with only 470 m in depth and 2054 m from shore, while the 23 km sample was collected over 1825 m in depth and 9150 m from shore (Supplementary Table 1). The aquafarm was located in Mt. Wynne Bay on the west coast of St. Vincent, in roughly 10–30 meters distance to the shoreline. As the aquafarm was attached to the ocean floor only via an anchor, it was able to swerve with the currents, resulting in a non-fixed position over time. The depth beneath the aquafarm, depending on its location, was 5–10 m. To assess whether Sargassum aquafarming affects the surrounding habitat, we sampled both the deployment bay in ~ 10 m distance to the aquafarm and the closest bay located south of the aquafarm (Figure 1). Both bays differed strongly in the habitat type: seagrass dominated the aquafarm bay, while sponges and corals were the main features in the control bay (Supplementary Figure 2). We collected four sets of samples over a period of 32 days, the last sample being taken two days after the aquafarm was removed from the water. A CTD (CTD90M, Sea & Sun Technology), equipped with an AMT pH sensor, an optical oxygen sensor, and a current meter with compass was submerged below the aquafarm from the 25th of April to the 14th of May, covering two of our sampling dates. Daily means of oxygen saturation, pH, salinity and temperature were calculated and plotted in R programming language () using the “ggplot2” package (Supplementary Figure 3). Current data are presented for one day as an illustrative example (Supplementary Figure 4).
Figure 1
2.2 Field data collection
To ensure sampled water was not contaminated by the sample collection or the boat, latex gloves were used, and samples were taken upstream of the boat. 5-liter water containers were submerged under the ocean surface until completely filled, sealed straight away, put on ice, and filtered within six hours after collection to keep DNA breakdown to a minimum. Each sample was filtered by hand using 3x 0.2µm Sterivex® filters (2x 2 liters and 1x 1 liter) and 50 ml syringes. The loaded filters were filled with 2 ml of RNAlater® (Thermo Fisher Scientific) and sealed with Parafilm® (Sigma-Aldrich). Samples were stored at -20 °C until DNA extraction. Visual observation did not find any larger animals or plants filtered onto the Sterivex filter. Figures.
2.3 eDNA extraction and analysis
For extraction the DNeasy PowerWater Kit, Cat.No./IF: 14900-100-NF (QIAGEN GmbH) was used with a slightly adapted protocol. The plastic cartridge around the Sterivex® filters was cracked using PVC forceps (Supplementary Figure 5). After opening the cartridge, the filter was cut into several pieces using a sterile scalpel and transferred into the manufacturer’s 5 ml PowerWater Bead Tubes via tweezers. After extraction of the filter the DNA content was analyzed with a NanoDrop 1000 (Thermo Fisher Scientific) and the extract was stored at -80 °C. To ensure assessment of potential contamination during the whole sample analysis, process blanks (sterile filters) were processed simultaneously with the sample batch. .
To amplify the 16S rRNA gene V4 hypervariable region primers V4-515F and V4-806R (; ) were used. Amplification of the 18SrRNA gene V7 hypervariable region was conducted by using the primers F-1183mod and R-1443mod (). Amplification was performed with KappaHifiPolymerase (Roche Diagnostics Deutschland GmbH, Germany) within the Mastercycler X50p – PCR-Thermocycler (Eppendorf SE, Germany). Preparation of the samples for the Illumina MiSeq sequencing platform required two amplification processes. The protocol for the first amplification process consisted of: initial denaturation at 95 °C for 5 minutes and 35 cycles of denaturation at 98 °C for 20 seconds, annealing at 55 °C for 15 second and extension at 72 °C for 1 minute. The subsequent index amplification protocol was identical, but cycles were reduced to 25. Samples were stored at 4 °C until further examination.
Of each of the resulting amplicons, as well as of GeneRuler 50bp DNA standard (ThermoScientific), 5 µl were mixed with 6x TriTrack loading dye (Thermo Scientific) and loaded on agarose gels. Gel electrophoresis was performed in a sodium borate buffer at 140V for 1 hour and 30 minutes. The gel was then transferred into an ultraviolet chamber (QUANTUM-1000/26M, Vilber Lourmat) and pictures of fluorescence were taken with the QUANTUM program. Fluorescence intensity of gel bands representing the target amplicon relative to fluorescence intensity of the standard was analyzed within ImageJ (), and samples were pooled in equifluorescent concentrations. Following a second electrophoresis of the pooled samples, they were extracted from the electrophoresis gel via the Zymoclean Gel DNA Recovery Kit (Zymo Research) according to the supplier’s manual. Following this procedure, nucleotide size distributions and concentrations of the pools were inspected on an Agilent 2100 Bioanalyzer System, using the DNA 7500 kit. The samples were sequenced with the MiSeq next generation sequencer using the MiSeq Reagent Kit v3 (600 cycles) sequencing kit. The pool was prepared for sequencing according to the Illumina protocol. PhiX (Illumina, San Diego, California, USA) was used to control sequencing quality, clustering and alignment. While commonly used in biodiversity assessment studies, the chosen metabarcoding approach considering 16S and 18S is constrained by limited taxonomic resolution at species level, incomplete reference databases, and technical biases that prevent reliable correlation between read abundance and original species abundance ().
Demultiplexed raw sequences were obtained from the sequencing facility and edited and assembled using the DADA2 pipeline as implemented in the qiime2 software package () within the Linux Environment. Sequences were assessed for quality, paired, dereplicated and chimeras were removed. Data was corrected for contamination and sequences were annotated at 100% coverage and 95% identity with reference sequences. The 16S RNA sequences were annotated using the SILVA database (). ASVs annotated as mitochondria or chloroplasts were discarded. 18S RNA sequences were annotated using the PhytoRef 2 (PR2) database (). The clean OUT table can be found in Zenodo (https://doi.org/10.5281/zenodo.19049226). ASV`s (contaminations) found in the process blank were discarded from the whole dataset. R Studio version 4.4.2 () was used to generate relative abundance plots on genus/species level and heatmaps. To generate heatmaps relative abundance of taxa was calculated and compared between sample types using Z-Scores. For quantification of alpha-biodiversity Fisher`s index was used due to its robustness against uneven number of samples between the sample types. The statistical method used to identify significant differences between sample types were ANOVA and Tukey`s Honest Significant Difference (HSD). The Evenness scores for the comparison of aquafarm and control bay were conducted by downloading calculated Simpson scores and species richness from Microanalyst and further data analysis in R programming language using the “ggplot2” and “vegan” packages in R. Samples were tested for normality (Shapiro-Wilk test) and homogeneity of variance (Levene test). Statistics and boxplots were done in using base R, the “car”, and “ggplot2” packages. To generate heatmaps, the relative abundance of taxa was calculated and compared between sample types using Z-Scores within the “ComplexHeatmap” and “magick” packages. To identify genera that describe differences between the water masses and thus, find potential biomarkers for Sargassum impacted waters, we conducted a linear discriminant analysis (LDA) effect size (LEfSe) analysis, combining non-parametric statistical testing with LDA to estimate effect size of each feature in the eDNA-based dataset, via MicrobiomeAnalyst ().
3 Results
High-throughput sequencing yielded 4,261,491 reads of 21,151 prokaryotic amplicon sequence variants (ASVs) and 437,288 reads encompassing 2167 eukaryotic ASVs.
3.1 Biodiversity of the microbiome in and around wild Sargassum mats
The prokaryotic community associated with wild Sargassum aggregations showed distinct taxonomic features when compared to the surrounding prokaryotic community in the open ocean. Fisher biodiversity was significantly increased within the Sargassum mat when compared to waters in 16 and 23 km distance (Post-hoc: p-values < 0.05, ANOVA: p-value <0.01, F-value = 23.811, Figure 2) and the predominant genera varied strongly between the different sample types. The water inside the wild Sargassum mat was uniquely enriched with Rivularia PCC 7116 (Nostocaceae).
Figure 2
Within all samples, open ocean and wild Sargassum mats, the twenty most predominant genera made up 75% or more of the relative abundance. All oceanic samples shared Cyanobiaceae (Next to Mat: 13%, 16 km: 25%, and 23 km: 24%), and the SAR86 clade (17%, 11%, and 16%) as families with highest relative abundance, while predominant families in the Sargassum mats were Nostocaceae (Rivularia PCC 7116) (20%), Cyanobiaceae (15%) and the SAR86 clade (9%) (Figure 3). Rare microbial families (<2% relative abundance) within all samples were SAR11 Clade II, Desulforominadia PB19, Parvibaculales OCS116 and Flavobacteriaceae (NS5). Rare microbial communities within the oceanic samples were Nostocaceae (all), Actinomarinaceae (16 km), Marinimicrobia (16 km), Bdellovibrionaceae (16 km), SAR11 Clade IV (16 km) and Trichodesmium (Next to mat). Additional rare microbial communities within the Sargassum mat were Actinomarinaceae, Bdellovibrionaceae OM27, Trichodesmium and Marinimicrobia (SAR406). The heatmap showed a variety of specific microbial taxa that are limited to or enriched in the wild Sargassum mat, distinguishing the habitat from oceanic waters. Some of these taxa were Negativicutes, Coriobacteriia, Denococci, Campylobacteria, Parcubacteria, Anaerolineae, and Bacilli (Figure 4). Additionally, the microbial community composition in the mat-adjacent water showed strongly enriched taxa, not only in comparison to the wild mat but also the open ocean waters, suggesting unstudied processes around the mats. Some of these taxa were Oligoflexia, Nitrososphaeria, Planctomycetes, Dehalococcoidia, Poribacteria, Fusobacteriia and Clostridia. While a high abundance of Cyanobacteria was not limited to the Sargassum mats, the genera enriched within Sargassum mats were different from those from the open ocean. While Rivularia was the main genus within the wild Sargassum, Synechococcus was the main genus found in the open ocean samples. The absence of Rivularia in Sargassum proximate waters suggests a strong association between this type of microorganism and the macroalgae.
Figure 3
Figure 4
LEfSe analysis detected four genera with linear discriminant analysis (LDA) scores above 4.5, indicating a strong variation between the samples. The genus that showed the highest LDA score, thus varying the strongest between the samples, was Rivularia PCC 7116, which was abundant within the Sargassum mat. Additionally, Vibrio and Synechococcus CC9902 were found to characterize the water masses at 16 km distance to the large Sargassum aggregation and the SAR85 clade characterized the waters surrounding the wild mat (Figure 5).
Figure 5
Within the eukaryotic data a non-significant decrease in biodiversity within the wild Sargassum mat was found when compared to the oceanic waters (p-value = 0.065, F-value = 3.834, Figure 2). The predominant families within the eukaryotic communities varied strongly between the wild mat and the open ocean samples. While the ten most abundant groups made up less than 75% of the relative abundance of the open ocean communities, they accounted for approximately 90% of the Sargassum-associated eukaryotes. All oceanic samples showed high relative abundances for the families Dinophyceae (Next to mat: 33%, 16 km: 31% and 23 km: 24%), Syndiniales Dino Group I & II (14%, 19% and 16%) and Bacillariaceae (15%, 7%, 5%), whereas the abundance of these families was rather low within the Sargassum mats (8%, 2% and 5%). In contrast, wild Sargassum showed high relative abundances for the families Ceramiales (16%) and Bryozoa (38%) that were found in only low abundances or not at all within the oceanic samples (Figure 6). Heatmap analysis supports these findings by showing Cnidaria, Florideophyceae (Ceramiales), Bryozoa and Phaeophyceae as some of the enriched taxa within the wild Sargassum. The main species found within these families were Clytia spp., Centroceras sp., Electra bellula and Sargassum thunbergii, the latter being only detected in the wild Sargassum mat with 0.8% of all reads. Furthermore, the heatmap not only shows strong variation between the Sargassum adjacent waters and the open ocean but also between the wild Sargassum and its adjacent waters (Figure 4). Within these waters Acantharea, Gregarinomorphea, Prymnesiophyceae, and Marine Stramenopiles and Ochrophyta (MOCH 4) were strongly enriched, creating a distinct habitat. The results suggest a distinct Sargassum-associated prokaryotic and eukaryotic community compared to the oceanic waters surrounding it.
Figure 6
3.2 Biodiversity around Sargassum aquafarms
The daily mean oxygen saturation ranged between 88.93% (26th of April) and 97.08% (5th of May). The pH varied between 8.1 and 8.18, salinity between 34.9 PSU and 35.9 PSU, and temperature between 27.3 °C and 28.1 °C. All measured variables showed slight differences between days. While on the 27th of April salinity was 35.18 PSU, temperature was 27.6 °C, oxygen saturation was 91.3% and pH was 8.11 on average, all of the parameters were higher on average on the 14th of May (35.48 PSU, 28.1 °C, 95.4% oxygen saturation and 8.18 pH, Supplementary Figure 3).
To test the first Sargassum aquafarm prototype, wild Sargassum was collected and kept in the prototype for 30 days. In order to investigate potential changes in the biodiversity of the bay, in which the aquafarm was tested, we analyzed water samples from aquafarm adjacent waters and compared them to aquafarm unimpacted waters of a bay located south of the aquafarm bay. Additionally, to test whether removal of the aquafarm would impact the biodiversity, we tested both the aquafarm and control bay three days after removal of the aquafarm.
While the prokaryotic microbial community did not differ significantly between the aquafarm and control bay during the trial, we were able to see a significant increase in the prokaryotic alpha-biodiversity of the aquafarm (p < 0.01, F-value = 12.32; Figure 2). The community around the aquafarm was dominated by ten families that made up more than 60% in all the samples. The families that showed highest abundance throughout the samples were the SAR86 clade and Cyanobiaceae (Figure 3). The nitrogen fixer Trichodesmium (Phormidiaceae) was very abundant in the control bay during the trial (26%), but its abundance around the aquafarm was low (1.2%) (Figure 3). While heatmap analysis showed similarities in the enriched species in the control and aquafarm bay during the trial, some species were enriched differently in the two bays. For example, the aquafarm bay showed higher read abundances for some sulphur-reducing bacteria (Desulfovibrionia and Desulfobacteria), while the control bay was enriched for Cyanobacteria and Dehalococcoidia. Additionally, Chlamydiae were strongly enriched within the aquafarm bay when compared to the control bay during the trial or both bays after removal of the algae and barrier. After the trial, the aquafarm bay was enriched in even more different sulphur-reducing bacteria (Desulfuromonadia, Desulfovibrionia and Desulfobulbia) as well as Alphaproteobacteria, while the control bay was enriched with Gammaproteobacteria and thermophilic orders (Thermoplasmata and Rhodothermia) (Figure 4). Additional analysis of the Evenness of the prokaryotic community did not show significant differences between the sample types (Supplementary Figure 6). During the aquafarm trial, mean Evenness was 0.172 ± 0.009 in the aquafarm bay and 0.153 ± 0.037 in the control bay. After the removal of the infrastructure and Sargassum biomass the mean Evenness was 0.165 ± 0.002 in the aquafarm bay and 0.173 ± 0.0002 in the control bay.
The biodiversity of the eukaryotic community did not differ significantly between the sample types (p-value = 0.51, F-value = 0.788; Figure 2). The top ten most abundant families constituted for more than 80% of the community composition within all samples. Dinophyceae made up more than 40% of the community composition within the aquafarm and control bay during and after the trial. The highest relative abundance of reads was found in the aquafarm bay after trial (57%), the lowest relative abundance was found in the aquafarm bay during the trial (43%). The biggest difference between the eukaryotic communities was the presence of the genus Bryozoa in the aquafarm bay but not in the control bay after the trial ended. The genus made up more than 8% of reads within the aquafarm bay during trial and decreased to 0.04% after removal of algae and aquafarm structures. Interestingly, Bryozoans made up 1.4% of reads within the control bay during the trial but dropped to complete absence of reads after the trial ended (Figure 6).
Heatmap analysis indicated a strong enrichment of a variety of classes within the aquafarm during the trial and after removal of the aquafarm when compared to the control bay. During the trial the aquafarm bay community composition was strongly enriched in Cnidaria, Bacillariophyceae, Sagenista and Bryozoa. After the trial, the aquafarm bay was enriched in Florideophyceae and Diplonemea, when compared to the other samples. Additionally, the heatmap indicated a larger difference in enriched species within the bays during and after the trial, when compared to one another (Figure 4), indicating a stronger impact by temporal variations than the deployed aquafarm. The LDA analysis did not identify any species as specifically associated with the aquafarm or control bay. Additional Evenness analysis showed no significant differences in calculated scores but a slight trend for a decrease in the control bay after the trial (Supplementary Figure 6). Calculated mean Evenness was 0.213 ± 0.014 in the aquafarm bay and 0.213 ± 0.011 in the control bay during the trial. After removal of the infrastructure and biomass, Evenness scores were 0.212 ± 0.004 in the aquafarm bay and 0.201 ± 0.0003 in the control bay.
4 Discussion
Pelagic Sargassum serves as an essential ecological habitat in the open ocean, providing critical ecological functions such as nutrient recycling and carbon uptake in otherwise nutrient-poor waters with low biomass. The floating macroalga supports complex prokaryotic and eukaryotic networks that form close associations within the macroalgal floating aggregations, creating dynamic environments that contribute to marine biodiversity. This study provides new insights into the biodiversity within and around wild Sargassum mats and Sargassum aquafarms to better understand the impact of this holopelagic macroalga on open ocean and coastal ecosystems.
4.1 Biodiversity in and around wild holopelagic Sargassum mats
Wild holopelagic mats of Sargassum off the coast of St Vincent and the Grenadines are rich in prokaryotic and eukaryotic biodiversity, with a significant increase in prokaryotic biodiversity inside wild Sargassum mats. Nevertheless, we could identify certain members of the communities surrounding Sargassum that did not appear in the open ocean. For example, Rivularia, a genus within the family Nostocaceae, made up roughly a fifth of all reads collected from the water in the wild Sargassum patches, while it was not detected in the open ocean samples, suggesting a close association of this nitrogen fixing cyanobacteria with Sargassum. This algae-bacteria interaction likely provides a critical nitrogen source for Sargassum, enabling the algae to thrive in nutrient-poor waters. Previous studies investigating Sargassum and bacteria association have identified the closely related genus Dichothrix (; ) within wild Sargassum mats. However, due to the morphological and genetic similarities between the two genera (), these studies might have misidentified these cyanobacteria. For better comparison, we tried to compare 16S sequences of both species, but only partial sequences are available. More recent studies also showed the importance of heterotrophic diazotrophs on the nutrient availability of Sargassum, with Vibrio (Gammaproteobacteria) and Filomicrobium (Alphaproteobacteria) being the main contributors to the nifH pool and a small proportion of Planctomycetes (). Sargassum mats are a major source of dissolved organic matter in the Atlantic Ocean (), a potential source of energy for heterotrophic diazotrophs. The enrichment of the heterotrophic diazotrophs in the Sargassum adjacent waters within our study suggests that these energy sources might be available further than the edges of the mats. Additionally, it shows a potential competition between the two classes of diazotrophs. While Rivularia, a solely epiphytic heterotrophic cyanobacterium, potentially uses Sargassum for dispersal and as a habitat, heterotrophic diazotrophs might benefit from organic substances released by Sargassum as an energy source that is available in no immediate vicinity of the algae’s surface. These circumstances could allow a cohabitation of Sargassum environments without the need for competition. Interestingly (), found the exact opposite trend, with heterotrophic diazotrophs being more abundant in the Sargassum biofilm when compared to adjacent waters. Therefore, nitrogen fixation in holopelagic Sargassum environments has yet to be understood, and future work should consider using metagenomic approaches to make more precise assumptions about the nitrogen-fixing bacteria on Sargassum.
The importance of Sargassum-associated Vibrio species in terms of their marine and human health implications when washed ashore remains heavily debated. While () reported a higher relative abundance of Vibrio in Sargassum microbiomes closer to shore (), found a stronger correlation to growth rates, with decreasing growth rates, and thus lower Vibrio abundances in coastal areas. While we were also able to identify Vibrio as part of the Sargassum community, we refrain from reporting a Sargassum-specific association due to the high prevalence of Vibrio in coastal oceanic waters. Unlike prior studies, which often lacked the sampling of near-shore waters unaffected by Sargassum aggregations, our findings revealed high Vibrio LDA (Linear Discriminant Analysis) scores in a sample collected in 16 km distance to the Sargassum aggregation. This sampling location was also the most proximate to shore (Figure 1), indicating a possible influx of Vibrio by human activities, such as ballast water discharge (). Coastal waters of the island of St. Vincent are subject to wastewater discharge from the capital without any pre-treatment and greywater discharge from hotels (; ). Additionally, the island is strongly impacted by the cruise industry of roughly 90 ships in the 2025–2026 cruise season (). These findings challenge the conclusions made by , who reported higher Vibrio abundance in coastal Sargassum aggregations and attributed them to Sargassum-mediated introduction. In contrast, our data showed less Sargassum-impacted waters (on the day of sampling) closer to shore to have higher counts of Vibrio reads, indicating that proximity to shore – and its associated anthropogenic factors – could be a more significant driver of prevalence of Vibrio than the presence of Sargassum. Our findings underscore the importance of considering a broader variety of influences when interpreting microbial dynamics in Sargassum-associated environments.
The eukaryotic communities associated with Sargassum mats reveal another dimension of its ecological importance. The mats support a variety of epiphytic organisms, including cnidarians (Clytia spp.), for which the polyp life stages are known to be epiphytic, bryozoans (Electra bellula), and other algae, such as Ceramiales (Centroceras sp.). The high relative abundance of reads of these taxa within the wild aggregations and visual observation of bryozoans on Sargassum surfaces (Supplementary Figure 8), coupled with their absence in surrounding waters, indicates that Sargassum provides a highly specialized habitat for these epiphytic organisms. As Sargassum forms large, dense mats of algae that can spread over a wide area, it facilitates transport of epiphytic organisms () across otherwise uninhabitable environments, serving as a refuge for marine life in the open ocean. However, the strong enrichment of specific taxa within the mats resulted in decreased alpha-biodiversity, highlighting the selective nature of this habitat. The limited exchange between Sargassum-associated communities and surrounding waters suggests that these mats act as isolated ecological niches, promoting persistence of specialized organisms while minimizing their dispersal to other environments.
Hydrodynamics are mediated by properties of obstacles in the water column. Holopelagic Sargassum has three major morphotypes (S. natans var. natans, S. natans var. wingeri, and S. fluitans var. fluitans) that differ by, e.g., their overall shape, size and shape of blades, and size of vesicles. As Sargassum mats commonly compose a sympatry of all three morphotypes, it is likely that the composition could lead to altered flow dynamics and transport of associated organisms within the mat. While () did not report differences in the attached microbiome for the different morphotypes, there could be differences in the surrounding water masses that were not examined in their study. While we are not able to report the composition of the wild mat sampled, we report the Sargassum aquafarm to be composed of 60% S. fluitans var. fluitans, 9% S. natans var. wingeri, and 31% S. natans var. natans within the first week of sampling and a shift during the growth phase to 60% S. fluitans var. fluitans, ~15% S. natans var. wingeri, and ~25% S. natans var. natans within the last week of sampling (Supplementary Table 2).
The potential use of Sargassum for carbon dioxide removal (CDR) raises important ecological and environmental considerations. The presence of encrusting marine epifauna, such as bryozoans, may reduce efficacy of Sargassum for CDR by contributing to calcification (). However, previous trials conducted in St. Vincent, reported a low PIC: POC ratio of wild Sargassum, including their epibionts (0.09 ± 0.15, Supplementary Table 3), suggesting minimal calcification by the epibionts. Our results support this observation, as E. bellula was found to be only slightly calcifying in Caribbean waters (). These results indicate that while Sargassum seems to be an important habitat provider for epibionts, the calcification rate and the resulting decrease in carbon sequestration potential in Caribbean waters is minimal.
4.2 Sargassum aquafarming and its impact on biodiversity
During the first one-month trial of Sargassum aquafarming in St. Vincent inside a floating barrier, we were not able to find significant differences in pro- and eukaryotic alpha-biodiversity between the area in which Sargassum cultivation took place and the proximate control bay, indicating that overall species richness was not affected by Sargassum aquafarming in a coastal environment within this short time period. However, we did see a significant increase in prokaryotic alpha-biodiversity after the removal of the aquafarm. While there are potential explanations for this by the removal of the aquafarm such as resource enrichment and release of tightly associated microbiota, have shown that despite physically and chemically stable conditions Puerto Rico coastal environments show highly dynamic microbiomes with strong changes during slight salinity variations. Additionally, Pacific waters in the San Pedro area show strong temporal shifts during seasons, with monthly varying microbiomes (). The environmental parameters from the surface water column show slight differences in temperature, oxygen saturation, pH, and salinity during the trial, which did not result in significant differences in alpha-biodiversity between the two covered sampling days (see Results section “Biodiversity around Sargassum Aquafarms” and Supplementary Figure 3). Despite not having environmental parameter data from the water column after the removal of the aquafarm, we do not expect the natural variability in the physicochemical parameters of the aquafarm bay to have caused the shift we observe in biodiversity after the removal of the aquafarm. To confirm this, future studies should continue monitoring water column environmental parameters for at least one month after removal of the aquafarm to test the hypothesis that natural variability of physicochemical parameters is not the cause of changes in biodiversity in the area where the aquafarm was deployed.
Interestingly, we did see a shift in community composition in both bays, with an increased abundance in reads of Trichodesmium in the control bay during the trial, when compared to the aquafarm bay. This is particularly interesting, as we were able to observe a large Trichodesmium bloom affecting both bays on May 3rd. Visual observation found the nitrogen-fixing cyanobacteria to get entangled in the Sargassum and die after a couple of days in the aquafarm bay (Supplementary Figure 7). The decline of these cyanobacteria, while being entangled in the Sargassum may have several explanations, including environmental stress (e.g. reduced water mixing or photostress) or a competition for nutrients with other Sargassum-associated cyanobacteria (e.g. Rivularia) that may better adapted to this unique habitat. Within our results on the impact of wild Sargassum mats on the surrounding water, we found Sargassum-associated bacteria (in this case Rivularia) to not be detected in the approximate waters (Figure 3). Therefore, we explain the lesser relative abundance of Trichodesmium in the aquafarm bay samples with the entanglement in the Sargassum and our samples being taken outside of the aquafarm. However, a small relative abundance of Rivularia was found in our samples collected during the trial with an increase after decommissioning of the aquafarm (Figure 3). While we do propose a limited impact of Sargassum aquafarming on the surrounding marine prokaryotic communities due to limited species exchange between the Sargassum and adjacent waters, further research is necessary to completely understand the water flow through Sargassum accumulations and the resulting species dispersal, especially of Rivularia. Our results also indicate a shift in enrichment of several nitrogen-fixing bacterial groups associated with Sargassum, including Alpha- and Gammaproteobacteria as well as Planctomycetes, during and after the aquafarm trial. As these groups were also found in the adjacent waters of wild Sargassum mats, the observed pattern likely reflects a change in the relative representation or dispersal of an established Sargassum-associated microbiome rather than the emergence of distinct farm-specific communities. Such variation might result from differences in biomass retention, energy source (potentially Sargassum-DOM) availability, microbial detachment from algae during decommissioning, or water exchange, all of which can alter the eDNA content in the surrounding waters. Additionally, the decommissioning of the aquafarm led to a great disturbance of the sediment, due to the heavy infrastructure being dragged out of the water in the aquafarm bay. The resuspension of the sediment could have led to the introduction of sediment-associated DNA and microorganisms in the water column and thereby increasing the biodiversity in eDNA samples. In this context, the observed enrichment in sulfur-reducing bacteria after decommissioning of the aquafarm may be consistent with disturbance-related release of taxa associated with anoxic or sedimentary habitats. Alternatively, the increase in biodiversity could reflect the displacement of Sargassum-associated microbiota during the removal process. However, because sampling was conducted two days after decommissioning, and environmental data (current data (Supplementary Figure 4) and visual observations of naturally occurring Sargassum inundation) indicated that water exchange occurs several times a day, we cannot make a precise suggestion as to why the microbiome community and biodiversity in the water column shifted. This interpretation is consistent with the stronger response of the prokaryotic biodiversity relative to eukaryotes to the decommissioning of the aquafarm, as bacterial communities and their extracellular DNA often respond more rapidly to short-term changes in their habitat. However, direct functional measurements would be required to determine whether the taxonomic shifts in prokaryotic community composition translate into broader changes in the microbial ecosystem processes. We strongly encourage further studies to investigate this phenomenon and assess the environmental impact of Sargassum aquafarms before, during, and after deployment.
The eukaryotic community composition also showed noteworthy differences between the control and aquafarm bays. The increased abundance of Electra bellula reads in the aquafarm bay reflects the strong association between these organisms and holopelagic Sargassum. The presence of Sargassum likely provided the structural habitat and nutrient-rich environment that facilitated the proliferation of Bryozoans (; ). However, after removing the Sargassum aquafarm, Bryozoans were no longer detected in the samples taken from the water`s surface in either of the investigated bays. This result suggests that their presence is strongly related to the availability of macroalgae substrates, and their absence post-removal indicates a loss of habitat that directly impacted their populations. The detection of Bryozoans within the control bay during the trial could be explained by the Bryozoans reproductive strategy, e.g. budding or release of planktonic larvae (). Here, the Evenness showed no difference in the community in the aquafarm bay during and after the trial. The decreased values in the control bay after the trial emphasize the importance of better collection and analysis of environmental water parameters in future studies.
The absence of significant differences in alpha-biodiversity of pro- and eukaryotes between the Bays during the trial suggests that Sargassum aquafarming does not inherently reduce species richness. This finding aligns with studies conducted in temperate and tropical regions, where macroalgae farms have been shown to support similar or even higher species richness compared to surrounding areas. For example, a study in coastal waters in Costa Rica showed an increase in biodiversity after implementation of seaweed plots (). Similarly, a study conducted in Japan showed Sargassum fusiforme farming to impact abiotic factors such as nutrient content and pH but still leading to increased phytoplankton biodiversity (). Our results align with findings from (), who found blooming events of Ulva rigida and Gracilaria vermiculophylla in coastal waters of Portugal to not change prokaryotic alpha-biodiversity, while changes in the community structure were dependent on the taxonomic levels considered. While we did find differences in community structure between the bays, it is important to note that the habitats within the two sampling sites, despite their proximate locations, were very different habitats. While the farming bay was primarily characterized as a seagrass habitat, the control bay was a sponge and coral habitat (Supplementary Figure 2). Our results show this by a strong enrichment of sponges within the control bay during and after the trial (Figure 4). Additionally, this study has a very limited temporal resolution of one month and is lacking investigation of the biodiversity prior to the deployment of the aquafarm. This means that long-term impacts and potential co-benefits given by Sargassum aquafarming need to be investigated in future studies. However, there are also concerns emphasizing potential risks associated with macroalgae farming. Some of these concerns are nutrient enrichment from organic matter deposition that might affect benthic communities (), which emphasizes the need to further investigate our findings of sulphur-reducing bacteria enrichment after the removal of the aquafarm. However, all of the above studies investigated impacts and challenges of benthic macroalgae farming that differs strongly from the approach of pelagic Sargassum aquafarming. This study serves as a preliminary assessment of the potential impacts and co-benefits of Sargassum aquafarming in coastal Caribbean waters.
This study compared the biodiversity around wild Sargassum mats, as well as Sargassum aquaculture systems. While both environments were associated with an increase in alpha-diversity, one within the wild Sargassum mat and the other after decommissioning of the aquafarm, the underlying drivers seem to differ. Wild Sargassum mats seem to enhance the alpha-biodiversity, most likely due to providing a stable and protected habitat within the floating biomass. In contrast, the increase in alpha-biodiversity after decommissioning of the aquafarm is likely due to disturbances of the sediment, rather than the dispersal of Sargassum-associated organisms. This hypothesis is supported by the limited taxonomic overlap between the two sample types, with only a few taxa being enriched in both, and many being system-specific. This taxonomic overlap is increased when comparing the samples collected in the aquafarm bay during the trial and the samples taken next to the mat. This result is sensible as samples collected in the aquafarm bay were taken from close to the Sargassum biomass but not within. While there is an increase in overlap between the two sample types, there is still a wide range of system-specific species that only occur in one of the two. Additionally, there are important differences in persistence and condition of biomass between wild Sargassum mats and aquafarmed Sargassum in coastal habitats. Whereas wild mats typically strand ashore and subsequently degrade, resulting in shifts in the community composition of the holobiont (), aquafarmed Sargassum can remain healthy and structurally intact, thereby maintaining a more stable habitat with less potential for shifts in the holobiont. Overall, our findings indicate that there are distinct differences between the wild Sargassum mats and aquafarm structures in coastal habitats with different implications for biodiversity, with the former acting as a structured habitat with limited species dispersal to outside waters and the latter primarily driving transient, disturbance-related changes in the environment.
5 Conclusion
Wild holopelagic Sargassum mats off St. Vincent and the Grenadines are habitat to a diverse pro- and eukaryotic community including cyanobacteria (e.g. Rivularia), bryozoans (e.g. Electra bellula), cnidarians and epiphytic algae. Despite significantly increased prokaryotic alpha-biodiversity within the mat, eukaryotic alpha-biodiversity was not increased within the mat, suggesting selective enrichment rather than overall diversity increase of eukaryotes. Strong mat-specific microbial signatures and limited species exchange with the adjacent waters indicate Sargassum mats as an ecological niche for species such as Rivularia. Our results indicated elevated Vibrio abundance in nearshore waters, rather than Sargassum mats, challenging prior assumptions of the magnitude of Sargassum-mediated transport of Vibrio.
This study investigated changes in the biodiversity and community composition during the first-ever trial of holopelagic Sargassum aquafarming using a unique aquafarm design. While there were no detectable changes in pro- and eukaryotic alpha-biodiversity between the aquafarm and control bays during cultivation, a temporary increase in prokaryotic alpha-biodiversity was found after the aquafarm removal. Detection of E. bellula during cultivation and its disappearance post-removal emphasize Sargassum`s role in providing essential habitat for sessile marine fauna and undermine our assumption, that the removal of the aquafarm does not impact the natural ecosystem after removal of the aquafarm. Notably, prokaryotic community shifts, including increases in sulphur-reducing bacteria post-removal emphasize the importance of distinguishing between natural variability and aquafarm-mediated change.
Overall, we hypothesize that short-term pelagic Sargassum aquafarming did not negatively impact coastal biodiversity off St. Vincent, supporting its potential as a sustainable aquaculture in coastal areas. Future studies should focus on long-term studies with baseline sampling and benthic impact assessment to fully evaluate ecosystem responses and potential co-benefits of Sargassum aquafarming.
Statements
Data availability statement
The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: https://doi.org/10.5281/zenodo.19049226, 10.5281/zenodo.19049226.
Author contributions
KP: Formal analysis, Data curation, Writing – original draft, Visualization, Methodology, Investigation, Writing – review & editing, Conceptualization. JS: Writing – original draft, Methodology, Investigation, Writing – review & editing. FE: Writing – review & editing, Investigation, Writing – original draft. LE: Resources, Validation, Writing – original draft, Writing – review & editing. SK: Writing – original draft, Resources, Writing – review & editing, Investigation, Validation. FW: Supervision, Writing – review & editing, Writing – original draft, Formal analysis, Resources, Data curation. MF: Conceptualization, Writing – review & editing, Supervision, Resources, Funding acquisition, Writing – original draft, Project administration.
Funding
The author(s) declared that financial support was received for this work and/or its publication. This research was supported by the C-CAUSE (Climate - Carbon Algae Utilization Sargassum Economy) project, which is funded by the German Federal Agency for Disruptive Innovation SPRIN-D Carbon to Value Challenge Phase I.
Acknowledgments
This research was carried out as part of a Master’s project in collaboration with the Alfred Wegener Institute for Polar and Marine Research and Seafields Solutions Ltd. We gratefully acknowledge the valuable support and dedication of the team based in St. Vincent and the Grenadines, whose efforts were instrumental in facilitating fieldwork and data collection.
Conflict of interest
KP reports previous employment with Seafields Solutions Ltd., as well as equity or stock ownership and travel support from the company. JS and MF-M report previous board membership at Seafields Solutions Ltd.; JS also reports travel reimbursement from the company, and MF-M reports equity or stock ownership. FE reports employment, equity or stock ownership, and travel support from Seafields Solutions Ltd., as well as consulting or advisory activities for Sayari Earth. MF-M also reports board membership at Macrocarbon.
The remaining author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The reviewer HSY declared a past co-authorship with the author FW to the handling editor.
Generative AI statement
The author(s) declared that generative AI was not used in the creation of this manuscript.
Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.
Publisher’s note
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Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmars.2026.1831822/full#supplementary-material
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Summary
Keywords
biodiversity impacts of seaweed aquaculture, Great Atlantic Sargassum Belt, holopelagic Sargassum, marine microbial communities, Sargassum Holobiont
Citation
Paetz K, Schnetzer J, Elmer F, Eggers SL, Kuenzel S, Weinberger F and Fernández-Méndez M (2026) Biodiversity and community dynamics of pelagic Sargassum: ecological and sustainable use implications of wild vs. cultivated aggregations. Front. Mar. Sci. 13:1831822. doi: 10.3389/fmars.2026.1831822
Received
16 March 2026
Revised
21 April 2026
Accepted
30 April 2026
Published
19 May 2026
Volume
13 - 2026
Edited by
Thanigaivel S., SRM Institute of Science and Technology, India
Reviewed by
Hwan Su Yoon, Sungkyunkwan University, Republic of Korea
Valerie Michotey, Aix-Marseille Université, France
Updates
Copyright
© 2026 Paetz, Schnetzer, Elmer, Eggers, Kuenzel, Weinberger and Fernández-Méndez.
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.
*Correspondence: Katharina Paetz, Katharina.paetz@awi.de
Disclaimer
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