Picoeukaryote Community Research Articles

Trophic strategies of picoeukaryotic phytoplankton vary over time and with depth in the North Pacific Subtropical Gyre.

In oligotrophic oceans, the smallest eukaryotic phytoplankton are both significant primary producers and predators of abundant bacteria such as Prochlorococcus. However, the drivers and consequences of community dynamics among these diverse protists are not well understood. Here, we investigated how trophic strategies along the autotrophy-mixotrophy spectrum vary in importance over time and across depths at Station ALOHA in the North Pacific Subtropical Gyre. We combined picoeukaryote community composition from a 28-month time-series with traits of diverse phytoplankton isolates from the same location, to examine trophic strategies across 13 operational taxonomic units and 8 taxonomic classes. We found that autotrophs and slower-grazing mixotrophs tended to prevail deeper in the photic zone, while the most voracious mixotrophs were relatively abundant near the surface. Within the mixed layer, there was greater phagotrophy when conditions were most stratified and when Chl a concentrations were lowest, although the greatest temporal variation in trophic strategy occurred at intermediate depths (45-100 m). Dynamics at this site are consistent with previously described spatial patterns of trophic strategies. The success of relatively phagotrophic phytoplankton at shallower depths in the most stratified waters suggests that phagotrophy is a competitive strategy for acquiring nutrients when energy from light is plentiful.

Distinct responses to warming within picoplankton communities across an environmental gradient.

Picophytoplankton are a ubiquitous component of marine plankton communities and are expected to be favored by global increases in seawater temperature and stratification associated with climate change. Eukaryotic and prokaryotic picophytoplankton have distinct ecology, and global models predict that the two groups will respond differently to future climate scenarios. At a nearshore observatory on the Northeast US Shelf, however, decades of year-round monitoring have shown these two groups to be highly synchronized in their responses to environmental variability. To reconcile the differences between regional and global predictions for picophytoplankton dynamics, we here investigate the picophytoplankton community across the continental shelf gradient from the nearshore observatory to the continental slope. We analyze flow cytometry data from 22 research cruises, comparing the response of picoeukaryote and Synechococcus communities to environmental variability across time and space. We find that the mechanisms controlling picophytoplankton abundance differ across taxa, season, and distance from shore. Like the prokaryote, Synechococcus, picoeukaryote division rates are limited nearshore by low temperatures in winter and spring, and higher temperatures offshore lead to an earlier spring bloom. Unlike Synechococcus, picoeukaryote concentration in summer decreases dramatically in offshore surface waters and exhibits deeper subsurface maxima. The offshore picoeukaryote community appears to be nutrient limited in the summer and subject to much greater loss rates than Synechococcus. This work both produces and demonstrates the necessity of taxon- and site-specific knowledge for accurately predicting the responses of picophytoplankton to ongoing environmental change.

Micromonas, a small pigmented flagellate, predominates the nanoflagellate and photosynthetic picoeukaryote communities in the northern South China Sea.

A small pigmented flagellate, Micromonas, is prevalently distributed in coastal and pelagic waters. However, there have been few studies conducted to quantify their abundance in the marginal seas of the Northwest Pacific Ocean. In this study, we used fluorescent in situ hybridization with tyramide signal amplification (TSA-FISH) to reveal the spatial distribution of Micromonas in the northern South China Sea (SCS). On average, the abundance of Micromonas was 317 cells mL-1, with the average proportions in the nanoflagellates (NF) and photosynthetic picoeukaryotes (PPE) communities being 10.94% and 15.39%, respectively. This indicates a wide distribution and dominance of this genus in the studied area. The relationships between Micromonas abundance and various environmental factors suggested that biotic correlations play more important roles than physicochemical filtering on Micromonas assemblage. This may indicate a broad environmental adaptation spectrum of this genus through its flexibility in terms of resource acquisition strategies. In summary, this study provides insight into the spatial distribution pattern of Micromonas and highlights its crucial contribution to the composition of NFs and PPE communities, which rely on biological interaction to respond to the changing environmental conditions in the northern SCS.

Effects of monsoons and storms on the structuring and diversity of picoeukaryotic microbial communities in a tropical coastal environment

Picoeukaryotes are key components in marine ecosystems that play crucial roles in food webs and biogeochemical cycles. Despite their significance, many aspects of their community ecology and diversity remain understudied. Here, we investigated the taxonomic and functional diversity of picoeukaryotic communities in response to monsoonal patterns and weather disturbances brought about by storms, characterizing tropical coastal regions. To do this, water samples were collected almost weekly or bi-weekly at a single location in a tropical coastal environment covering the late northeast (NE) and southwest (SW) monsoons. We then performed high-throughput amplicon sequencing of the V4 region of the 18S rRNA gene to generate taxonomic profiles of the communities across time. Clustering based on environmental parameters grouped our samples into months associated with NE monsoon, SW monsoon, and stormy SW monsoon, demonstrating seasonality influenced by monsoons and storms, typically observed in tropical coastal waters. In comparison, clustering based on abundance only grouped the samples into NE and SW monsoon, with most communities during storm period joining the NE monsoon samples. These samples exhibited greater diversity, with smaller taxa such as Syndiniales, Prymnesiophyceae, Picozoa, Cercozoa, Stramenopiles, and Chlorophytes being the most abundant groups present. In contrast, SW monsoon samples have lower diversity but have become generally dominated by large-celled taxa, mostly diatoms. Multivariate and correlation analyses both revealed nitrate as the strongest environmental driver of the picoeukaryotic community structuring. Meanwhile, network analysis grouped the taxa into three modules, more consistent with the clustering based on environmental parameters, implying that although storms may not significantly change the community composition, they may however influence the dominating taxa. Each module was composed of a unique set of co-occurring taxa, highlighting high turnover of picoeukaryotic communities between each season. In addition, our results showed that SW monsoon-associated module had higher interconnectivity than other modules, suggesting that the interactions during this period may be less species-specific, thus, more adaptable than during NE monsoon. However, we observed that extreme fluctuations caused by storms could have possibly allowed for selection of dominant taxa. Shotgun metagenomic sequencing of representative samples from each monsoon period also revealed that differently abundant functional genes, particularly genes associated to nitrogen metabolism, might have also helped in adaptation to the changing nutrient conditions. Our observations provide new insights on the potential trajectory of microbial communities under environmental stresses, which are important in understanding the implications of emerging threats such as coastal eutrophication and climate change.

Microbial Communities Are Shaped by Different Ecological Processes in Subtropical Reservoirs of Different Trophic States.

Understanding microbial community structure and the underlying control mechanisms are fundamental purposes of aquatic ecology. However, little is known about the seasonality and how trophic conditions regulate plankton community in subtropical reservoirs. In this study, we study the prokaryotic and picoeukaryotic communities and their interactions during wet and dry seasons in two subtropical reservoirs: one at oligotrophic state and another at mesotrophic state. Distinct microbial community compositions (prokaryotes and picoeukaryotes) and seasonal variation pattern were detected in the oligotrophic and mesotrophic reservoirs. The interactions between prokaryotic and picoeukaryotic communities were more prevalent in the oligotrophic reservoir, suggesting enhanced top-down control of small eukaryotic grazers on the prokaryotic communities. On the other hand, the microbial community in the mesotrophic reservoir was more influenced by physico-chemical parameters and showed a stronger seasonal variation, which may be the result of distinct nutrient levels in wet and dry seasons, indicating the importance of bottom-up control. Our study contributes to new understandings of the environmental and biological processes that shape the structure and dynamics of the planktonic microbial communities in reservoirs of different trophic states.

Composition and genetic diversity of picoeukaryotes in the northeastern South China Sea during the Luzon winter bloom

We analyzed picoeukaryotes during the Luzon winter bloom, in combination with remote sensing, flow cytometry and high throughput sequencing. Satellite images showed a seasonal increase in phytoplankton in the study area during winter, and the in situ campaign observed an extremely high abundance of photosynthetic picoeukaryotes, which were the main contributors to the total phytoplankton biomass. Auto- and mixotrophic picoalgae (Chlorophyta, Ochrophyta, and Dinophyta), heterotrophic flagellates (Radiolaria) and parasites (Syndiniale) were found to be potentially numerous components of picoeukaryote communities. Overall, auto- and mixotrophic photosynthetic picoeukaryotes were mainly distributed in the upper layers (0–100 m) while heterotrophic flagellates and parasites dominated the deep layers (200–300 m). The Kuroshio-induced upwelling stimulated the growth of photosynthetic picoeukaryotes, which mainly belonged to Chlorophyta, especially the class Mamiellophyceae.

Global beta diversity patterns of microbial communities in the surface and deep ocean

AbstractAimDispersal and environmental gradients shape marine microbial communities, yet the relative importance of these factors across taxa with distinct sizes and dispersal capacity in different ocean layers is unknown. Here, we report a comparative analysis of surface and deep ocean microbial beta diversity and examine how these patterns are tied to oceanic distance and environmental gradients.LocationTropical and subtropical oceans (30°N–40°S).Time period2010–2011.Major taxa studiedProkaryotes and picoeukaryotes (eukaryotes between 0.2 and 3 μm).MethodsBeta diversity was calculated from metabarcoding data on prokaryotic and picoeukaryotic microbes collected during the Malaspina expedition across the tropical and subtropical oceans. Mantel correlations were used to determine the relative contribution of environment and oceanic distance driving community beta diversity.ResultsMean community similarity across all sites for prokaryotes was 38.9% in the surface and 51.4% in the deep ocean, compared to mean similarity of 25.8 and 12.1% in the surface and deep ocean, respectively, for picoeukaryotes. Higher dispersal rates and smaller body sizes of prokaryotes relative to picoeukaryotes likely contributed to the significantly higher community similarity for prokaryotes compared with picoeukaryotes. The ecological mechanisms determining the biogeography of microbes varied across depth. In the surface ocean, the environmental differences in space were a more important factor driving microbial distribution compared with the oceanic distance, defined as the shortest path between two sites avoiding land. In the deep ocean, picoeukaryote communities were slightly more structured by the oceanic distance, while prokaryotes were shaped by the combined action of oceanic distance and environmental filtering.Main conclusionsHorizontal gradients in microbial community assembly differed across ocean depths, as did mechanisms shaping them. In the deep ocean, the oceanic distance and environment played significant roles driving microbial spatial distribution, while in the surface the influence of the environment was stronger than oceanic distance.

Seasonal and Spatial Variations in Synechococcus Abundance and Diversity Throughout the Gullmar Fjord, Swedish Skagerrak.

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3–V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 × 104 cells ml–1. This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord.

Global diversity and distribution of aerobic anoxygenic phototrophs in the tropical and subtropical oceans.

The aerobic anoxygenic phototrophic (AAP) bacteria are common in most marine environments but their global diversity and biogeography remain poorly characterized. Here, we analyzed AAP communities across 113 globally-distributed surface ocean stations sampled during the Malaspina Expedition in the tropical and subtropical ocean. By means of amplicon sequencing of the pufM gene, a genetic marker for this functional group, we show that AAP communities along the surface ocean were mainly composed of members of the Halieaceae (Gammaproteobacteria), which were adapted to a large range of environmental conditions, and of different clades of the Alphaproteobacteria, which seemed to dominate under particular circumstances, such as in the oligotrophic gyres. AAP taxa were spatially structured within each of the studied oceans, with communities from adjacent stations sharing more taxonomic similarities. AAP communities were composed of a large pool of rare members and several habitat specialists. When compared to the surface ocean prokaryotic and picoeukaryotic communities, it appears that AAP communities display an idiosyncratic global biogeographical pattern, dominated by selection processes and less influenced by dispersal limitation. Our study contributes to the understanding of how AAP communities are distributed in the horizontal dimension and the mechanisms underlying their distribution across the global surface ocean.

Oxygen gradients shape the unique structure of picoeukaryotic communities in the Bay of Bengal

Picoeukaryotic communities respond rapidly to global climate change and play an important role in marine biological food webs and ecosystems. The formation of oxygen minimum zones (OMZ) is facilitated by the stratification of seawater and higher primary production in the surface layer, and the marine picoeukaryotic community this low-oxygen environment is topic of interest. To better understand the picoeukaryotic community assembly mechanisms in an OMZ, we collected samples from the Bay of Bengal (BOB) in October and November 2020 and used 18S rDNA to study the picoeukaryotic communities and their community assembly mechanisms that they are controlled by in deep-sea and hypoxic zones. The results show that deterministic and stochastic processes combine to shape picoeukaryotic communities in the BOB. We divided the water column into three vertical layers: the upper oxycline (UO), the OMZ, and the lower oxycline (LO), based on dissolved oxygen concentrations (dissolved oxygen: UO > LO > OMZ) at vertical depths (from 5 m to 2000 m). Deterministic processes controlled the picoeukaryotic community in the UO, while the picoeukaryotic communities in the OMZ and LO were dominated by stochastic processes. The OMZ had a stronger diffusional limitation and the habitat niche breadth in the UO was wider than that in OMZ and LO. We classified the picoeukaryotic community into three functional composition types (phototrophic, mixotrophic, and heterotrophic); heterotrophs were most abundant in the surveyed area, and the proportion of decreased significantly with increasing depth and decreasing dissolved oxygen. The picoeukaryotes in the investigated area also correlated with temperature, salinity, and nutrients (phosphate, silicate, nitrate, nitrite, and ammonium). These findings contribute to a better understanding of picoeukaryotic communities in deep-sea and low-oxygen environments, their functional structuring, as well as the effects of environmental changes on their community structure.

Diversity, Composition, and Activities of Nano- and Pico-Eukaryotes in the Northern South China Sea With Influences of Kuroshio Intrusion

Nano- and pico-eukaryotes play important roles in the diversity and functions of marine ecosystems. Warm, saline, and nutrient-depleted water that originates in the Kuroshio Current seasonally intrudes into the northern South China Sea (NSCS) from autumn to spring. To clarify the mechanisms in shaping the community structure of nano- and pico-eukaryotes as well as impacts of the Kuroshio intrusion on the NSCS ecosystem, genomic DNA and RNA were co-extracted from samples collected at two depths from nine stations, and then the V9 region of 18S rDNA and rRNA was sequenced with high-throughput sequencing. Our results showed that Dinophyceae was the most diverse and abundant nanoeukaryotic group during the study period revealed by both DNA and RNA surveys. In contrast, the relative read abundance of MAST, Pelagophyceae, and Dinophyceae in the size fraction of picoeukaryotes might be largely underestimated by the DNA survey. The RNA survey was the more reliable method to investigate the eukaryotic community structure. Environmental filtering played an important role in shaping the community structure, and the sampling depth became the governing factor of the beta diversity under the environmental setting of stratification during the study period. The spatial variations in the diversity of nanoeukaryotes were subject to the dispersal limitation under the size rule. The effects of the Kuroshio intrusion on the nanoeukaryotic community structure might also be explained by the dispersal limitation. Overall, neutral processes are critical in shaping the community structure of nanoeukaryotes. The relative metabolic activities of nanoeukaryotes were relatively stable in accordance with the high similarity of community structure between sampling sites. The responses of the relative metabolic activities of picoeukaryotes to environmental factors displayed two distinct patterns: positive correlations with salinity and nutrients and negative with temperature for Dinophyceae, MAST, and Pelagophyceae, while reversed patterns for Mamiellophyceae and Radiolaria. Our findings improve the understanding of the nano- and pico-eukaryotic communities in the NSCS and the mechanisms of their assembly.

Seasonality and episodic variation in picoeukaryote diversity and structure reveal community resilience to disturbances in the North Pacific Subtropical Gyre

AbstractWe examined variability in the euphotic zone (0–175 m) picoeukaryotic community based on time‐series observations (2011–2013) at Station ALOHA in the North Pacific Subtropical Gyre. By sampling over scales ranging from daily to approximately monthly over 2.25 years, we evaluated the resilience of the picoeukaryotic community to seasonal‐ to episodic‐scale physical disturbances, such as convective mixing and mesoscale processes, respectively. We quantified the frequency and intensity of disturbances that altered upper ocean light and nutrients in the context of the Hawaii Ocean Time‐series program climatology, and evaluated picoeukaryotic community resilience based on shifts in dissimilarity in community structure at different depths in the euphotic zone. Our results suggest that in this stratified habitat, picoeukaryote communities are resilient on timescales of days to weeks in response to these physical disturbances, and that the juxtaposition of mesoscale and submesoscale disturbances on more predictable seasonality requires spatially and temporally resolved assessment of community response and resilience. We highlight the value of examining recent (days to weeks) physical forcing of the upper ocean for insight into the influences of physical habitat alterations that structure the contemporaneous plankton community.

Diversity and co-occurrence networks of picoeukaryotes as a tool for indicating underlying environmental heterogeneity in the Western Pacific Ocean

Picoeukaryotes are an essential component of microbial communities and play key roles in marine ecosystems. In this study, surface water picoeukaryotes were investigated at 32 stations along a latitudinal cross-section of the Western Pacific (WP) in 2015. Multivariate analyses demonstrated that there were clear spatial patterns in picoeukaryotic community structures which were consistent with the distributions of environmental variables. The spatial patterns of community structures and diversity indices were all significantly correlated with multiple environmental parameters, especially nutrients. Co-occurrence networks linked community variability to environmental heterogeneity. In summary, the construction of picoeukaryotic communities in the WP was significantly affected by numerous environmental variables, and certain variables were revealed as key forcing factors responsible for the main similarities between picoeukaryotic communities. This study details the relationships between the picoeukaryotes and environmental parameters in the WP, and provides insight for application of using picoeukaryotes as indicator in future bioassessment for open waters.

Comparison of picoeukaryote community structures and their environmental relationships between summer and autumn in the southern Chukchi Sea.

Picoeukaryotes constitute an important component of the living biomass of oceanic communities and play major roles in biogeochemical cycles. There are very few studies on picoeukaryotes found in the Chukchi Sea. This work shows the relationship between community distribution and composition of picoeukaryotes residing in water masses and physicochemical factors in the southern Chukchi Sea studied in both midsummer (July) and early autumn (September), 2012. Illumina 18S V4 rDNA metabarcoding were used as the main tool. In July, Mamiellophyceae, Dinophyceae, and Trebouxiophyceae were the main microbial classes, with Micromonas, Prasinoderma, Telonema, Amoebophrya, Bathycoccus, Picomonas, and Bolidomonas representing the main genera. In September, Trebouxiophyceae surpassed Dinophyceae and was the second main microbial class, with Micromonas, Prasinoderma, Bathycoccus, Bolidomonas, Telonema, Choricystis, and Diaphanoeca representing the main genera. Water mass was the primary factor determining the community composition and diversity of picoeukaryotes. Abundance of Bathycoccus was found to be highly correlated with Alaskan Coastal Water and that of Prasinoderma, Bolidomonas, and Diaphanoeca with Bering Seawater. Nitrate and phosphate content of water in midsummer and dissolved oxygen (DO) and temperature in early autumn were the main factors that shaped the abundance of the picoeukaryote community.

Seasonal dynamics of natural Ostreococcus viral infection at the single cell level using VirusFISH.

Ostreococcus is a cosmopolitan marine genus of phytoplankton found in mesotrophic and oligotrophic waters, and the smallest free-living eukaryotes known to date, with a cell diameter close to 1μm. Ostreococcus has been extensively studied as a model system to investigate viral-host dynamics in culture, yet the impact of viruses in naturally occurring populations is largely unknown. Here, we used Virus Fluorescence in situ Hybridization (VirusFISH) to visualize and quantify viral-host dynamics in natural populations of Ostreococcus during a seasonal cycle in the central Cantabrian Sea (Southern Bay of Biscay). Ostreococcus were predominantly found during summer and autumn at surface and 50 m depth, in coastal, mid-shelf and shelf waters, representing up to 21% of the picoeukaryotic communities. Viral infection was only detected in surface waters, and its impact was variable but highest from May to July and November to December, when up to half of the population was infected. Metatranscriptomic data available from the mid-shelf station unveiled that the Ostreococcus population was dominated by the species O. lucimarinus. This work represents a proof of concept that the VirusFISH technique can be used to quantify the impact of viruses on targeted populations of key microbes from complex natural communities.

Decoupling of biotic and abiotic patterns in a coastal area affected by chronic metal micronutrients disturbances

Coastal systems are highly productive areas for primary productivity and ecosystem services and host a large number of human activities. Since industrialization, metal micronutrients in these regions have increased. Phytoplankton use metals as micronutrients in metabolic processes, but in excess, had deleterious effects. In coastal systems, picoeukaryotes represent a diverse and abundant group with widespread distribution and fundamental roles in biogeochemical cycling. We combined different approaches to explore picoeukaryotes seasonal variability in a chronically metal polluted coastal area at the south-eastern Pacific Ocean. Through remote and field measurements to monitor environmental conditions and 18S rRNA gene sequencing for taxonomic profiling, we determined metal chronic effect on picoeukaryote community's structure. Our results revealed a stable richness and a variable distribution of the relative abundance, despite the physicochemical seasonal variations. These results suggest that chronic metal contamination influences temporal heterogeneity of picoeukaryote communities, with a decoupling between abiotic and biotic patterns.

Comparison of Deep-Sea Picoeukaryotic Composition Estimated from the V4 and V9 Regions of 18S rRNA Gene with a Focus on the Hadal Zone of the Mariana Trench.

Diversity of microbial eukaryotes is estimated largely based on sequencing analysis of the hypervariable regions of 18S rRNA genes. But the use of different regions of 18S rRNA genes as molecular markers may generate bias in diversity estimation. Here, we compared the differences between the two most widely used markers, V4 and V9 regions of the 18S rRNA gene, in describing the diversity of epipelagic, bathypelagic, and hadal picoeukaryotes in the Challenger Deep of the Mariana Trench, which is a unique and little explored environment. Generally, the V9 region identified more OTUs in deeper waters than V4, while the V4 region provided greater Shannon diversity than V9. In the epipelagic zone, where Alveolata was the dominant group, picoeukaryotic community compositions identified by V4 and V9 markers are similar at different taxonomic levels. However, in the deep waters, the results of the two datasets show clear differences. These differences were mainly contributed by Retaria, Fungi, and Bicosoecida. The primer targeting the V9 region has an advantage in amplifying Bicosoecids in the bathypelagic and hadal zone of the Mariana Trench, and its high abundance in V9 dataset pointed out the possibility of Bicosoecids as a dominant group in this environment. Chrysophyceae, Fungi, MALV-I, and Retaria were identified as the dominant picoeukaryotes in the bathypelagic and hadal zone and potentially play important roles in deep-sea microbial food webs and biogeochemical cycling by their phagotrophic, saprotrophic, and parasitic life styles. Overall, the use of different markers of 18S rRNA gene allows a better assessment and understanding of the picoeukaryotic diversity in deep-sea environments.

Diversity of picoeukaryotes in the eastern equatorial Indian Ocean revealed by metabarcoding

We used 18S rRNA gene metabarcoding to investigate picoeukaryotic diversity and distribution at the surface and deep chlorophyll maximum (DCM) of 4 stations in the eastern equatorial Indian Ocean (EEIO). The results showed that picoeukaryotic communities were dominated by 5 phyla: Dinoflagellata, Radiolaria, Chlorophyta, Ochrophyta and Ciliophora. The picoeukaryotic communities were classified into 3 groups matching their water mass origins and depth: (1) Group I was in the surface waters of the Bay of Bengal, which had low salinity, and was dominated by Radiolaria Group A, Spirotrichea and marine stramenopiles; (2) Group II was in the DCM within the intrusion of Arabian Sea high salinity water, in which Chloropicophyceae and Pelagophyceae were more abundant; and (3) Group III was located in the 0°-5°S surface water, which was enriched by Dinophyceae. In addition, Caecitellaceae paraparvulus was abundant at 4°S, where weak vertical mixing occurred. This study provides the first baseline of picoeukaryotic diversity in the EEIO.

Seasonal Variations of Picoeukaryote Community Structure in Zhoushan Fishing Ground, East China Sea

Picoeukaryotes (<2–3 µm) are major oceanic primary producers and play a crucial role for the functioning of marine ecosystems. However, the community structure of such communities remains poorly understood, especially in the East China Sea (ECS). We investigated the seasonal variations of abundance and diversity of picoeukaryotes, and recorded environmental variables, along a transect in Zhoushan Fishing Ground. High-throughput sequencing was used for sequencing the V4–V5 variable region within the 18S rRNA gene to analyze genetic diversity and relative abundance of picoeukaryotes. A total of 40 phyla, 68 classes, 99 orders, 126 families, and 140 genera were observed. The estimated richness and diversity indices were both higher in each season. The composition and dominant picoeukaryotes changed significantly at the class level with the seasons alternating. Basidiomycota, Ciliophora, Ascomycota and Cryptomonadales were observed throughout the year. The Linear Discriminant Analysis (LDA) revealed the predominant discriminant taxa of four seasonal groups. Redundancy analysis (RDA) showed that NO2− and salinity played very important roles to picoeukaryotes for all the spring samples and DS5 sample in winter, PO43− and pH affected mostly picoeukaryotes for winter samples and autumn samples.

Distinct oxygen environments shape picoeukaryote assemblages thriving oxygen minimum zone waters off central Chile

AbstractOxygen minimum zones (OMZs) support ocean biogeochemical cycles of global importance. The OMZ off central Chile is characterized by seasonally variable oxygen concentrations due to upwelling events. Bacterial and archaeal communities from this area have been previously described; however, picoeukaryote communities remain largely unexplored. In order to improve our knowledge on picoeukaryote ecology and the effect of controlling factors on its community structure, environmental parameters and 18S rRNA metabarcoding analyses were performed in water samples collected at several depths at a time series station on the continental shelf in March, May and August. Our results showed that oxygen, nitrate, silicate and temperature are relevant factors shaping the picoeukaryote community structure. Overall, according to our sequence dataset, the OMZ was dominated by Dinophyceae members including marine parasitic dinoflagellates. Moreover, dysoxic and suboxic conditions were enriched by fungi and phagotrophic protists from Ustilaginomycetes, Bicoecea and Choanoflagellatea. The latter is particularly relevant in the understanding of metazoan evolution and the origins of multicellularity in low-oxygen environments. Picoeukaryote communities changed significantly over the 3 months sampled with variations in water column stratification, including the occurrence of a winter bloom of Mamiellales. Altogether, this study reveals a great diversity and dynamics of picoeukaryotes inhabiting a coastal OMZ.