Marine picoplankton populations are influenced by bottom-up control (resource availability, temperature) and top-down processes (e.g. grazing, viral lysis); however, seasonal variability in the bottom-up and top-down controls of picoplankton remains unclear, especially in cold-season studies, which remain scarce due to sampling challenges. To gain insight into the role of bottom-up and top-down control as determinants of picoplankton population abundance in the cold season, we examined bacterial, Synechococcus spp., and picoeukaryotic phytoplankton growth and mortality in a subtropical coastal region during the cold season (<25 degrees C) using modified dilution experiments. Results showed bacterial and Synechococcus spp. production rates were positively correlated with temperature, with growth rates ranging from 0.02 to 4.91 d(-1) (bacteria) and 0.29 to 6.28 d(-1) (Synechococcus spp.). Nanoflagellate grazing accounted for 43-80% of bacterial and 64-448% of Synechococcus spp. production, with carbon losses ranging from 2.07 to 18.82 mg C m(-3) d(-1) (bacteria) and from 1.94 to 92.93 mg C m(-3) d(-1) (Synechococcus spp.), while viral lysis was minimal for both but more significant for picoeukaryotic phytoplankton. Moreover, the 3 microbial groups exhibited distinct patterns: (1) Bacterial production was nearly balanced by total mortality (grazing and viral lysis) across all temperatures; (2) Synechococcus spp. contributed significantly to picophytoplankton production but declined sharply below 20 degrees C as grazing exceeded production; (3) Picoeukaryotic phytoplankton production surpassed Synechococcus spp. below 20 degrees C. These findings underscore temperature-driven microbial production and grazing-dominated mortality, suggesting reduced viral lysis due to increased lysogeny.
Browning and eutrophication threaten freshwater biodiversity. However, the independent and joint effects of browning and nutrient enrichment on algal communities remain poorly known. Our aim was to investigate such effects on benthic diatoms using a field experiment with freshwater microcosms including 5 levels of water color (clear to humic) and 2 levels of nutrients (control and nutrient enrichment). We also compared diatom communities in the experimental pools to those of surrounding natural ecosystems. We found that the experimental pools had low species richness, most probably due to dispersal limitation, showing no clear response to the independent or combined effects of water color and nutrients. Water color was significantly associated with community composition, but only in the nutrient control pools on the first sampling occasion. Nutrients were not significantly related to community composition. The communities in the experimental pools and surrounding natural waterbodies were highly dissimilar. The low diversity in the experimental pools suggests that environmental conditions and dispersal limitation may jointly limit the number of species. The fact that richness did not show a clear response to color or nutrients suggests that tolerant species were dominant in the pools. The results further imply that nutrient enrichment may weaken the effect of browning, potentially leading to diatom community homogenization. Our findings indicate that the diatoms colonizing new habitats may possibly originate from the surrounding terrestrial habitats, in addition to aquatic ecosystems. Our study highlights the importance of investigating multiple environmental pressures on algal communities.
The pervasive use of herbicides in aquatic environments has heightened the need to assess their specific effects on photosynthetic organisms such as microalgae and macrophytes. In freshwater ecosystems, species interactions, nutrient availability, and pollutants such as paraquat can significantly alter community dynamics. We investigated the effect of paraquat on allelopathic interactions between Chlorella sorokiniana and Lemna minor. In monocultures, C. sorokiniana exhibited increased growth and chlorophyll content at lower paraquat concentrations (0.01-1 mg l-1). However, higher concentrations (10 and 50 mg l-1) triggered oxidative stress characterized by elevated peroxidase (POD) activity and lipid peroxidation (malondialdehyde, MDA). Similarly, L. minor showed reduced chlorophyll content and biomass at higher paraquat doses, whereas its antioxidant activity increased at sublethal concentrations. Co-culturing of the 2 species intensified oxidative stress responses, particularly when paraquat was present, with both species exhibiting heightened hydrogen peroxide (H2O2) levels and altered chlorophyll synthesis. Co-cultivation with C. sorokiniana enhanced the chlorophyll production by L. minor in the absence of paraquat, indicating potential synergistic interactions. These findings reveal that paraquat exposure disrupts the physiology of individual species and alters allelopathic interactions, with potential implications for ecosystem structure and function. Understanding these complex dynamics is essential for developing strategies to mitigate the ecological risks posed by herbicide contamination of aquatic environments.
Microcystis blooms frequently occur in eutrophic freshwater bodies, where epiphytic bacteria interact with Microcystis colonies through cooperative or competitive relationships. Declining colonies are sedentary and are always found in the lower water layer, whereas healthy, mobile colonies reside exclusively in the upper layer. In this study, we compared the composition of epiphytic bacterial communities associated with Microcystis colonies in the upper and lower water layers of an aquaculture pond. A dominant Flavobacterium strain was isolated from the lower epiphytic bacterial community, and its colony-degrading potential and carbohydrate-active enzyme (CAZyme)-encoding genes were analyzed. The results showed significant differences in community composition between the upper and lower epiphytic bacterial communities. Compared to the upper epiphytic bacterial community, the relative abundance of Flavobacterium in the lower community increased by 691%, whereas that of Cetobacterium decreased by 92.84%. The dominant Flavobacterium strain isolated from the lower community was identified as F. columnare TN-1. When Microcystis colonies were treated with F. columnare TN-1 for 12 h, both the size of individual colonies and the extracellular polysaccharide (EPS) content per cell significantly decreased, indicating that F. columnare TN-1 could degrade Microcystis EPS and lead to colony disaggregation. Genomic analysis revealed that F. columnare TN-1 harbors 274 CAZyme-encoding genes, representing 8.5% of its total gene repertoire, including glycoside hydrolase and polysaccharide lyase family members implicated in EPS degradation.
Salt marshes have a high capacity to stabilize organic carbon in their soils due to their high primary production and the prevailing anoxic conditions. However, halophytes transport oxygen to their rhizosphere, creating a heterogeneous environment where oxic, microoxic and anoxic micro-niches coexist. Understanding which mechanisms, beyond anoxia, protect organic matter (OM) from microbial consumption is crucial for explaining its persistence in the soil. We assessed the potential of soil microbial communities to consume carbon sources and to identify compounds that are preserved by mechanisms other than anoxia. Soil samples were collected at several depths, up to 89 cm, in 3 distinct halophyte communities and bare sediments in the Ria de Aveiro (Portugal). EcoPlatesTM microbial incubations and Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) were successfully applied to explore OM microbial mineralization (38 samples). The soil samples showed a high spatial homogeneity in microbial potential consumption, regarding both depth and halophyte species. The high potential aerobic consumption of all samples showed that, in the case of soil erosion, aerobic degradation of the accumulated soil OM may begin with the existent microbial community. Two of the carbon sources that were not consumed were likely embedded within larger molecules-d-galacturonic acids in hemicellulose and 4-hydroxybenzoic acids in phenolic compounds. The latter may have been accumulated, thereby contributing to the role of salt marshes as carbon sinks. Our study shows the potential of microbial incubations coupled with FTIR to greatly increase our understanding of OM dynamics in salt marsh soils by exploring the accumulation of specific compounds.
Measuring extracellular hydrolytic enzyme activity can provide a fundamental understanding of the biogeochemical cycling of organic matter in freshwater and marine basins. We investigated the level of leucine aminopeptidase, lipase activity, and their spatial and seasonal variability across 3 sandy beaches on the southern Baltic Sea. These beaches differed in the degree of anthropogenic pressure. The results showed that lipase activity was about 4-10 times higher compared to leucine aminopeptidase activity in the sand of all studied beaches. The activity level of these 2 hydrolases depended on the degree of anthropogenic pressure. In the sand of the beach subjected to high anthropopressure, the activity of both hydrolytic enzymes was the highest. The enzymatic activity also varied along the horizontal and vertical profiles of the beaches. The 2 studied enzymes typically showed their maximum activity in the dry zones and minimum in the wet zones of the profile perpendicular to the shoreline. The highest level of leucine aminopeptidase and lipase activity in all studied beaches was determined in the surface sand layer of the vertical profile. A seasonal variation in the activity of the 2 studied hydrolases was also documented.
Rising temperature, variation of light levels, and changes in nutrient availability are among the top environmental factors known to have profound effects on growth, grazing, and photosynthesis in mixotrophic species. Polar species, in particular, are facing rapid changes in their environment. In the present study, phagotrophic and phototrophic responses of an Arctic chrysophyte, Dinobryon faculiferum, were investigated at different temperatures (2, 4, and 6 degrees C), light levels (0-200 mu mol photons m-2 s-1), and nutrient concentrations (12.5 and 50% f/2 + Si nutrient media in 32 PSU artificial seawater). Bacterivory by D. faculiferum, estimated by ingestion of fluorescent proxies, increased with rising temperature, while photosynthetic rates, determined by 14C uptake, were low under all experimental regimes; phagotrophy appeared to contribute more to this species' carbon budget than phototrophy. Furthermore, D. faculiferum survived long periods of darkness, implying it may not be an obligate phototroph, as found for some other species in the genus. In experiments limiting nitrogen and/or phosphorus, there was no effect of limiting phosphorus on either phototrophy or phagotrophy, but both were reduced in treatments with limited nitrogen, presumably impeding metabolic processes when strongly limited. Our results demonstrate the influences of abiotic environmental factors on phagotrophic and phototrophic responses, providing further understanding of how mixotrophic protists in polar systems may respond to a changing climate.
Significant lake shrinkage in arid and semi-arid regions has profoundly altered microbial communities crucial to biogeochemical cycles. This study characterized fungal diversity and community structure across 10 saline-alkaline lakes on the Ordos Plateau, China, using internal transcribed spacer rRNA gene amplicon sequencing. The lakes spanned a salinity gradient (0.34-431 g l-1), alkaline conditions (pH 8.51-10.25), and elevated Na+, Cl-, CO32-, and SO42- concentrations. Ascomycota dominated the fungal communities (58.32%), followed by Basidiomycota (7.63%) and Mucoromycota (2.04%), while 31.54% of operational taxonomic units remained unclassified, indicating substantial undocumented fungal diversity. Both fungal alpha diversity (Shannon index: 1.9-4.62) and richness (Chao index: 107-547) exhibited significant negative correlations with salinity, potentially associated with ionic stress from Na+, Cl-, and CO32-. Canonical correspondence analysis demonstrated that salinity, CO32-, total phosphorus, and Ca2+ collectively explained 68.26% of community variation. These findings highlight the susceptibility of fungal communities to the combined effects of ionic and nutrient stressors in shrinking plateau lakes, emphasizing the urgency of monitoring salinity-nutrient interactions to conserve ecological functions in fragile semi-arid aquatic ecosystems.
Denitrification is a primary mechanism for nitrogen removal in aquatic environments; however, few studies have measured epiphytic denitrification rates in shallow eutrophic lakes. In this study, we collected submerged plant species from 2 bays within Lake Taihu (Gonghu Bay and Xukou Bay) during July and October and measured epiphytic denitrification using the acetylene blockage technique. Epiphytic denitrification rates ranged from 0.04 +/- 0.01 to 20.13 +/- 2.25 nmol N2O-N h-1 per gram of plant dry weight in July and from 0.92 +/- 0.40 to 2318.98 +/- 666.74 nmol N2O-N h-1 in October. Denitrification rates were higher in Gonghu Bay, where nitrogen concentrations exceeded 0.77 mg l-1, than in Xukou Bay, where nitrogen levels were below 0.56 mg l-1. Significant differences were observed among host plants and their growth stages, with higher rates of denitrification in senescent plants (in October) than in mature ones (in July). These differences varied by growth state and morphological structure (submerged vs. canopy-type plants). Epiphytic biomass and denitrification rates were positively correlated with environmental factors including total nitrogen, organic carbon and particulate matter in the water. Our study indicates that nitrogen and organic carbon concentrations in the water promote the development of epiphyton, which exhibits consistent denitrifying activity under variable conditions. Additionally, the denitrification function of epiphyton in Lake Taihu is influenced by the host plant species and their growth stages, highlighting the complex interplay between biological and environmental factors in regulating nitrogen removal processes in eutrophic lakes.
Microbial plankton communities play key roles in biogeochemical cycling and primary production in marine coastal ecosystems. Given the impacts of global change on Norwegian coastal waters, there is a need to understand the drivers of microbial plankton community structure and diversity. Environmental drivers such as temperature, salinity, and light influence the dynamics of microbial community structure and abundance in temperate oceans. Here we characterize the summer diversity of protist and prokaryotic plankton communities, using DNA metabarcoding and light microscopy, in the surface, upper mixed layer, and deep waters of Spindsfjorden, a South Norwegian fjord facing the North Sea. The sampling site was vertically stratified throughout the summer, with compositionally variable communities dominated by phototrophs and mixotrophs in the surface and upper mixed layers, and stable communities dominated by heterotrophs and parasites in the deep layer. Late summer blooms were dominated by the diatom Cerataulina pelagica in the surface and dinoflagellates of Tripos spp. in the upper mixed layer. Positive co-occurrences between certain diatom taxa and flavobacteria, and between diatoms and Alphaproteobacteria suggest potential symbiotic relationships or overlapping environmental preferences. Negative associations were observed between certain dinoflagellate groups and Syndiniales, possibly due to parasitic interactions. Temperature emerged as a key environmental driver of community change, underscoring its role in shaping the dynamics of microbial communities in temperate, coastal ecosystems.
Phytoplankton are essential components of marine food webs and biogeochemical cycles; thus, elucidating the factors that mediate their growth and composition are essential for understanding the marine environment. Vitamins have been found to be a fundamental requirement for the growth of many marine phytoplankton. Here, 20 incubation experiments were conducted over the course of 1 yr in a coastal ecosystem to examine the addition of thiamin (B1) and its components 4-methyl-5-thiazoleethanol (HET) and 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP) on net phytoplankton growth rates. While vitamin addition periodically resulted in significant changes in growth rates, no uniform compound-specific or temporal patterns in rate changes were observed. Additionally, no significant changes in rates were found to correlate with starting phytoplankton abundance. Together, these findings suggest that the composition of the starting phytoplankton community likely influenced the population level response to vitamin addition and highlights the complexity of understanding the ramifications of vitamin fluxes in the marine environment.
Ammonia-oxidizing bacteria (AOB) are responsible for the first step of nitrification and are thereby important players in the global nitrogen cycle. Microorganisms in aquatic environments are frequently exposed to oxidative stress from metabolic byproducts and photochemical processes. Heterotrophic bacteria aid AOB under oxidative stress; however, the impacts of oxidative stress on the heterotrophic communities in co-culture with the autotrophic microorganisms have not been investigated in detail. In this study, we exposed 2 AOB enrichment cultures to oxidative stress via hydrogen peroxide (H2O2) and evaluated the effects of H2O2 on the activity of the AOB and the heterotrophic communities. Cultures received different frequencies of exposure to 10 or 100 mu M H2O2 as well as time to recover after exposure to H2O2. Ammonia-oxidizing activity was unaffected by 10 mu M H2O2; however, an increase in lag phase was observed in the presence of 100 mu M H2O2. The heterotrophic community structure changed with repeated exposure to 100 mu M H2O2. Microorganisms such as Xanthobacter sp. and Variovorax sp. increased in abundance in the presence of 100 mu M H2O2, and Methyloversatilis sp. stayed stable under all conditions. It is likely that these microorganisms were resistant to the H2O2 stress and supported the AOB. Ammonia-oxidizing activity recovered from repeated exposure to H2O2 stress immediately after removal of the H2O2 stress. In contrast, the heterotrophic community needed repeated exposure to non-stress conditions to change back towards pre-stress conditions, indicating that different heterotrophic communities can provide the same support to the ammonia oxidizers.
Primary productivity in the Bay of Bengal remains relatively low throughout the year, yet it harbors the fourth most intense oxygen minimum zone (OMZ) in the ocean. Despite the unique characteristics of the OMZ in the Bay, including trace oxygen levels, the microbial community and its biogeochemical role remain understudied. We investigated prokaryotic diversity in the euphotic zone and the OMZ of the Bay of Bengal. Alpha diversity was significantly higher in the OMZ compared to the surface waters. Community structures varied significantly between the coastal and the open ocean and within the OMZ across different oxygen levels. Among bacterial phyla, Proteobacteria dominated, followed by Cyanobacteria and Actinobacteria, while Thermoplasmata dominated among Archaea. Cyanobacteria, Actinobacteria, and Bacteroidota predominated in the euphotic zone. In contrast, higher abundances of Marinimicrobiota, Marine Group B, Crenarchaeota, Planctomycetota, Chloroflexi, Verrucomicrobiota, Nitrospinota, and Acidobacteriota were observed in the OMZ. At the genus level, Prochlorococcus, Candidatus (Ca.) Actinomarina, clade Ia, and SAR86 dominated in the euphotic zone, whereas SAR324, SAR406 clade, Ca. Nitrosopumilaceae, and SUP05 were prevalent in the OMZ. A positive correlation between cyanobacterial abundances and nitrogen fixation rates emphasized their potential role in diazotrophy. SUP05 likely contributes to sulfur oxidation and the reduction of nitrate and nitrite in the OMZ. Predictive functional gene analysis suggests that denitrifiers, anammox bacteria, and sulfur oxidizers prevailed in the OMZ. Our study highlights distinct prokaryotic diversity across the water column and their contributions to the nitrogen and sulfur cycling in the Bay of Bengal.
Cyanobacteria are the dominant primary producers in many marine waters, and are intimately connected with the cyanophages that infect them. The most commonly isolated marine cyanophages form a monophyletic group based on several marker genes including g20, a gene that codes the capsid assembly protein. Based on morphology, these viruses are typically referred to as cyanomyoviruses. Here, we used g20 sequences to interrogate the diversity of cyanomyoviruses at 5 locations in the waters of the northwestern Arabian Gulf. The diversity and richness of g20 sequences varied among locations and were highest at the southernmost sites. Most sequences belonged to a small number of operational taxonomic units (OTUs), with the rest belonging to low abundance rare OTUs. Phylogenetic analysis revealed that the most abundant genotypes fell within the ubiquitous cyanomyovirus Cluster II, while others clustered with metagenome assembled pelagimyophage sequences and other environmental sequences across a broad diversity of clades. This study revealed a diverse community of cyanomyoviruses in the northwestern Arabian Gulf that is dominated by a few relatively abundant but phylogenetically diverse taxa.
Seagrass meadow ecosystems offer several valuable ecosystem services in coastal regions around the world. Recent studies have suggested that one such important service is reduction of pathogenic bacteria, specifically Vibrio spp., in adjacent waters. The specific mechanisms of pathogen reduction remain unclear, although increased sedimentation has been suggested as one likely process for pathogens to be quenched from the water column. Whether Vibrio spp. persist in the sediment or in other compartments of the seagrass meadow is currently unknown, but it has been shown that marine surface biofilms can function as reservoirs of pathogenic vibrios. This general feature may also apply to seagrass and sediment surfaces. In this study, we investigated the relative abundance and community ecology of Vibrio spp. bacteria in Baltic Sea seagrass meadows using both culturing and culture-independent methods. While we did not detect a significant reduction of Vibrio spp. in the water column above unvegetated sites as compared to seagrass meadows, we observed high relative abundances of Vibrio spp. on seagrass roots. This supports previous observations that marine surfaces are selectively colonized by Vibrio spp., implying that these habitats are important for the persistence and possibly release of Vibrio spp. into the water column. Our results emphasize the need to understand the interactions of pathogenic bacteria with coastal habitats, including interactions with host organisms such as seagrasses that provide biofilm microenvironments, in order to understand how diseases associated with these organisms develop.
Seawater microorganisms impact ecological and biogeochemical cycling on coral reefs and are sensitive indicators of ecosystem status. Microbialization, a shift towards trophic collapse and resultant high microbial biomass, is a global concern on coral reefs. Indeed, macroorganisms can influence microbial processes and community composition on reefs, which is best understood as increased macroalgae resulting in copiotrophic microbial growth and oxygen reduction. Whether or not smaller-scale changes in macroorganisms influence the overlying seawater microbial communities is largely unknown. Here, we assessed seawater microorganisms across 3 coral reefs to understand their connection to reef site and within-reef benthic characteristics. At 3 coral reefs in St. John, US Virgin Islands, we collected 60 ml seawater samples 2 cm above the seafloor, spaced 2 m apart in a grid pattern, and assessed bacterial and archaeal communities via sequencing of small subunit ribosomal RNA genes. Benthic cover within 1 m of each sample was determined at 10 cm resolution through photogrammetry. Our results reveal that overall reef site overwhelmingly shapes microbial community structure, while within-reef benthic cover surrounding sample locations has minimal influence. However, ecospheres as areas that reflect the small-scale effects of benthic cover directly under each sample, significantly explain as much as 12.1% of within-reef microbial variation and may even outweigh variation attributable to reef site alone. These findings provide new insights into fine-scale spatial variability in reef seawater microbiomes that are crucial for the use of microorganisms as indicators of microbialization and coral reef health.
A growing body of literature has highlighted the importance of phytoplankton-bacterial associations to marine and estuarine ecological and biogeochemical function, but their population linkages remain sparsely characterized within urban estuaries. Since many developed coastlines are heavily impacted by anthropogenic nutrient inputs, elucidating their phytoplankton-bacterial dynamics provides insight into nutrient cycling, productivity, and can help inform water quality management. This study compared surface (0.5 m depth) physical water quality, cell abundances of major phytoplankton taxa and bacteria, as well as concentrations of chlorophyll a (chl a) and dissolved organic matter (DOM) in the nitrogen (N)-enriched Western Long Island Sound (WLIS), USA, between mid-channel and shore sites (in 2020 and 2021). Shore bacterial and phytoplankton abundances as well as DOM concentrations (primarily dissolved organic N and carbon [DOC]), were significantly higher than mid-channel, especially during summer, indicative of terrestrial loading influencing microbial assemblages as well as N and C cycling. Abundances of key phytoplankton taxa were better indicators of bacterial abundances than chl a, as bacterial abundances positively and significantly correlated with those of dinoflagellates, especially the most common genera Prorocentrum (mid-channel, shore) and Heterocapsa (shore only), but not with diatoms. However, pennate diatom abundances negatively and significantly correlated with DOC concentrations in the mid-channel. Results highlight the impact of terrestrial inputs on WLIS microbial assemblage dynamics, presumably by favoring bacteria and dinoflagellate population coupling, as well as shed new ecological insight into how phytoplankton and bacterial communities respond to nutrient loadings in urban estuaries.
Cryptophytes are eukaryotic microalgae found in a variety of aquatic environments, from tea-colored ponds and lakes to the blue-water open ocean. To broaden the range of their spectral absorption beyond the limits of chlorophyll a, cryptophytes contain phycobiliprotein (PBP) accessory pigments. Hemiselmis pacifica contains the PBP cryptophyte-phycocyanin 577 (Cr-PC 577), which allows it to absorb green to orange wavelengths of light. Here, we characterized variability in PBP absorbance and growth rates of H. pacifica when this species was grown in nutrient-rich environments of differing spectral quality but equal light intensity. Two weeks after a shift from white to green light, H. pacifica altered the absorbance of its Cr-PC 577 to increase capture of green photons. Further, these complementary shifts were reversible when cultures were returned to the white-light environment, and the timescale of the reversal was faster than the original shift (similar to 1 wk). Growth rates of H. pacifica in green light (0.25 d(-1)) were lower than in white-light controls (0.32 d(-1)), but not significantly different from cells grown in red light (0.27 d(-1)). The ability to adjust quickly to changes in light quality may confer an ecological advantage to cryptophytes when their environment is affected by processes such as eutrophication, deforestation/afforestation, or browning.
Phytoplankton play crucial roles in aquatic ecosystems, serving as the foundation of marine food webs and being responsible for similar to 50% of the world's oxygen production. Predation by microzooplankton and viral lysis are the major sources of phytoplankton mortality, with the balance between these 2 processes affecting microbial food webs and biogeochemical cycles. However, determining the dominant mortality process in time and space remains an open question. This study investigated microzooplankton grazing and viral lysis rates during a mesocosm experiment in western Norway. High-resolution measurements were determined on phytoplankton groups using flow cytometry to observe changes in mortality rates and carbon flow during phytoplankton blooms. Digital droplet PCR was employed to detect Emiliania huxleyi and Micromonas spp. and associated viruses within environmental samples, to explore its use for determining mortality processes and understanding the impact on the phytoplankton community. The results showed that grazing and viral lysis dominated mortality at different times, with only one significant instance of both processes being observed. Microzooplankton grazing primarily affected picoplankton, while nanoeukaryotes and E. huxleyi were more susceptible to viral lysis. Molecular detection did not always match with abundances or rates determined by flow cytometry; however, it did provide insights into their dynamics throughout the mesocosm. These findings provide insights into the complex interactions between microzooplankton, viruses and phytoplankton communities. Understanding the balance between microzooplankton grazing and viral lysis can contribute to a more comprehensive understanding of carbon flow in aquatic ecosystems, which has significant implications for food webs and biogeochemical cycles.
With a ubiquitous presence in marine ecosystems, Labyrinthulomycetes protists (LP) play critical ecological roles in oceanic habitats. Recently, some LP strains have been suggested to survive in low-salinity environments, but their distribution in freshwaters was largely unknown. This study investigated LP abundance and diversity dynamics along a fresh-saltwater gradient in 2 seasons. LP were detected in all samples. Although LP abundance in freshwaters (typically 10 4 to 10 5 copies l -1 ) was significantly lower than that in saline waters, their abundance still corresponded to that of previously reported LP in some coastal waters, suggesting their potentially essential roles in riverine ecosystems. High-throughput sequencing analyses identified 110, 54, and 146 LP amplicon sequence variants (ASVs) in fresh, brackish, and saline waters, respectively. Canonical correspondence analysis and variance partitioning analysis further indicated that salinity and temperature were the most significant environmental factors to affect LP community structure. Notably, most of the dominant ASVs in fresh/brackish waters were annotated to a rarely reported Labyrinthulida family, Amphitraemidae, and a newly identified river cluster of the order Thraustochytrida, which were significantly different from those of saline waters. Finally, the metabolic capabilities of the detected LP genera suggest that LP likely play diverse ecological roles in riverine ecosystems.
