Anticyclonic eddies (ACEs) restructure resources and drive trophic cascades, yet depth-resolved effects on nanoflagellate bacterivory remain unclear. We quantify bacterivory by phago-mixotrophic nanophytoplankton (PMNP) and heterotrophic nanoflagellate (HNF) using fluorescent-bead incubations in a warm-core ACE in the South China Sea. Despite a 16% decline in nanoeukaryotes, PMNP abundance increases significantly (2.8× surface, 1.4× deep chlorophyll maximum (DCM), shifts toward larger cells, and elevates bacterial turnover and phago-mixotrophic production. HNF community grazing also increases at both euphotic depths despite similar per-cell rates, yielding impacts comparable to PMNP. Structural equation models link PMNP grazing to surface water-mass properties and nutrients and DCM turbidity, whereas HNF grazing tracks water-mass and prey/virus dynamics. In the mesopelagic zone, HNFs dominate; grazing HNF abundance increases 2.7× at 500 m and is associated with particulate beam attenuation coefficient, an in situ particle proxy. These findings reveal a depth-partitioned response coupling mesoscale forcing to microbial carbon processing.

Salt stress is a major abiotic factor limiting grapevine growth and yield. To elucidate the physiological and molecular regulatory mechanisms underlying salt tolerance in grapevine, this study used ‘Carménère’ (Vitis vinifera) and ‘Pinot Noir’ (Vitis vinifera) as experimental materials. Under 200 mmol/L NaCl stress, the physiological response characteristics of the two cultivars were systematically compared, and transcriptome sequencing combined with qRT-PCR analysis was conducted to explore the molecular basis of their differences in salt tolerance. The results showed that salt stress significantly impaired photosynthetic performance and disrupted cellular homeostasis in grapevine; however, the reductions in relative chlorophyll content (SPAD value), maximum photochemical efficiency of photosystem II (Fv/Fm), and photosynthetic performance were significantly smaller in ‘Carménère’ than in ‘Pinot Noir’, indicating greater stability of the photosynthetic apparatus in ‘Carménère’. Meanwhile, ‘Carménère’ maintained higher activities of antioxidant enzymes and higher levels of non-enzymatic antioxidants, effectively reducing reactive oxygen species accumulation and membrane lipid peroxidation. In addition, under salt stress, ‘Carménère’ accumulated greater amounts of osmotic adjustment substances and maintained lower Na+ content and higher K+ content, demonstrating a more efficient capacity for osmotic regulation and ion homeostasis. Transcriptomic analysis revealed that the plant hormone signal transduction, MAPK signaling, and glutathione metabolism pathways were significantly enriched in ‘Carménère’, with multiple key genes being coordinately upregulated under salt stress. Taken together, these findings indicate that ‘Carménère’ achieves enhanced salt tolerance through a multilayered signaling regulatory network that coordinates physiological defense responses. This study provides a theoretical basis for elucidating the mechanisms of salt tolerance in grapevine and for the molecular breeding of salt-tolerant cultivars.

Salt stress is one of the major abiotic constraints limiting the growth of grapevine (Vitis vinifera L.). Although seaweed-based biostimulants have been widely reported to enhance plant stress tolerance, the physiological and molecular mechanisms underlying their foliar application-mediated alleviation of salt stress in grapevine remain poorly understood. In this study, 1-year-old grapevine ('Cabernet Sauvignon') seedlings were grown to the 15-20 leaf stage prior to treatment. The seedlings were then exposed to 200 mmol & centerdot;L-1 NaCl with foliar spraying of three doses of seaweed-based biostimulant: low (SLF, 1:1200 dilution), medium (SMF, 1:800 dilution), and high (SHF, 1:500 dilution) concentrations of a seaweed-based biostimulant via foliar spraying. Physiological and biochemical parameters were determined, and transcriptomic analysis was performed to elucidate the regulatory mechanisms involved. The results showed that the low-concentration treatment exhibited the most pronounced mitigating effect, significantly reducing malondialdehyde and hydrogen peroxide contents by 35.47% and 27.53%, respectively, while markedly enhancing the activities of superoxide dismutase, catalase, and ascorbate peroxidase. In addition, SLF treatment effectively maintained Na+/K+ ionic homeostasis and preserved the normal functioning of the photosynthetic system under salt stress. Transcriptomic analysis revealed that 1482 differentially expressed genes (DEGs) were identified between the SLF and salt-stressed groups, including 593 upregulated and 869 downregulated genes. These DEGs were significantly enriched in pathways related to photosynthesis, hormone signal transduction, and antioxidant detoxification, indicating their active involvement in salt stress responses. Furthermore, weighted gene co-expression network analysis identified several candidate genes closely associated with these physiological processes, including VvAOC4, VvGBSS1, and VvARR9, suggesting a strong linkage between transcriptional regulation and physiological alleviation effects. Overall, this study provides novel insights into the coordinated physiological and molecular mechanisms by which foliar application of a seaweed-based biostimulant enhances salt stress tolerance in grapevine seedlings.

Subsurface anticyclonic eddies (SAEs) reshape ocean stratification and nutrient-light regimes, yet their microbial and biogeochemical impacts remain poorly resolved. We combined CTD-ADCP hydrography, nutrient profiles, microscopy and flow-cytometry, and 18S/16S rRNA transcript sequencing to examine a long-lived SAE in the northern South China Sea. The lens-shaped eddy featured a strong velocity core (maximum 0.46 m s(-1) at 90 m) and a deepened euphotic zone from exterior reference to edge to center (107.9 -> 117.9 -> 124.5 m). Depth-integrated chlorophyll-a (25-150 m) was higher inside the eddy (19.7 mg m(-2)) than at reference waters (18.2 mg m(-2)), peaking at the center (20.8 mg m(-2)), indicating subsurface intensification of autotrophic biomass. Alpha-diversity enhanced within the eddy, especially at its edge, while vertical turnover of protistan and bacterial assemblages exceeded center-edge differences, contrasting with surface-intensifying anticyclonic eddies that often homogenize communities. Distance-based redundancy analyses identified water mass, nutrient, viral, bacterial, and nanoflagellate gradients as key correlates of community structure, reflecting light-nutrient colimitation modulated by top-down control. Ecotype-resolved patterns supported this framework that high-light Ostreococcus tauri declined, whereas low-light Prochlorococcus MIT9313 was similar to 5-fold enriched just above the core and toward the center. Protist-bacteria co-occurrence network formed depth- and edge-specific modules and hub taxa included protist Stramenopiles, free-living Alphaproteobacteria and particle-associated Deltaproteobacteria. Collectively, the deepened euphotic zone and elevated subsurface chlorophyll-a, together with spatially structured community assembly, demonstrate that the eddy functions as a localized biogeochemical reactor enhancing subsurface productivity and microbial recycling in oligotrophic waters-linking eddy physics to ecosystem function.

Protists form the foundation of aquatic food webs and drive global nutrient cycles, yet distinguishing which species photosynthesize, graze or do both remains a major challenge because most are uncultivable and community surveys seldom resolve species-level traits. We developed a field grazing-scPCR framework that integrates short-term grazing assays, single-cell microscopy and 18S rRNA sequencing to link morphology, fluorescence-based trophic indicators, ingestion evidence and phylogenetic identity in 21 individually isolated protistan cells spanning freshwater to oceanic ecosystems. Using a conservative, phylogeny-informed classification, this approach confirmed constitutive mixotrophs (Cryptomonas curvata, Poterioochromonas malhamensis), identified a candidate non-constitutive mixotroph within Katablepharidaceae, and showed that prey-derived fluorescence can overestimate mixotrophy in natural assemblages. Two C. curvata isolates exhibited contrasting states, an active grazer with plastid autofluorescence and a non-grazing, aflagellate cyst retaining plastid fluorescence, highlighting the limits of single-time-point assays. Linking single-cell observations to MetaPR2 and Tara Oceans exact-match records further placed trophically characterized taxa in a broader biogeographic context. This framework advances species-level resolution of protistan trophic diversity in nature while underscoring the need to interpret fluorescence and ingestion signals in phylogenetic and ecological context.

Soil salinization imposes severe ionic and osmotic stress on plants, threatening ecosystem sustainability. Biochar (BC) and arbuscular mycorrhizal fungi (AMF) have shown potential to alleviate salinity stress, but their interactive effects on the soil-plant-microbe continuum have not been fully elucidated. This study utilized partial least squares path modeling (PLS-PM) to assess the individual and combined contributions of BC and AMF to sodium (Na+) homeostasis in Suaeda salsa. Results indicate that BC primarily acted as a soil conditioner, reducing Na+ bioavailability by 49.19% (compared to the unamended saline control) (as indicated by the soil Na+/K+ ratio) through enhanced adsorption and cation exchange, thereby contributing to improved soil health (including an 87.23% increase in SOC). In contrast, AMF elicited a plant physiological response, characterized by the upregulation of antioxidant enzymes (such as a 114.85% increase in catalase) and osmotic regulators (e.g., a 90.22% increase in soluble sugars), which collectively mitigated oxidative stress and promoted vacuolar sequestration of Na+. A strong synergistic effect (E > 0) was observed. The PLS-PM model quantitatively delineated the causal pathways, revealing that BC and AMF function through complementary soil-soil solution-root-leaf-vacuole transport routes. This study reveals the mechanism underlying the interaction between biochar and AMF, while quantifying the pathways through which they synchronously regulate the rhizosphere chemical environment and internal detoxification in plants, providing new insights into how halophytes cope with environmental stress challenges.

This study investigates the seasonal dynamics, environmental drivers, and assembly mechanisms of picoeukaryotic communities in the hydrographically complex Changjiang River Estuary and adjacent East China Sea. Using 18S rRNA gene high-throughput sequencing and concurrent environmental profiling, we found pronounced differences in picoeukaryotic assemblages between the winter and summer surveys that were associated with contrasting hydrodynamic conditions. During winter, strong vertical mixing weakened environmental differentiation among water masses, and geographical distance was identified as the strongest correlate of picoeukaryotic community variation. Community assembly was predominantly associated with stochastic processes, particularly dispersal limitation and ecological drift, together with broader ecological niche breadths. In contrast, summer stratification strengthened environmental differentiation among water masses and increased the contribution of heterogeneous selection to community assembly. Taxonomic and network analyses revealed that communities were predominantly composed of Syndiniales-affiliated sequences. Co-occurrence networks demonstrated that parasitic Syndiniales and other potential parasitic groups acted as crucial keystone taxa (hubs and connectors) across seasons, suggesting significant host-parasite co-adaptation and their potential role in carbon transfer within the microbial loop. Notably, these keystone taxa transitioned from widespread, cosmopolitan distributions during winter mixing to highly compartmentalized, water-mass-specific niches during summer stratification. Collectively, our observations suggest that the contrast between winter mixing and summer stratification can alter the selective pressures and spatial gradients governing picoeukaryotic communities. This study underscores the critical importance of seasonal sampling to evaluate microbial interactions and assembly mechanisms in highly dynamic estuary-sea systems.

The interactions between epiphytic organisms and Ulva prolifera play a crucial role in regulating species communities within pelagic ecosystems and in the dynamics of U. prolifera-dominated green tides. Despite diatoms and ciliates being common epiphytic microorganisms, their relationships with U. prolifera remain poorly understood. In this study, we conducted two experiments to investigate these relationships. Our study identified the diatoms Cylindrotheca closterium, Nitzschia sp., small pennate diatoms (<20 μm), and the euplotid ciliates (Euplotes spp.) as epiphytic microorganisms associated with U. prolifera. Conversely, scuticociliates, although previously reported to increase in abundance during U. prolifera blooms, were not directly associated with U. prolifera. The presence of these diatoms and ciliates did not affect the growth of healthy U. prolifera. However, the health status of U. prolifera differentially affected the diatoms. Healthy U. prolifera promoted the growth of C. closterium by releasing phosphate, but likely suppressed the growth of Nitzschia sp. and small pennate diatoms (<20 μm) through allelopathy. In contrast, decayed U. prolifera provided adhesion substrates and benefited the growth of Nitzschia sp. and small pennate diatoms. Euplotid ciliates likely consumed propagules released by U. prolifera, suggesting a trophic link to the algal reproductive process.

Mixotrophy, the ability to integrate autotrophy and heterotrophy within a single cell, has been documented in a diverse array of phytoplankton taxa in aquatic ecosystems. However, our understanding of how mixotrophic strategies fluctuate in response to spatial and temporal changes in estuarine-coastal environments, characterized by strong environmental gradients, remains limited. This study employed a fluorescent bead-based grazing approach to explore the spatiotemporal variations in mixotrophic nanophytoplankton and the environmental factors shaping their strategies. It was found that mixotrophic nanophytoplankton exhibited distinct spatiotemporal variations. In spring, mixotrophs exhibited significantly higher abundance, proportion, and average cell size compared to summer, alongside lower ingestion rates. Furthermore, an inverse relationship between ingestion rate and the proportion of mixotrophs revealed a seasonal trade-off in nutritional strategies. Mixotrophs prioritized autotrophy in spring under higher nutrients and irradiance, while they drove a shift toward heterotrophy in summer, compensating for reduced autotrophic productivity with increased turbidity, bacterial abundance, and temperature. Spatially, mixotrophs displayed higher abundance and lower clearance rates in the plume relative to those in coastal waters during spring. The observed temporal and spatial patterns were shaped by biotic factors, including the abundance and size of nanophytoplankton and bacterial prey, as well as abiotic factors, such as temperature, turbidity, irradiance, and nutrient concentrations. This study highlights the environmental drivers and adaptive strategies of mixotrophic nanophytoplankton, providing significant insights into the adaptability of mixotrophs to dynamic estuarine-coastal ecosystems.

The diurnal cycle of light significantly impacts microbes, making diurnal investigations crucial for understanding microbial communities. Zhubi Reef is known to harbor exceptionally rich biodiversity, with both zooplankton and seawater properties demonstrating diurnal patterns. However, microbial community structures and their potential diurnal dynamics remain largely unexplored. This study is the first to utilize flow cytometry and high-throughput sequencing to investigate prokaryotic and microeukaryotic communities in the Zhubi lagoon, focusing on diurnal variations under different light intensities. The picophytoplankton cell abundance and the microbial community structures both exhibit clear diurnal variations. Light is identified as the primary driver of diurnal variations in the picophytoplankton cell abundance. The diurnal variation in microbial community diversity is driven by changes in the cell abundance of two dominant picocyanobacterial groups. Our findings reveal the diurnal variation in microbial community structures is mediated by the light-driven fluctuation of dominant cyanobacterial populations, and the diurnal variation patterns of specific populations may vary with habitats and sampling timepoints. This research provides valuable insights into the microbial community structure within the Zhubi lagoon.

Grapevine (Vitis vinifera L.) is highly sensitive to soil salinization, which severely restricts its cultivation in salt-affected areas. In this study, "Pinot Noir" (V. vinifera "Pinot Noir") was micro-grafted onto the salt-tolerant rootstock "Kangzhen No. 3" to explore the mechanisms by which rootstock-mediated micrografting enhances scion salt tolerance. Grafted seedlings, un-grafted scions, and rootstocks were irrigated with 200 mmol/L NaCl for 6 days. Physiological assessments and transcriptomic analysis revealed that grafted plants exhibited significantly improved salt tolerance compared to ungrafted "Pinot Noir." Differentially expressed genes were mainly enriched in plant hormone signal transduction, MAPK signaling, and phenylpropanoid biosynthesis pathways. Two key genes, VvFLS and VvGSTU14, were selected for functional validation. Overexpression in grapevine calli enhanced antioxidant capacity and significantly improved salt tolerance. These findings demonstrate that micrografting with a salt-tolerant rootstock can enhance scion performance under saline stress by modulating key signaling and metabolic pathways, providing a theoretical foundation for grapevine improvement and sustainable production on saline soils.

This review summarizes the latest research on the response mechanisms and alleviation measures of grapevines under salt stress. As an important economic crop, grapes are widely cultivated worldwide, but salt stress poses a significant threat to their growth, development, and fruit quality. The article discusses the impact of salt stress on key physiological processes in grapes, including photosynthesis, ion homeostasis, and antioxidant enzyme defenses. It also explores the genetic regulatory mechanisms related to salt tolerance, with a focus on the roles of genes such as VviExo70B and VaSAP15 in enhancing salt tolerance. Additionally, strategies for alleviating salt stress, such as grafting, gene editing technologies, and the application of exogenous substances, are reviewed. This paper aims to provide theoretical support for grapevine salt stress research and practical guidance for grapevine cultivation management, while also looking ahead to future research directions, particularly the potential of multi-omics technologies and CRISPR in improving salt tolerance.

Biochar and endophytic bacteria have synergistic effects in improving saline soil properties and plant growth. However, the regulatory pathways underlying this effect, particularly concerning Na+ translocation and salt stress responses, remain unclear and unquantified. This study investigated the influence of biochar prepared from Enteromorpha prolifera and the endophytic bacteria (Sphingomonas prati) on Na+ transfer process, growth and physiological responses of Suaeda salsa through pot experiments and multivariate analysis. Results showed that the combination of biochar and Sphingomonas prati resulted in a significant reduction of exchangeable Na+ content in both soil (by 24.9

Salt stress is a typical abiotic stress in plants that causes slow growth, stunting, and reduced yield and fruit quality. Fertilization is necessary to ensure proper crop growth. However, the effect of fertilization on salt tolerance in grapevine is unclear. In this study, we investigated the effect of nitrogen fertilizer (0.01 and 0.1 mol L−1 NH4NO3) application on the salt (200 mmol L−1 NaCl) tolerance of grapevine based on physiological indices, and transcriptomic and metabolomic analyses. The results revealed that 0.01 mol L−1 NH4NO3 supplementation significantly reduced the accumulation of superoxide anion (O2.-), enhanced the activities of superoxide dismutase (SOD) and peroxidase (POD), and improved the levels of ascorbic acid (AsA) and glutathione (GSH) in grape leaves compared to salt treatment alone. Specifically, joint transcriptome and metabolome analyses showed that the differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) were significantly enriched in the flavonoid biosynthesis pathway (ko00941) and the flavone and flavonol biosynthesis pathway (ko00944). In particular, the relative content of quercetin (C00389) was markedly regulated by salt and nitrogen. Further analysis revealed that exogenous foliar application of quercetin improved the SOD and POD activities, increased the AsA and GSH contents, and reduced the H2O2 and O2.- contents. Meanwhile, 10 hub DEGs, which had high Pearson correlations (R2 > 0.9) with quercetin, were repressed by nitrogen. In conclusion, all the results indicated that moderate nitrogen and quercetin application under salt stress enhanced the antioxidant system defense response, thus providing a new perspective for improving salt tolerance in grapes.

Microeukaryotes play a vital role in shaping marine ecosystems, especially in marine productivity, the microbial food web, and carbon cycle. The Indian Ocean is one of the largest oligotrophic areas in the world, but little is known about the biodiversity of microeukaryotes in the area. The community composition and geographical distribution of microeukaryotes collected from the surface (SUR) and deep chlorophyll maximum (DCM) layers in the southwestern Indian Ocean were studied using high-throughput sequencing of the 18S rRNA gene. The metagenomic data helped quantify the impact of environmental factors on microeukaryotic communities. The relative abundance of different taxa groups exhibited distinct patterns between SUR and DCM layers, except for the most dominant Dinoflagellata that accounted for more than 40.6% abundance in each sample. Radiolaria was much more abundant in the nutrient-rich DCM layer than the SUR layer. The community similarity of microeukaryotes decreased with increasing of geographic distance, whereas the temperature and inorganic nitrogen were the most important environmental parameters to community structure. Abundant communities were more influenced by dispersal limitations and rare communities were more responsive to environmental factors. Correlation network analyses revealed strong biotic interactions indicative of parasitism, predation and competition, and their contribution to microeukaryotic population in diverse environments. Overall, this study provided insights into the biodiversity of microeukaryotes by characterizing the differences between water layers and identifying the driving factors in the ocean.

Coastal wetlands possess significant carbon storage capabilities. However, in coastal soil-plant systems augmented with biochar and microorganisms, the mechanisms of these amendments and carbon participation remain unclear. This study utilized pot experiments to explore how Enteromorpha prolifera biochar and Arbuscular mycorrhizal fungi (AMF) affect soil organic carbon (SOC), carbon-related microbes, photosynthetic and osmotic system of Suaeda salsa. The results showed biochar reduced exchangeable sodium percentage by 6.9% through adsorption and ion exchange, and increased SOC content by 34.4%. The abundance of carbon-related microorganisms (Bacteroidota and Chloroflexi) was increased and carbon metabolizing enzyme (cellulase and sucrase) activity in the soil was enhanced. AMF significantly improved plant growth compared with CK, as evidenced by the enhanced dry weight by 2.34 times. A partial least squares pathway model (PLS-PM) and correlation analysis suggested that the combined effect of biochar and AMF could be outlined as two pathways: soil and plant. Biochar increased SOC, improved the growth of soil carbon metabolizing microorganisms, and further promoted the activity of carbon-related enzymes. Additionally, AMF facilitated nutrient absorption by plants through root symbiosis, with biochar further enhancing this process by acting as a nutrient adsorber. These combined effects of biochar and AMF at soil and plant level enhanced the photosynthetic process of Suaeda salsa. The transport of photosynthetic products to the roots can increase the carbon storage in the soil. This study provides quantitative evidence supporting the increase of carbon storage in coastal wetland soil-plant systems through a combined application of biochar and AMF.

ABSTRACTMicroeukaryotic plankton (0.2–200 µm), which are morphologically and genetically highly diverse, play a crucial role in ocean productivity and carbon consumption. The Pacific Ocean (PO), one of the world’s largest oligotrophic regions, remains largely unexplored in terms of the biogeography and biodiversity of microeukaryotes based on large-scale sampling. We investigated the horizontal distribution of microeukaryotes along a 16,000 km transect from the west to the east of the PO. The alpha diversity indices showed a distinct decreasing trend from west to east, which was highly correlated with water temperature. The microeukaryotic community, which was clustered into the western, central, and eastern PO groups, displayed a significant distance-decay relationship. Syndiniales, a lineage of parasitic dinoflagellates, was ubiquitously distributed along the transect and dominated the community in terms of both sequence and zero-radius operational taxonomic unit (ZOTU) proportions. The prevailing dominance of Syndiniales-affiliated ZOTUs and their close associations with dinoflagellates, diatoms, and radiolarians, as revealed by SparCC correlation analysis, suggested that parasitism may be an important trophic strategy in the surface waters of the PO. Geographical distance and temperature were the most important environmental factors that significantly correlated with community structure. Overall, our study sheds more light on the distribution pattern of both alpha and beta diversities of microeukaryotic communities and highlighted the importance of parasitisms by Syndiniales across the tropical PO.IMPORTANCEUnderstanding the biogeographical and biodiversity patterns of microeukaryotic communities is essential to comprehending their roles in biogeochemical cycling. In this study, planktonic microeukaryotes were collected along a west-to-east Pacific Ocean transect (ca. 16,000 km). Our study revealed that the alpha diversity indices were highly correlated with water temperature, and the microeukaryotic communities displayed a distinct geographical distance-driven pattern. The predominance of the parasitic dinoflagellate lineage Syndiniales and their close relationship with other microeukaryotic groups suggest that parasitism may be a crucial survival strategy for microeukaryotes in the surface waters of the Pacific Ocean. Our findings expand our understanding of the biodiversity and biogeographical pattern of microeukaryotes and highlight the significance of parasitic Syndiniales in the surface ocean.

Disentangling microbial dynamics in the mesopelagic zone is crucial due to its role in processing sinking photic production, affecting carbon export to the deep ocean. The relative importance of photic zone processes versus local biogeochemical conditions in mesopelagic microbial dynamics, especially seasonal dynamics, is largely unknown. We employed rRNA gene transcript-based high-throughput sequencing on 189 samples collected from both the photic and mesopelagic zones, along with seasonal observations, to understand the South China Sea’s protistan-bacterial microbiota diversity, drivers, and mechanisms. Mesopelagic communities displayed unexpectedly greater seasonal but less vertical dynamics than photic counterparts. Temperature, dissolved oxygen, nutrients, and bacterial abundance drove mesopelagic communities vertically. Photic zone processes (using net community production and mixed layer depth as proxies) of past seasons, coinciding with strong monsoon periods, shaped seasonal fluctuations in mesopelagic communities, indicating a time-lag effect. Furthermore, certain microbes were identified as indicators for beta diversity by depth and season. This investigation deepens our understanding of how and why mesopelagic communities vary with season and depth. Recognizing the time-lagged effect of photic zone processes on mesopelagic communities is crucial for understanding the current and future configurations of the ocean microbiome, especially in the context of climate change and its effect on carbon export and ocean storage.

The waters near the Antarctic Peninsula have always been a hot spot because of their variable and unique oceanographic conditions. To distinguish the differences in phytoplankton communities, we conducted a mesoscale survey during the austral summer of 2018. Samples were collected at 14 stations located in the Drake Passage, South Shetland Islands (SSI) and South Orkney Islands (SOI). A total of 615 operational taxonomic units were extrapolated using V4-5 18S rDNA sequencing collected from the chl a(max) layers. Three groups (I, II, and III) were divided at a similarity level of 60% by multivariate statistical analysis, and there were significant differences between groups II and III. Generally, diatoms and dinoflagellates predominated among all three groups. Chlorophyta was found in greater abundance near the SSI, whereas pennate diatoms contributed more around the SOI. The different abundance of particular dinoflagellates was the reason for grouping by SIMPER analysis. And Pseudochattonella farcimen, Micromonas sp. and Pyramimonas sp were screened out which inducing dissimilarity were less mentioned in other literatures. Temperature and salinity among stations showed little difference in our study. Phytoplankton assemblage was a complex issue and the factors induced the difference of phytoplankton communities need to further analyze.

To clarify the changes in phytoplankton community and influencing factors in short-term nutrient-addition experiments in the Equatorial Eastern Indian Ocean, we conducted three experiments (one in situ-like experiment, one on-deck experiment with deep seawater, and one on-deck experiment with surface seawater). Our findings indicate that when nutrients were added, there was a more significant increase in the chlorophyll a (chl a) concentrations of microphytoplankton (>20 μm) compared to those of nanophytoplankton (2-20 μm) and picophytoplankton (<2 μm). The chl a concentrations for phytoplankton <20 μm only exhibited significant increases in the on-board incubation of surface seawater collected at 1300 hr when grazing stress have likely been weak. In picophytoplankton, occasional increases in the abundances of Synechococcus were found, while the abundances of Prochlorococcus and eukaryotic picophytoplankton (Peuk) did not increase significantly. It results likely from the preference of grazing effect by herbivores and bottle effects. Additionally, the Prochlorococcus from 75 m was more adapted to weak light, thus its abundance sharply decreased when incubated under high light. We suggest that the nutrient effects have greater influence on microphytoplankton, but other factors, such as grazing and light, might contribute more to <20 μm phytoplankton. Furthermore, bottle effects should be considered when conducting incubation experiments.