Based on survey data collected in the spring and summer of 2023, this study reveals the seasonal variations in the phytoplankton community structure and its driving factors in the Pearl River Estuary. In spring, the phytoplankton community was overwhelmingly dominated by the nitrogen-fixing Cyanophyta Trichodesmium thiebautii, whereas in summer, it was jointly dominated by Bacillariophyta and Dinophyta, accompanied by a significant increase in community diversity. Redundancy analysis indicated that the spring community was primarily driven by nutrient levels, while the summer community mainly responded to changes in water temperature, reflecting the high sensitivity of subtropical estuarine ecosystems to land–sea interactions and seasonal environmental fluctuations.

Coccolithophores are important calcifying phytoplankton that play a dual role in the marine carbon cycle through photosynthesis, which contributes to the organic carbon pump, and calcification, which drives the carbonate counter pump. Despite their considerable ecological significance, investigations into their community dynamics in the Bohai Sea-a marginal sea heavily influenced by anthropogenic activities-remain notably limited. In this study, we investigated the diversity and distribution of living coccolithophores in Bohai Bay during the summer and autumn of 2020, and integrated historical data to elucidate long-term trends. A total of 13 and 11 species were identified in summer and autumn, respectively, with Gephyrocapsa oceanica and Gephyrocapsa huxleyi consistently dominating the assemblage (accounting for >80% of total abundance). Cell abundance was significantly higher in autumn (mean: 4.33 × 104 cells/L) than in summer (mean: 9.01 × 103 cells/L), with high-abundance areas persistently concentrated in the southern mariculture zone. Redundancy analysis revealed pronounced seasonal shifts in environmental drivers: temperature and nitrite dominated in summer, whereas silicate and nitrite emerged as the primary regulators in autumn, reflecting post-diatom bloom nutrient regimes. A historical comparison over the past decade revealed a dramatic increase in coccolithophore abundance, coinciding with documented warming, shifts in nutrient regimes (increasing N/P ratio and decreasing Si/N ratio), and decreasing turbidity. The dominance of G. oceanica, together with its positive response to nitrogen and negative correlation with silicate, establishes this species as a sensitive indicator of eutrophication and post-diatom bloom conditions. This study provides essential baseline data for understanding coccolithophore ecology in anthropogenically influenced coastal systems and highlights their value as integrative indicators of environmental change.

The Western Pacific Ocean (WPO), a vast oligotrophic system, plays a critical role in global carbon cycling, with its biogeochemical processes heavily influenced by complex hydrodynamics. This study investigated the controlling factors of phytoplankton distribution, biomass, and community composition across nine distinct water masses using HPLC-CHEMTAX (chemical taxonomy) analysis of diagnostic pigments. Prochlorophytes were the dominant phytoplankton community, comprising over 50% of the total biomass in most water masses, followed by Chlorophytes, Haptophytes, Cryptophytes, and Chrysophytes. Nutrient availability, primarily nitrate (NO3-N) and phosphate (PO4-P), emerged as the principal controlling factors of phytoplankton community structure. Divergence and episodic divergence event facilitated nutrient upwelling, enhancing phytoplankton biomass. River runoff inputs near the New Guinea Island elevated nitrate concentrations, promoting the aggregation of phytoplankton among the coastal zone. The prevalence of picophytoplankton under oligotrophic conditions underscored a community structure with implications for carbon export efficiency. These findings elucidate mechanistic links between physical forcings, nutrient dynamics, and phytoplankton ecology, providing a process-oriented understanding of natural and anthropogenic nutrient inputs and carbon cycling in the WPO with broader relevance to oligotrophic marine systems.

This study contains integrated in situ observations from three research cruises (2019–2020), one of which encountered the tropical cyclone Sinlaku. The results show that the depth-integrated phytoplankton abundance and carbon biomass in the open sea was more than eightfold and sixfold, respectively, than those in the shelf. Diatoms were the major contributors in shelf areas. In the open sea, Trichodesmium was dominant, exhibiting abundances more than sixteen times higher than shelf areas and exceeding diatom and dinoflagellate abundances by three to nine orders of magnitude near the surface. In the period affected by tropical cyclone Sinlaku, total phytoplankton abundance and carbon biomass increased markedly compared to non-cyclone conditions, with horizontal and vertical distributions showing significant fluctuations due to the patchy distribution of high-density aggregations. Dinoflagellates were the dominant carbon biomass contributor at the 25 m layer, accounting for 71.13

With the increasing detection of micro- and nanoplastics (MNPs) in marine environments and the expanding body of related research, their environmental behavior and ecological effects have become central topics in marine environmental science. This review addresses the growing concern over MNP pollution in the marine realm, encompassing their primary sources, spatial accumulation and distribution, environmental transport and transformation dynamics, and ecotoxicological effects on marine organisms and ecosystems, as well as the ecological risks they pose within key habitats such as seagrass beds and coral reefs. We synthesize evidence on the biological impacts of MNPs, including oxidative stress, tissue accumulation, metabolic disturbances, and immune impairment, as well as the heightened risk of pathogen transmission facilitated by the so-called “Plastisphere”. Moreover, we explore the potential implications of MNP exposure on oceanic carbon cycling and net primary productivity. The reviewed literature suggests that MNPs are capable of long-range transport and progressive fragmentation into ultrafine particles, which are readily ingested and retained by a wide array of marine organisms, subsequently inducing toxicological effects and compromising both organismal health and ecological integrity. Such disturbances may undermine critical ecosystem services, including carbon sequestration capacity and food web stability. Finally, based on the current research landscape, we outline future research priorities: improving environmental detection and toxicological evaluation of MNPs, elucidating their long-term effects at the ecosystem scale, and investigating their interactions with co-occurring pollutants under complex, multi-stressor scenarios. These efforts are essential to support science-based assessment and effective management strategies for marine MNP pollution.

Marine heterotrophic protists (MHP) are significant components of marine microbial food webs and play key roles in organic matter remineralization and carbon cycling. However, their diversity and vertical distribution in the Bay of Bengal (BoB) and the Eastern Indian Ocean (EIO) remain poorly characterized. In the present study, we showed the first molecular assessment of depth-resolved MHP communities across the epipelagic (5 m, DCM, 100 m), mesopelagic (200 m, OMZ, 1000 m), and bathypelagic (1500-2000 m) zones in the BoB and EIO using high-throughput sequencing. We observed distinct depth-related MHP patterns. The epipelagic zone was dominated by autotrophic and mixotrophic genera, including Ostreococcus, Bathycoccus, Gyrodinium, Scrippsiella, Collosphaera, and Hexacontium. The mesopelagic zone harboured more parasitic genera, with major genera including Hexacontium, Syndiniales, Thalassiosira, and Neobodo. The bathypelagic zone was characterized by parasitic radiolarians such as Cladococcus and Hexacontium, as well as bacterivorous taxa including Monosiga. The detection of bacterivorous Monosiga and parasitic Syndiniales associated with radiolarians highlights the prevalence of bacterivory and parasitism in deep-water communities. Redundancy analysis (RDA) indicated a limited influence of environmental variables on the MHP community at mesopelagic and bathypelagic depths. Correlation network analysis revealed a decrease in connectivity and an increase in modularity with depth, indicating more compartmentalized interactions in deeper layers. This study provides the first comprehensive characterization of MHP communities in the BoB and EIO across vertical depth gradients, revealing distinct taxonomic assemblages and interaction patterns with depth. These findings provide critical baseline information on MHP diversity and community assemblies in these oligotrophic and stratified oceanic regions.

This study investigated the diversity and metabolic potential of microbial communities in the Eastern Indian Ocean (EIO) through 16S rDNA gene sequencing and metagenomics analyses. Water samples were collected from the surface waters (5 m depth) and 150 m depth layer in the EIO between March 20th and June 6th, 2019. This study reveals microbial-driven biogeochemical dynamics in the oligotrophic Eastern Indian Ocean, where vertically stratified communities (Cyanobacteria/Proteobacteria-dominated surface vs. diversified Proteobacteria at 150 m) and latitudinal diversity gradients reflect nutrient limitations. Metagenomics identified four carbon fixation strategies: the Calvin cycle dominated epipelagic CO2 assimilation, while the 3-hydroxypropionate bicycle showed elevated surface activity, alongside reductive citrate and Wood-Ljungdahl pathways involving novel Actinobacteria. Nitrogen cycling exhibited spatial heterogeneity: nifH-dominated nitrogen fixation in the surface waters, prevalent narGHI nitrate reduction, and divergent nirS/nirK/nosZ distributions tied to nutrient gradients. Proteobacteria and Actinobacteria were key nitrogen fixers, with novel Actinobacteriota diazotrophs expanding known diversity. Elevated nosZ abundance in the Bay of Bengal underscored regional nitrous oxide consumption hotspots. These findings underscore microbial mediation of carbon-nitrogen fluxes in oligotrophic systems, providing genomic insights into ecosystem responses to climate-driven ocean changes.

As a crucial component of ecosystems, the distribution of zooplankton is closely related to environmental conditions and ocean currents. Despite its unique role in global circulation, zooplankton data for the Eastern Indian Ocean (EIO) remain scarce. To better characterize zooplankton composition and distribution in EIO, we conducted a survey in the region between 5 degrees S and 14 degrees N and 80 degrees-93 degrees E from March-May 2022. To our knowledge, this is among the first applications of ecological models to reveal the preliminary characteristics of zooplankton community assembly mechanisms in EIO. Through microscopic examination, we identified 427 species of adult zooplankton and 24 zooplankton larvae taxa. The zooplankton abundance in the surveyed ranged from 137 to 1,326 ind./m3. Among them, copepods, particularly small-bodied species, were the dominant components, contributing most significantly to the abundance and species richness of EIO zooplankton. Redundancy Analysis (RDA) and Random Forest (RF) results indicated that Dissolved oxygen (DO), temperature, and chlorophyll a were the primary factors influencing the abundance and diversity of EIO zooplankton. According to cluster analysis, the EIO zooplankton could categorized into three ecological groups: Group A (GA), Group B (GB) and Group C (GC). The results of Neutral Community Model revealed that the community assembly of EIO zooplankton was primarily influenced by stochastic processes. However, certain deterministic factors still influence the community assembly mechanisms of GA and GB. Correlation analyses between environmental factors and characteristic species in each group showed that deterministic assembly in GA is mainly driven by interspecific interactions, whereas in GB it is primarily driven by environmental selection.

Nanoplastics (NPs) and polycyclic aromatic hydrocarbons (PAHs) are recognized as persistent organic pollutant (POPs) with demonstrated physiological toxicity. When present in aquatic environments, the two pollutants could combine with each other, resulting in cumulative toxicity to organisms. However, the combined impact of NPs and PAHs on microorganisms in seawater is not well understood. In this study, we conducted an exposure experiment to investigate the individual and synergistic effects of NPs and PAHs on the composition, biodiversity, co-occurrence networks of microbial communities in seawater. Exposure of individuals to PAHs led to a reduction in microbial community richness, but an increase in the relative abundance of species linked to PAHs degradation. These PAHs-degradation bacteria acting as keystone species, maintained a microbial network complexity similar to that of the control treatment. Exposure to individual NPs resulted in a reduction in the complexity of microbial networks. Furthermore, when PAHs and NPs were simultaneously present, the toxic effect of NPs hindered the presence of keystone species involved in PAHs degradation, subsequently limiting the degradation of PAHs by marine microorganisms, resulting in a decrease in community diversity and symbiotic network complexity. This situation potentially poses a heightened threat to the ecological stability of marine ecosystems. Our work strengthened the understanding of the combined impact of NPs and PAHs on microorganisms in seawater.

In order to study the dynamics of marine phytoplankton communities in response to anticipated in temperature and CO2, a shipboard continuous culture experiment (Ecostat) was conducted. The experiment involved simulations under current atmospheric CO2 concentrations (400 ppm) and projected year-2100 CO2 levels (1000 ppm), as well as varying temperature under present (22 °C) versus increased temperature (26 °C) in the Yellow Sea during the summer of 2020. The results showed that both the increased pCO2 and temperature had significant effects on microphytoplankton and picophytoplankton, with the warming effect proving to be more significant. The different responses of various species to acidification and warming and their coupling effect led to the changes in microphytoplankton and picophytoplankton community structure. Elevated temperature and greenhouse treatments promoted the growth of dominant diatoms and Synechococcus, such as Guinardia flaccida and Pseudo-nitzschia delicatissima. This phenomenons widened the ecological niche, and the changes in the growth patterns of dominant species consequently influenced the content of cellular elements. Mantel's analysis further demonstrated that both warming and greenhouse promoted the growth of diatoms and Synechococcus. Projections of marine phytoplankton community trends by the end of the century based on Growth Rate Ratio (GRR), indicated that not only would species with GRR < 1 decrease, but also numerous species with growth rates >1 at elevated pCO2 levels would be ousted from competition. This experiment demonstrates the need to investigate whether extended exposure to increased pCO2 and temperature over more extended time scales would similarly induce shifts in the biological and biogeochemical dynamics of the Yellow Sea.

Dimethyl sulfur compounds including dimethylsulfoniopropionate (DMSP), dimethyl sulfide (DMS), and dimethyl sulfoxide (DMSO), play a crucial part in global sulfur cycling. The eastern Indian Ocean (EIO), characterized by its remarkable diversity of biomes and climate dynamics, is integral to global climate regulation. However, the regulation mechanism of DMS (P, O) in the EIO remains to be elucidated in detail. This paper presented a field survey aimed at investigating the spatial distribution of DMS (P, O) and their relationships with environmental and biological factors in the EIO. The surface concentrations of DMS, DMSPt, and DMSOt varied from 0.07 to 7.37 nmol/L, 0.14 to 9.17 nmol/L, and 0.15 to 3.32 nmol/L, respectively, and their distributions are attributed to high Chl-a concentration near Sri Lanka and the influence of ocean currents (Wyrtki jets, Bay of Bengal runoff). Higher concentrations of DMS (P) and DMSOt were predominantly observed in water columns shallower than 75m and deeper than 75m deep, respectively. The monthly DMS fluxes in the study area peaked in August. Temperature and Dissolved Silica Index (DSI) were the key environmental determinants for DMS distribution, while nitrate (NO3-) was the primary factor for both DMSPt and DMSOt. In terms of biological factors, Prochlorococcus and Synechococcus were significant contributors to DMS (P, O) dynamics. Synechococcus was the dominant influence on the DMS source and DMSPt sink, whereas Prochlorococcus primarily consumed DMSOt. Furthermore, the structural equation modeling (SEM) revealed the relationship between DMS, DMSPt, DMSOt, and the key environmental/biological factors, as well as among them, and together they formed a co-regulatory network in the EIO. This contributes significantly to the advancement of global ecosystem models for DMS (P, O).

The distribution characteristics, biomass, and communities of phytoplankton in the western Pacific Ocean (WPO) were investigated using high-performance liquid chromatography (HPLC)-CHEMTAX analysis. The results revealed significant differences in the distribution of phytoplankton communities among different water masses in the WPO. Haptophytes were the dominant group, followed by Prochlorophytes, Cyanobacteria, Prasinophytes, and Diatoms. The distribution of phytoplankton communities was primarily determined by the level of nitrate, phosphate, and silicate, while temperature showed a negative correlation with major phytoplankton communities. In the 130°E section, the divergence caused by Halmahera Eddy (HE) and Mindanao Eddy (ME) provided the abundant nutrients, making them the primary environmental influence factor near the equator. This divergence brought relatively eutrophic deep seawater into the euphotic layer, resulting higher biomass of phytoplankton communities. In the 20°N section, the distribution of phytoplankton was mainly influenced by the invasion of Kuroshio Current and its offshore flow. Additionally, due to the low surface-to-volume ratios, microphytoplankton dominated the phytoplankton community in this section instead of nanophytoplankton or picophytoplankton. In summary, this study confirms previous findings on distribution characteristics of phytoplankton and provides new insights into the environmental and biological regulations of phytoplankton communities in the WPO.

The Haihe River system is a major conduit of land-based pollution into the Bohai Sea in North China, This study presented the first-ever observation of hypoxia in the Haihe River estuary, providing valuable insights into the complex interplay between physical and biochemical factors influencing hypoxia in estuarine systems. The research found seasonal variations in DO distribution in the estuaries of the Haihe and Yongdingxin Rivers. Hypoxia emerged in summer at the bottom of the Haihe River estuary and dissipated in autumn, with the minimum DO concentration of 1.85 mg/L recorded at the bottom of the HH-2 station. Within the hypoxic zone, the average DO concentration was 4.02 mg/L, and the average of DO saturation was less than 0.4. This study also identified the primary drivers of summer hypoxia as a combination of physical and biochemical factors, with water stratification due to hydrodynamic factors limiting exchange and reoxygenation between different water layers. In addition, the decomposition of organic matter in the bottom consumed a significant amount of dissolved oxygen.The significantly higher DO concentration and saturation in autumn compared to summer suggested the possible existence of seasonal hypoxia in the Haihe estuary. This study of hypoxia in the Haihe River estuary contribute to a better understanding of the factors that influence seasonal hypoxia. The findings have important implications for the management and remediation of hypoxia in the Haihe estuary and other estuarine systems.

Ocean warming and acidification interactively affect the coccolithophore physiology and drives major biogeochemical changes. While numerous studies investigated coccolithophore under short-term conditions, knowledge on how different transitional periods over long-exposure could influence the element, macromolecular and metabolic changes for its acclimation are largely unknown. We cultured the coccolithophore Chrysotila dentata, (culture generations of 1st, 10th, and 20th) under present (low-temperature low-carbon-dioxide [LTLC]) and projected (high-temperature high-carbon-dioxide [HTHC]) ocean conditions. We examined elemental and macromolecular component changes and sequenced a transcriptome. We found that with long-exposure, most physiological responses in HTHC cells decreased when compared with those in LTLC, however, HTHC cell physiology showed constant elevation between each generation. Specifically, compared to 1st generation, the 20th generation HTHC cells showed increases in quota carbon (Qc:29%), nitrogen (QN :101%), and subsequent changes in C:N-ratio (68%). We observed higher lipid accumulation than carbohydrates within HTHC cells under long-exposure, suggesting that lipids were used as an alternative energy source for cellular acclimation. Protein biosynthesis pathways increased their efficiency during long-term HTHC condition, indicating that cells produced more proteins than required to initiate acclimation. Our findings suggest that the coccolithophore resilience increased between the 1st-10th generation to initiate the acclimation process under ocean warming and acidifying conditions.

Heavy metal contamination has been the focus of many studies owing to its potential risk on the health of coastal ecosystems. The Bohai Bay (BHB) is the second largest bay of Bohai Sea and subjected to serious anthropogenic perturbations. The aim of this study was to evaluate the distribution and pollution status of toxic heavy metals in seawater with two fractions (dissolved and suspended particulate phases) and surface sediments of this coastal system. Therefore, several hydrochemical parameters and concentration of seawater metals and sediment metals were measured at two cruises of 2020 summer and autumn. The spatial distribution and potential ecological risks were examined and their inter-element relationships were analyzed to identify potential geochemical processes. By comparing historical data since 1978, we find declining trends in contents of most trace metals in seawater and sediments, suggesting that recent pollution control in BHB have an effect on diminishing metal pollution. Dissolved metals showed no significant dependence on their particulate phase. The seawater posed a moderate to high level of ecological risk. The hydrochemical factors mainly had a greater impact on dissolved metals during summer, whereas they influenced suspended metals more significantly during autumn. These results provide fundamental information to support environmental quality management and ecological protection in coastal systems.

Distribution and carbon biomass of planktonic foraminifera were investigated from the euphotic zone of the Eastern Indian Ocean during a two-month cruise, ‘Shiyan I’ (10 April–13 May 2014). Foraminifera species were collected through plankton net sampling at 44 locations (80.00°–96.10° E, 10.08° N–6.00° S). The temperature (°C) ranged between 12.82 and 31.8 °C, the salinity ranged between 32.5 and 35.5, and chlorophyll-a concentrations ranged between 0.005 µg/L and 0.89 µg/L. A total of 20 taxa were identified based on the spherical chamber shell, spines, and a final whorl which were examined under light microscopy and scanning electron microscopy. Dominant species that were characterized by the high dominant index Y > 0.14–0.46 were Globigerina bulloides, Globigerinoides ruber white, Globigerinella siphonifera, Turborotalita quinqueloba, and Globigerinella calida, contributing to the community up to 86%. The shell size of collected taxa was from 51 to 508 μm and the total carbon biomass was estimated to be between 0.062 µg C m–3 and 26.52 µg C m–3. The high carbon biomass was recorded at two stations in the equator zone. Due to its large size, Globorotalia menardii had total carbon biomass of 3.9 µg C m–3, followed by G. calida 0.68 µg C m−3, Trilobatus sacculifer 0.38 µg C m–3, Orbulina universa 0.56 µg C m–3, and G. ruber white 0.22 µg C m–3, respectively. The Pearson correlation analysis showed that the temperature and chlorophyll-a were two explanatory environmental variables that were found to be highly significant (p < 0.05) and that triggered the distribution and abundance of dominant foraminifera species in the study region. Overall, high abundances and carbon biomass were derived from the euphotic zone and equatorial region of the Eastern Indian Ocean.

Phytoplankton are known as important harbingers of climate change in aquatic ecosystems. Here, the influence of the oceanographic settings on the phytoplankton community structure in the western South China Sea (SCS) was investigated during two seasons, i.e., the winter (December 2006) and summer (August–September, 2007). The phytoplankton community was mainly composed of diatoms (192 taxa), dinoflagellates (109 taxa), and cyanobacteria (4 taxa). The chain-forming diatoms and cyanobacteria Trichodesmium were the dominants throughout the study period. The phytoplankton community structure displayed distinct variation between two seasons, shifting from a diatom-dominated regime in winter to a cyanobacteria-dominated system in summer. The increased abundance of overall phytoplankton and cyanobacteria in the water column during the summer signifies the impact of nutrient advection due to upwelling and enriched eddy activity. That the symbiotic cyanobacteria–diatom (Rhizosolenia–Richelia) association was abundant during the winter signifies the influence of cool temperature. On the contrary, Trichodesmium dominance during the summer implies its tolerance to increased temperature. Overall, the two seasonal variations within the local phytoplankton community in the western SCS could simulate their community shift over the forthcoming climatic conditions.

Marine chromophytic phytoplankton are a diverse group of algae and contribute significantly to the total oceanic primary production. However, the spatial distribution of chromophytic phytoplankton is understudied in the West Pacific Ocean (WPO). In this study, we have investigated the community structure and spatial distribution of chromophytic phytoplankton using RuBisCO genes (Form ID rbcL). Our results showed that Haptophyceae, Pelagophyceae, Cyanophyceae, Xanthophyceae, and Bacillariophyceae were the dominant groups. Further, chromophytic phytoplankton can be distinguished between upwelling and non-upwelling zones of the WPO. Surface and 75 m depths of a non-upwelling area were dominated by Prochlorococcus strains, whereas chromophytic phytoplankton were homogenously distributed at the surface layer in the upwelling zone. Meanwhile, Pelagomonas-like sequences were dominant at DCM (75 m) and 150 m depths of the upwelling zone. Non-metric multidimensional scaling (NMDS) analysis did not differentiate between chromophytic phytoplankton in the upwelling and non-upwelling areas, however, it showed clear trends of them at different depths. Further, redundancy analysis (RDA) showed the influence of physicochemical parameters on the distribution of chromophytic phytoplankton. Along with phosphate (p < 0.01), temperature and other dissolved nutrients were important in driving community structure. The upwelling zone was impacted by a decrease in temperature, salinity, and re-supplement of nutrients, where Pelagomonas-like sequences outnumbered other chromophytic groups presented.

The variation of diazotrophs has been elusive in multiple SCS and WPO regions due to insufficient data. Therefore, the dynamics of diazotrophic composition and distribution were investigated in this study, based on high-throughput sequencing and quantitative PCR of the nifH gene. We found that Proteobacteria dominated the diazotrophic community in the river-impacted SCS and cyanobacteria and Proteobacteria were more abundant in the ocean-dominated SCS and WPO. The qPCR analysis showed that cyanobacterial Trichodesmium was abundant in the Pearl River plume and in the SCS basin influenced by the Kuroshio intrusion, and it also thrived in the subequatorial region of the WPO. Unicellular cyanobacteria UCYN-A were mainly detected in the river-impacted area, UCYN-B was abundant in the WPO, UCYN-C had a relatively high abundance in the ocean-dominated area, and a preponderance of γ-Proteobacteria γ-24774A11 was observed in the ocean-dominated SCS and pelagic WPO. Diazotrophic communities had significant distance–decay relationships, reflecting clear biogeographic patterns in the study area. The variations of diazotrophic community structure were well explained by dissolved inorganic nitrogen, dissolved inorganic phosphate by an eigenvector spatial variable PCNM1. These results provide further information to help determine the ecological mechanism of elusive diazotrophic communities in different ocean ecosystems.