Marine snow is a concentrated microbial habitat that can modify bacterial growth and mortality in surrounding seawater. To examine short-term warming effects on these interactions, we conducted 24 h modified dilution experiments using natural coastal seawater incubated at 26 and 29 C, with and without artificial marine snow produced from axenic Thalassiosira pseudonana cultures in roller bottles. Bacterial growth rates in the free-living fraction increased from 4.28 d–1 to 4.52 d–1 with warming alone. The addition of marine snow reduced apparent free-living bacterial growth rates by 74% and elevated the attached bacterial fraction to 58% under in situ conditions, reflecting substantial cell redistribution onto aggregates. Grazing mortality in the surrounding water declined in the presence of marine snow under warming, consistent with protist migration toward particle-associated prey. Viral lysis was the dominant bacterial loss pathway across all treatments, intensifying under warming to 224% of free-living bacterial production. The attached viral fraction reached 82% under in situ conditions but declined markedly with warming, suggesting temperature-driven viral release into the free-living compartment. These results demonstrate that marine snow fundamentally restructures free-living and particle-associated microbial interactions and plays a critical role in mediating short-term microbial responses to coastal warming events

Coccolithophores are vital to marine ecosystems and play a central role in the global carbon cycle. Although abundant in the temperate North Atlantic and Barents Sea, their long-term bloom dynamics remain poorly understood. Using two decades of satellite observations, we reveal a substantial decline in coccolithophore blooms between 2003 and 2022, with a 36.1% reduction in bloom area and a 51.2% decrease in bloom frequency. The decline is most pronounced in the temperate North Atlantic and western Barents Sea, while the eastern Barents Sea shows notable increases. Decreasing sea surface temperatures (SSTs) in the North Atlantic and warming in the eastern Barents Sea are identified as key drivers of these trends. However, SST changes alone do not explain the asymmetric patterns across the Barents Sea, suggesting additional environmental drivers and phytoplankton competition. Our study provides the most spatially and temporally resolved assessment of coccolithophore bloom trends in this region to date, with broader implications for marine ecosystems and carbon cycling.

Abstract Marginal seas contribute disproportionately to the ocean carbon cycle but remain poorly constrained due to strong spatial and seasonal variability. Here, we combine newly collected in situ particle imagery with machine learning to reconstruct monthly, depth‐resolved climatologies of particle biovolume and size distribution in the South China Sea, the largest tropical marginal sea in the North Pacific. Particle biovolume peaks in winter due to monsoon‐driven mixing, with secondary summer maxima in regions influenced by upwelling and river plumes. Although particle size generally covaries with biovolume, seasonal decoupling occurs in plume‐dominated zones. Applying a regionally optimized size‐based model, we estimate an annual carbon export of 111.7 ± 6.0 Tg C yr−1, corresponding to a high export efficiency of 17.5 ± 0.9%, exceeding typical low‐latitude values. Our study integrates observational‐modeling approaches to highlight the diverse physical and biogeochemical drivers of particle heterogeneity and an elevated biological pump efficiency in complex marginal sea.

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.

Marine net primary productivity underpins ocean food webs, global fisheries, and the ocean’s role in the carbon cycle, yet its global magnitude and long-term evolution remain highly uncertain. Here, we compile an expanded global database of field net primary productivity measurements determined by carbon isotope incubation, increasing observational coverage by ~50% relative to existing compilations. We observe spatially varying agreement between satellite products and in situ observations, motivating the development of an observationally constrained model that integrates field measurements with environmental predictors. This model demonstrates improved predictive skill and yields revised global net primary productivity estimate ~10–20% lower than conventional satellite products. Reconstructions over the satellite era further reveal a pronounced hemispheric redistribution of net primary productivity, characterized by increasing spatial heterogeneity in the Northern Hemisphere and progressive homogenization in the Southern Hemisphere. These findings refine both the magnitude and spatial structure of marine productivity and have implications for understanding the ocean carbon cycle under ongoing climate change. Global marine net primary production is 10–20% lower than estimated by conventional satellite products with a pronounced hemispheric redistribution over the past two decades, according to a global database of field measurements determined by carbon isotope incubation.

Abstract. The oligotrophic subtropical gyres, vast yet nutrient-poor, pose challenges to our understanding of efficient carbon sequestration. Here, we integrate taxonomic, sediment trap, and metagenomic analyses to investigate the mechanisms underlying regionally heterogeneous and efficient diatom-mediated carbon export in the western North Pacific Subtropical Gyre. We discovered that within a vertically stratified nutrient regime, diatom communities displayed clear niche partitioning: Navicula and Rhizosolenia were enriched in the nutrient-depleted surface mixed layer, while Nitzschia, Chaetoceros, and Thalassiosira tended to dominate the deep chlorophyll maximum – reflecting hydrographic control over community assembly. This trait-based community structuring directly influenced the composition and magnitude of diatom carbon export, with fluxes ranging from 10³ to 10⁵ cells m⁻² d⁻¹ and an estimated 0.13–194.85 μg C m⁻² d⁻¹. Total carbon export and export efficiency (carbon exported relative to production) was markedly enhanced at station affected by the Kuroshio (K2b), which was mainly driven by the large, carbon-rich Rhizosolenia, delineating a distinct regional hotspot. Critically, metagenomic analysis revealed a limited presence of bacteria genes encoding key carbohydrate-active enzymes capable of degrading diatom-derived fucose-containing sulfated polysaccharides (FCSP), indicating a key biochemical mechanism that may reduce organic matter remineralization and enhance flux preservation. Our findings establish a multi-process framework wherein hydrodynamic regimes select for export-prone diatom communities with specific functional traits (e.g., size, carbon content), and the biochemical resistance of their organic byproducts may synergistically promote efficient carbon export. This study deciphers the interacting controls on carbon sequestration heterogeneity in the oligotrophic ocean, with crucial implications for predicting the biological pump's response to global change.

Marine snow is a concentrated microbial habitat that can modify bacterial growth and mortality in surrounding seawater. To examine short-term warming effects on these interactions, we conducted 24 h modified dilution experiments using natural coastal seawater incubated at 26 and 29 C, with and without artificial marine snow produced from axenic Thalassiosira pseudonana cultures in roller bottles. Bacterial growth rates in the free-living fraction increased from 4.28 d–1 to 4.52 d–1 with warming alone. The addition of marine snow reduced apparent free-living bacterial growth rates by 74% and elevated the attached bacterial fraction to 58% under in situ conditions, reflecting substantial cell redistribution onto aggregates. Grazing mortality in the surrounding water declined in the presence of marine snow under warming, consistent with protist migration toward particle-associated prey. Viral lysis was the dominant bacterial loss pathway across all treatments, intensifying under warming to 224% of free-living bacterial production. The attached viral fraction reached 82% under in situ conditions but declined markedly with warming, suggesting temperature-driven viral release into the free-living compartment. These results demonstrate that marine snow fundamentally restructures free-living and particle-associated microbial interactions and plays a critical role in mediating short-term microbial responses to coastal warming events

The subtlety of oceanic color shifts, compounded by noise in satellite records, has made it uncertain whether the open ocean is undergoing a change akin to terrestrial greening. Using optical indices derived from a new, stringently screened MODIS-Aqua remote sensing reflectance dataset, we show that the ocean (60° S–60° N) has become markedly greener over the past two decades, with 73% of latitudinal bands exhibiting a shift toward greener hues and 24% showing statistically significant trends. By separating optical signals from different constituents, we attribute this greening primarily to increases in phytoplankton pigments, accompanied by smaller increases in other optically active components. We suggest the observed greening results from a complex interplay of warming-driven competitive advantages of picophytoplankton, mixed layer dynamics and atmospheric dust deposition. Importantly, we find that previously perceived chlorophyll-a concentration (Chl) decline in mid-low latitudes (40° S–40° N) are largely driven by radiance data of lower-certainty; removing these data reverses the trend. Moreover, the water corresponding to these data is not warming, further challenging the conventional causal mechanism that links warming to reduced Chl. Collectively, these results reveal that Earth’s largest biome is undergoing a subtle yet detectable greening in response to climate change.

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.

Atmospheric nitrogen deposition is an important external nitrogen source to the ocean that can fuel export production, yet its origin and contribution remain uncertain in the nitrogen-limited North Pacific Subtropical Gyre (NPSG). We present aerosol nitrate and reduced nitrogen (RN) concentrations and nitrogen isotopic composition (delta 15N), along with air-mass back trajectories, across the NPSG in summer and winter. High delta 15N values (-0.4 parts per thousand to 3 parts per thousand) of aerosol nitrate and RN suggest that natural sources dominate in both seasons, contributing only modestly to the local external nitrogen supply. A synthesis of historical observations reveals pronounced zonal gradients in aerosol nitrogen concentrations and delta 15N between the NPSG and transition zone, indicating enhanced anthropogenic influence in the latter, where nitrogen limitation is weaker. We estimate that lateral ocean transport from the transition zone increases external nitrogen inputs to the NPSG by 18%, highlighting an indirect pathway linking human emissions to oligotrophic ocean productivity.

Silicon (Si) plays a central role in regulating marine primary productivity and mediating interactions between carbon (C) and nitrogen (N) cycling through its control on diatom growth. The stable Si isotopic composition (δ30Si) of biogenic silica (BSi) preserved in sediments has therefore been widely used to reconstruct past nutrient utilization and productivity. However, its applicability in low-productivity ocean regions, where diatoms contribute only modestly to total primary production, remains poorly constrained. Here, we present the first high-resolution, seasonally resolved datasets of coupled Si, C, and N isotope systems from the upper 200 m at two stations in the oligotrophic South China Sea (SCS) basin.Surface waters exhibit relatively heavy δ30Si signatures of dissolved silicic acid (DSi; δ30SiDSi; +2.6‰ to +3.2‰) and of BSi (δ30SiBSi; +1.9‰ to +2.1‰), reflecting near-complete DSi utilization by diatoms. Both Rayleigh-derived fractionation factors (30ℇDSi) based on δ30SiDSi data and apparent fractionation factors (Δ30Si, δ30SiBSi_obs. − δ30SiDSi_obs.) show a strong seasonal variability. 30ℇDSi in the euphotic zone ranging from −1.1‰ to −1.3‰ in summer and −0.6‰ to −0.8‰ in winter, and Δ30Si in the surface mixed layer ranging from −1.0‰ to −1.1‰ in summer and −0.5‰ to −0.8‰ in winter, with no clear spatial variability. Seasonal shifts in diatom productivity influence the distribution and export of biogenic particles. While the overall contribution to bulk primary production remains limited in the picophytoplankton-dominated SCS basin, diatoms can enhance BSi export and particulate organic carbon and nitrogen (POC and PON) production in winter. Coupled analyses of δ30SiBSi and C and N isotopic compositions of POC (δ13CPOC) and PON (δ15NPON) in the euphotic zone reveal a significant positive correlation between δ30SiBSi and δ13CPOC during the winter mixing period, indicating a strong linkage between the Si and C cycles under favorable environmental conditions. In contrast, no correlations are observed between δ30SiBSi and δ15NPON in either season, suggesting that Si and N cycles are largely decoupled due to complex N cycling processes. These findings indicate that δ30SiBSi holds potential as a proxy for tracing primary production in oligotrophic settings and highlight the value of integrating multiple isotope systems to disentangle cycling of different nutrients and improve palaeoceanographic reconstructions in low-productivity ocean regions.

Global warming intensifies coastal phytoplankton blooms (CPBs) and marine heatwaves (MHWs), elevating risks to marine ecosystem health. However, the impacts of regional warming on CPBs in the coastal seas around China (CSAC) remain inadequately quantified, which impedes the development of targeted strategies to mitigate the increasing bloom frequency. To address this gap, we analyzed 1 km-resolution daily CPB records (2003-2020) from the CSAC, combined with concurrent abiotic data sets, to quantify their responses to warming. Our results indicate that bloom frequency increased in 72.1% of the affected CSAC areas, with 57.6% of these increases exhibiting a positive correlation with rising sea surface temperature. Blooms typically expanded in coverage during temperate springs and tropical autumns under moderate-intensity, long-duration MHWs. Key hotspots, such as the Pearl River Delta and Leizhou Bay, experienced earlier bloom timing and higher bloom frequency, as well as greater spatial extent during these seasonal MHW events. Our results highlight that both gradual warming and discrete MHW events are key drivers of the increased frequency and expanded spatial coverage of blooms in productive coastal zones. Therefore, although climate warming is projected to strengthen water column stratification and reduce nutrient availability, efforts to reduce coastal eutrophication remain crucial for mitigating future CPB intensification.

Phosphorus (P) availability is closely associated with algal bloom types, yet filamentous attached algae frequently proliferate in clear waters or low-P systems. To examine associations between algal dominance and microbial P-cycling potential, sediment–periphyton assemblages from a low-P artificial water-diversion canal were incubated for 120 days under three water matrices representing oligotrophic, lake-like, and eutrophic conditions. Algal dominance was assessed by microscopy and metagenomic relative abundance profiling, together with sediment P fractions, bacterial communities, and P-cycling genes. Filamentous attached algae remained dominant under low-P treatments, accompanied by depletion of sediment P, especially redox-sensitive BD-P, and higher relative abundances of genes associated with P scavenging and organic P utilization, including aphA and glpQ. In contrast, eutrophic treatments were associated with planktonic algae accounting for more than 70% of the algal community at the bloom stage and with distinct bacterial assemblages and P-cycling gene profiles. Candidate phosphate-solubilizing taxa, including Ideonella, Runella, Sphingopyxis, and Gemmatimonas, were statistically associated with community variation. These results suggest that the level of P in the water, microbial P-cycling potential, and sediment P dynamics may jointly contribute to algal niche differentiation at the water–sediment interface, providing a framework for understanding filamentous algal proliferation in low-P aquatic systems.

Abstract Marine heatwaves (MHWs) profoundly impact marine ecosystems, yet it is unclear how their thermal characteristics shape phytoplankton responses. Using satellite and reanalysis data validated with a multi‐year in situ data set from the East China Sea, we show that the intensity and duration of MHWs can contribute to contrasting outcomes, alongside co‐varying background conditions, rather than being explained solely by nutrient control. Relative to chlorophyll‐a climatology, short‐intense events stimulate phytoplankton when surface nitrate exceeds 2.8 mmol m−3, whereas long‐weak MHWs suppress phytoplankton unless nitrate rises above 7.3 mmol m−3. At equal cumulative intensity, rapid warming boosts biomass more effectively than gradual warming, but only in nutrient‐rich waters. Similar patterns emerge across the global ocean. Because the duration of MHWs tends to increase faster than intensity under global warming, our findings indicate amplified negative impacts from long‐weak MHWs, with cascading effects on marine food webs and carbon cycling.

Particulate C:N:P:Si ratios and their variations in the upper ocean are crucial for understanding carbon export and its coupling with nutrient dynamics and phytoplankton community composition associated with nutrient limitation. Here, we present the first dataset of size-fractionated biogenic particle concentrations and their elemental ratios, including particulate organic carbon (POC), particulate nitrogen (PN), particulate phosphorus (PP), and biogenic silica (BSi), in the upper 500 m of the water column in the subtropical western North Pacific. The highest POC, PN, and PP concentrations consistently occurred in surface water and then decreased with increasing depth, whereas BSi concentration was frequently highest in the subsurface chlorophyll maximum layer at similar to 120 m. The small size fraction (SSF, 0.8 or 1-51 mu m) dominated the total pool of POC, PN, and PP but contributed less to the total BSi pool than the large size fraction (LSF, >51 mu m). This feature was accompanied by lower C:N, C:P, and N:P ratios and higher C:Si ratios in the SSF than in the LSF. In the euphotic zone, total particulate C:N, C:P, N:P, and C:Si ratios averaged 8 +/- 1, 146 +/- 30, 17 +/- 3, and 120 +/- 48 mol:mol respectively, and exceeded the canonical Redfield and Brzezinski ratio. The three former ratios aligned with cellular C:N:P ratios of Prochlorococcus and Synechococcus. Moreover, these ratios exhibited observable latitudinal gradients; they were generally higher in the gyre center than in its southern boundary occupied by the North Equatorial Current. Below the euphotic zone down to 500 m, there was a distinct increase in SSF molar C:N, C:P, and N:P ratios with depth, while total particulate molar C:Si ratios remained relatively constant. Combined with data collected in the subtropical eastern South Pacific and North Atlantic, our results demonstrate that the composition of the phytoplankton community primarily controls particulate molar C:N:P:Si stoichiometry in the euphotic zone of ocean gyres, in particular in the SSF, below which preferential remineralization of various bioelements plays an important role.

Phytoplankton forms the foundation of marine food webs, and their seasonal dynamics shape ocean ecosystem functioning and carbon cycling. In low-latitude basins such as the South China Sea (SCS), these dynamics have traditionally been regarded as stable and primarily controlled by warm-cold seasonal oscillations, often evaluated using single metrics such as chlorophyll-a concentration. However, this paradigm fails to capture the significant regional heterogeneity within a single basin and the complex, sometimes contradictory, responses of different phytoplankton community parameters. To address this, we conducted a comprehensive analysis of a 20-year multi-platform dataset (remote sensing, ship-based, and Biogeochemical-Argo) to reveal how regional physical processes drive the seasonal dynamics of phytoplankton chlorophyll-a, community composition, and primary production in the basin of SCS. In the northern basin, winter monsoon coupling with strong Kuroshio intrusion elevated surface chlorophyll-a by 200% compared to summer, yet contributed only 31.2% to the annual primary production, revealing a significant decoupling driven by light limitation and low-temperature suppression of photosynthesis. In the southwestern basin, summer upwelling stimulated diatom blooms and sustained primary production comparable to its winter levels. Niche models confirmed that regional physical processes (Kuroshio intrusion vs. upwelling) select for distinct phytoplankton assemblages within the basin. We conclude that regional physical forcing, rather than basin-wide monsoon seasonality alone, is the primary driver of phytoplankton dynamics and carbon cycling in this low-latitude basin, supplementing the traditional seasonal oscillation paradigm for such systems.