Limnology and Oceanography

Sinking marine particles, one pathway of the biological carbon pump, transport carbon to the deep ocean from the oceans surface, thereby contributing to atmospheric carbon dioxide modulation and benthic food supply. Few in situ measurements exist of sinking particles in the Northern Gulf of Alaska (NGA); therefore, regional carbon flux prediction is poorly constrained. In this study, we aim to (1) characterize the magnitude and efficiency of the biological carbon pump and (2) identify drivers of carbon flux in the NGA. We deployed drifting sediment traps to simultaneously collect bulk carbon and intact sinking particles in polyacrylamide gels and measured net primary productivity from deck-board incubations. Through deployments during the summer of 2019, we found high carbon flux magnitude, low attenuation with depth, and high export efficiency. We quantitatively attributed carbon flux between ten particle types, including various fecal pellet categories, dense detritus, and aggregates using polyacrylamide gels. The contribution of aggregates to total carbon flux (41 - 93%) and total carbon flux variability (95%) suggests that aggregation processes, not zooplankton repackaging, played a dominant role in carbon export during the summer of 2019 in the NGA. Furthermore, efficient export correlated significantly with the proportion of chlA > 20 {micro}m, total aggregate flux, and proportion aggregate flux. These results suggest that this stratified, small-cell-dominated ecosystem can have sufficient aggregation to allow for a strong and efficient biological carbon pump. These are the first measurements of carbon flux and the first integrative description of the BCP in this region. Significance StatementO_ST_ABSNovelty and significanceC_ST_ABSWe use a comprehensive approach that brings together sediment trap sampling and imaging, optically measured distribution of sinking and suspended particles, and incubations to make the first description of the biological carbon pump in the Northern Gulf of Alaska. We found high carbon flux magnitude, low attenuation with depth, and high export efficiency with a phytoplankton community consisting of mostly pico-and nanoplankton. Notably, just 25% of carbon flux out of the euphotic zone was as recognizable fecal pellets; instead, we demonstrate that aggregation processes were the main driver of carbon flux. Additionally, size-fractionated chlorophyll-a (> 20 {micro}m) strongly correlated with export efficiency across our region. These results lead us to question our expectations about what conditions and processes can create strong and efficient flux events in the Gulf of Alaska. Breadth of InterestThis study is the first description of the biological carbon pump in the Northern Gulf of Alaska and greatly improves biogeochemical constraints on this system. We report observed primary production, carbon flux, export ratio, carbon flux attenuation, and carbon flux by 10 particle types, which can be used to test regional climate models. This study builds on previous studies published in L&O: Strom et al. 2007; Ebersbach & Trull 2008; McDonnell & Buesseler 2010, 2012; and Durkin et al. 2016. Author contribution statementSO, SS, and AM: conceptualization, methodology, and investigation. SS, AM, GH: funding acquisition and project administration. SO, TK, AM: formal analysis. GH, TK, and AM: supervision. SO: visualization, writing-original draft preparation. SO, GH, TK, SS, and AM: writing-reviewing and editing.

The sinking of large particles (i.e., marine snow) has long been recognized as a key pathway for efficient particulate organic carbon (POC) export to the ocean interior during the decline of spring diatom blooms. Recent work has suggested that particles smaller than marine snow can also substantially contribute to POC export. However, a detailed characterization of small and large sinking particles at the end of blooms is missing. Here, we separately collected suspended and small and large sinking particles using Marine Snow Catchers and assessed their biogeochemical composition after the North Atlantic spring bloom in May 2021. During the three weeks of sampling, when four intense storms (maximum wind speeds 37 - 50 kts) created high turbulent energy dissipation rates and deepened the mixed layer, we observed two distinct sedimentation episodes. During the storm periods, sinking particles were dominated by small (diameter < 0.1 mm), slow-sinking (~18 m d-1), silica-rich particles that carried a moderate POC flux (< 6 mmol C m-2 d-1) to 500 m depth. Once the storms ceased, the volume of large (diameter > 0.1 mm), fast-sinking (> 75 m d-1), carbon-rich marine snow aggregates (not fecal pellets) increased exponentially and POC fluxes at 100 m depth were more than fourfold greater (30{+/-}12 mmol C m-2 d-1) than those during the previous event. The aggregates consisted of a mixed post-bloom plankton community. Our data suggest that the intense storms determined the timing, type, and magnitude of POC flux at the end of a spring phytoplankton bloom.

The marine biological carbon pump is mainly driven by the interplay between aggregate sinking velocity and remineralization. Sinking velocity of natural marine aggregates is not routinely measured but often calculated using Stokes law, which does not consider size-dependent changes in porosity. We analyzed the flow fields around 81 in situ-formed aggregates using Particle Image Velocimetry (PIV) to determine the factors controlling aggregate settling. Using an independently derived scaling of porosity with size, we predicted the sinking velocity of laboratory-formed and in situ-formed aggregates with known densities. Small aggregates (<500 {micro}m) have relatively lower porosities than large aggregates, and their increased compactness and density leads to higher size-specific settling velocities, and generally higher carbon-to-volume ratios. Applying our scaling approach to a global data set of vertical aggregate abundance and size distribution, we found that small aggregates contribute 40-70% to total carbon fluxes in situ. TeaserImproved sinking velocity prediction for marine aggregates highlights the contribution of small aggregates to carbon sequestration.

Many dinoflagellates are mixotrophic and regulate their vertical position to navigate dynamic gradients in light, nutrients, and prey. Here, it is shown that the obligate kleptoplastidic mixotroph, Dinophysis acuminata, transitions from diel vertical migration to formation of a stationary, sub-surface thin layer in response to prolonged prey deprivation. An inshore bloom within a salt marsh kettle pond was recorded through continuous in-situ imaging, automated oxygen and fluorescence depth profiling, and targeted water chemistry measurements. During the blooms initial development, D. acuminata cells were photosynthetically active and divided vegetatively while vertically migrating. As photosynthesis and growth slowed, vertical migration ceased and cells formed a stable thin layer that promoted conditions for local acidification and nitrogen remineralization. Surface avoidance by the thin layer drove selective retention of cells within the relatively deep kettle hole. Together, these findings illustrate linkage of metabolic state and swimming behavior in D. acuminata and show how swimming behavior can drive development of toxic blooms within inshore systems. They also illustrate how D. acuminata and other eurytolerant bloom-forming species can exploit and shape physicochemical gradients associated with coastal eutrophication.

The sea-surface microlayer (SML) is a crucial ocean-atmosphere interface involved in gas exchange and nutrient cycling. Slicks, i.e., viscous surface layers, common in coastal regions serve as microbial hotspots. We studied microbial abundance, surfactants, dissolved organic carbon (DOC), and net community production (NCP) of O2 in slick and non-slick SMLs and underlying water (ULW) in the coastal Baltic Sea. Slicks often showed higher surfactant levels compared to the ULW. Microbial respiration often exceeded production, resulting in net O2 consumption, although some ULW sites exhibited net O2 production. The SML was enriched with pico- and nanophytoplankton, with cyanobacteria being negatively correlated with total dissolved nitrogen. In contrast, microphytoplankton accumulated in the ULW, indicating niche separation with depth. Microscopy revealed ciliates and juvenile sporophytes dominating a slicks >100 {micro}m fraction. In eutrophic coastal systems, slicks influence plankton communities and O2 dynamics, supporting their role in surface biogeochemical cycling and climate-driven changes.

The post-detachment drifting phase of macrophytes, during which they can be alive, dead, or senescent, plays a crucial ecological and biogeochemical role by influencing long-range dispersal, transporting rafting species, affecting carbon sequestration, promoting blooms, and leading to beaching events. In order to predict the dispersal of macrophytes and macroplastic particles and where they will affect the ecosystem, it is important to be able to model how their drift velocities are influenced by hydrodynamic and aerodynamic factors. In this study, we investigated the drift velocity of macrophytes with diverse morphologies and macroplastic particles in a racetrack flume under different current conditions, in combination with and without wind in the same direction as the water current. Our data show that the drift velocity of macrophytes is highly dependent on their buoyancy and affected by morphological characteristics. Wind increased the velocity of the surface water, which in turn increased the drift velocity of both macrophytes and macroplastic particles. However, wind-induced turbulences reduced the overall effect, especially for macrophytes, which protruded minimally above the water surface in comparison to macroplastic particles. For positively buoyant specimens, an existing particle model was experimentally confirmed to predict macrophyte and macroplastic particle drift velocities reliably, irrespective of shape. For negatively buoyant species, we propose a novel equation to predict drift velocity, incorporating the diverse shapes of macrophytes, as well as their interaction with the bottom. These results represent the first step toward the development of trait-based models that represent macrophytes more realistically in dispersal simulations. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=135 SRC="FIGDIR/small/709487v1_ufig1.gif" ALT="Figure 1"> View larger version (54K): org.highwire.dtl.DTLVardef@1ab9f6aorg.highwire.dtl.DTLVardef@6ef75dorg.highwire.dtl.DTLVardef@132334forg.highwire.dtl.DTLVardef@c6a3d8_HPS_FORMAT_FIGEXP M_FIG C_FIG

Microbial degradation of suspended and sinking organic carbon regulates long-term oceanic carbon storage by controlling the efficiency of the biological pump. Yet microbial controls on carbon export and remineralization remain poorly constrained, limiting predictions of how ocean carbon cycling will respond to climate change. Here, we combined in situ sampling with ship-based incubations to quantify prokaryote-driven removal rates of suspended and sinking total organic carbon (TOC). Samples were collected below the mixed layer during three stages of a spring Phaeocystis pouchetii bloom in the Labrador Sea. Phaeocystis blooms can dominate regional phytoplankton biomass and are expected to increase under future climate. Removal rates were used as a proxy for carbon lability and combined with 16S rRNA metabarcoding and carbon composition analyses to link microbial community structure with substrate characteristics. Removal rates of sinking particles (0.02-0.06 d-1) were an order of magnitude higher than those of suspended TOC (0.002 d-1) during bloom-decline and non-bloom. In contrast, during late-bloom, suspended carbon exhibited rates of 0.01 d-1, comparable to sinking particles, and was enriched in exopolymer-rich colonies. Prokaryotic community composition varied primarily among bloom stages rather than carbon fractions, indicating that bloom stage-- and thus particle origin and composition--was the dominant control on bacterial degradation and assembly. Bacterial diversity peaked where carbon was refractory and originated from mixed phytoplankton. Together, these results demonstrate that suspended Phaeocystis-derived carbon can be rapidly remineralized when blooms produce exopolymer-rich colonies and highlight bloom stage as key regulator of microbial carbon processing and biological pump efficiency.

Rhizaria are a diverse supergroup of large marine protists that are often overlooked due to their fragility, lower abundances, and wide size range relative to other plankton. Despite their global distribution, Rhizaria ecology and biogeography is poorly understood due to a paucity of datasets and use of differing methodologies. Here we present the first characterization of Rhizaria ecology in the northern Gulf of Alaska (NGA), a variable yet productive subarctic ecosystem with important fisheries that is experiencing long-term warming. Seawater samples were collected from CTD-secured Niskin bottles at stations within the NGA Long-Term Ecological Research study area during summer 2023. We report some of the highest Rhizaria abundances (25 cells L-1) from any ocean environment to date and thus suggest a restructuring of the current biogeographical paradigm that posits highest abundances at the equator and decreases at higher latitudes. Acantharia was the most ubiquitous subgroup. Distinct depth niches were also revealed: Foraminifera dominated surface waters, Radiolaria exhibited a cosmopolitan distribution, and Phaeodaria were the deepest living. Prey captures and algal interactions primarily occurred offshore in the upper water column. A wide range of taxa had captured prey while the hosts to presumptively symbiotic algae were mainly Foraminifera and Acantharia. We highlight Rhizaria as key players in NGA food web dynamics as evidenced by their wide depth distributions, taxonomic diversity, and variable nutrition strategies. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=97 SRC="FIGDIR/small/652060v1_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@1cb948corg.highwire.dtl.DTLVardef@857c74org.highwire.dtl.DTLVardef@1adcaeborg.highwire.dtl.DTLVardef@e5332b_HPS_FORMAT_FIGEXP M_FIG Graphical Abstract.Distribution of Rhizaria subgroups in the northern Gulf of Alaska (left). Proposed revision of the biogeographical distribution of Rhizaria in the Pacific and Southern Oceans (right). C_FIG HighlightsO_LIThe N. Gulf of Alaska contains some of the highest Rhizaria abundances yet reported C_LIO_LIAcantharia was the most abundant taxon C_LIO_LIRhizaria subgroups inhabited distinct depth niches C_LIO_LIA wide range of taxa had captured prey C_LIO_LIForaminifera and Acantharia were the most common hosts to algal cells C_LI

While phytoplankton dynamics in the annual North Atlantic spring bloom have been well characterized, the physiological underpinnings driving these changes and their net impact on the biogeochemistry of the region are less understood. Phytoplankton metabolism is both affected by, and influences the regions nutrient cycling, primary production, and ultimately, the fate of carbon export. Thus, developing an understanding of these processes is critical. Phytoplankton biomass, biological rates, and gene expression data along with associated environmental parameters were measured as part of the NASA EXport Processes in the Ocean from RemoTe Sensing programs campaign to the North Atlantic to evaluate the relationships amongst these processes within the four most dominant phytoplankton groups (diatoms, dinoflagellates, haptophytes, and chlorophytes) during the spring bloom. We observe a transition from a period dominated by active diatom growth (defined as Phase I) to a period dominated by non-diatom phytoplankton groups (Phase II). Silicic acid depletion appears to limit overall production and reduce competition from diatoms, likely leading to enhanced contributions of haptophytes in Phase II. Expression of key protein-encoding genes involved in cell maintenance, photosynthesis, and nitrogen and vitamin metabolisms varied amongst the taxa throughout the observation period. Expression patterns of diatom genes involved in silicon transport suggest an apparent uncoupling between genes involved in nitrate uptake and photosynthesis, resulting in an increase in silicification independent growth. Our analysis demonstrates the utility in combining gene expression with biological rate processes to provide a more holistic view of phytoplankton bloom dynamics and phenology.

The second field campaign of the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program was conducted in the late spring of 2021 within the vicinity of the Porcupine Abyssal Plain (49.0{degrees}N, 16.5{degrees}W) in the North Atlantic Ocean. Observations from EXPORTS support previous characterizations of this system as highly productive and organic matter rich, with the majority of primary production occurring in large cells ([&ge;] 5 {micro}m) such as diatoms that are primarily utilizing nitrate. Rates of total euphotic zone depth-integrated net primary production ranged from 36.4 to 146.6 mmol C m- 2 d-1, with an observational period average f-ratio of 0.74, indicating predominantly new production. Substantial variability in the contribution of small (<5 {micro}m) and large cells occurred over the observation period, coinciding with the end of the annual spring phytoplankton bloom. Physical changes associated with storms appear to have impacted the integrated production rates substantially, enhancing rates by [~]10%. These disturbances altered the balance between contributions of the different phytoplankton size fractions, thus highlighting the important role of mixed layer variability in nutrient entrainment into the upper water column and production dynamics. In diatoms, inputs of silicic acid related to deepening of the mixed layer increased silicic acid uptake rates yet concomitant increases in NPP in large cells was not observed. This campaign serves as the high productivity endmember within the EXPORTS program and as such, elucidates how nutrient concentrations and size class play key roles in both low and high productivity systems, but in differing ways.

Eutrophication in the Baltic Sea has caused an imbalance in the inorganic nitrogen (N) to phosphorus (P) ratio, leaving excess phosphate (PO4) after the phytoplankton spring bloom that terminates after N-depletion. Using monitoring data, we demonstrated that the PO4 concentration has continued to increase in the outermost Gulf of Finland during past decades. We further investigated the fate of such excess PO4 in a two-week mesocosm (1.2 m3) experiment. The starting concentration of PO4 was 0.66 {micro}M, and treatments included a non-treated control (control), nitrate addition (N-add; 3.6 {micro}M), glucose addition (C-add; 25 {micro}M) and combined nitrate and glucose addition (N+C-add). The addition of N both in N-add and N+C-add treatments stimulated nano- and microphytoplankton, while the picophytoplankton abundance increased only after N-depletion. Also, the copepod biomass was positively affected by the N-addition. N2-fixing cyanobacteria were present but in low abundance. Carbon addition did not enhance heterotrophic bacterial uptake of PO4 contrary to our expectations, nor did it affect the phyto- or zooplankton community composition. The PO4 concentration was reduced to [~]0.4 {micro}M in the control and C-add treatments and to 0.16 {micro}M in the two N-amended treatments, with an inorganic N:P uptake ratio of 6.7. These results underscore the role of picophytoplankton in reducing the excess PO4 pool after the spring bloom, a function traditionally ascribed to bloom-forming diazotrophic cyanobacteria in the Baltic Sea.

Prokaryotes represent a major fraction of marine biomass and play a key role in the global carbon cycle. We studied the vertical profiles (from surface down to the bathypelagic realm) of abundance, cytometric signatures, and activity of prokaryotic communities along a productivity gradient in the subtropical and tropical Atlantic to assess whether there is a vertical linkage between surface productivity regimes and deep ocean prokaryotic communities. We found that latitudinal changes in the vertical patterns of cytometric variables were coupled with surface productivity: higher prokaryotic abundances and viabilities, and smaller cell sizes were observed below highly productive surface waters, an effect reaching down to the bathypelagic layer. On the contrary, leucine uptake rates in deep waters showed no clear relationship with surface productivity. Changes in resource and energy allocation to growth vs. maintenance in hostile environments, cell-size-dependent metabolic requirements and variability in leucine to carbon conversion may all be part of the array of factors involved in controlling prokaryotic activity patterns that were measured. Our work adds to the recent findings that highlight the importance of vertical connectivity for prokaryotic communities in the dark ocean.

Tropical coral reef ecosystems worldwide are being impacted by combined pressures of climate change and human activities that introduce large quantities of nutrients and sediments into coastal areas. In this context, phytoplankton represent a critical link between dissolved inorganic nutrients and coral reef food webs, yet their role in these ecosystems remains understudied. We investigated ecological responses of the summer phytoplankton community of Shiraho Reef (Ishigaki Island, Okinawa, Japan) to nutrient enrichment using field-based microcosm experiments under natural light and temperature conditions in September 2022 and 2023. Treatments included single and combined additions of nitrogen, phosphorus, and silicon. Chlorophyll a (Chl a) concentrations increased after three days under combined nutrient conditions, whereas single-nutrient additions produced limited responses, indicating a strong co-limitation by nitrogen and phosphorus in the reef. Analysis of size-fractionated Chl a revealed shifts from picophytoplankton that typically dominate tropical oligotrophic ecosystems toward larger groups supported by enhanced nutrient availability. Our results show short-term impacts of nutrient enrichment events on phytoplankton size structure and biogeochemical cycling in coral reefs, and highlight the importance of pelagic processes in coral reef carbon dynamics under nutrient-enrichment.

Phosphorus is a vital nutrient required for the functioning of living organisms. In aquatic environments, dissolved inorganic phosphate is considered its most bioavailable form. However, phosphate can be scarce, which has the potential to limit microbial metabolism and ecosystem functioning. To overcome phosphate scarcity, microbes produce alkaline phosphatase (AP) to access dissolved organic phosphorus (DOP). Here, we conducted a year-long study of alkaline phosphatase activity (APA) at the Ellen Browning Scripps Memorial Pier, a nutrient-rich coastal site. APA was observed throughout the year despite phosphate-replete conditions, suggesting that the role of APs in microbial nutrition is not completely understood. We tested the hypothesis that APA may promote acquisition of organic carbon liberated from DOP hydrolysis by growing the heterotrophic marine bacterium Ruegeria pomeroyi on three DOP compounds as sole carbon sources and assessing APA. Controlling for carbon concentration, all DOP sources supported growth, but at lower levels than glucose, with the highest growth observed on glucose-6-phosphate (G6P), followed by adenosine monophosphate (AMP) and adenosine triphosphate (ATP). Moreover, cell-specific APA was significantly enhanced in carbon-deplete conditions and during growth on G6P, relative to cultures grown on replete glucose or nucleotides. These findings suggest alkaline phosphatases (APs) are part of a generic carbon stress response and likely play a role in acquiring certain forms of organic carbon by R. pomeroyi, with implications for other taxa. Overall, this study helps advance the current state of knowledge regarding microbial phosphorus cycling and carbon utilization in aquatic environments.

Marine particle forensics frequently uncover information on composition, age, size, and ecological history. Zooplankton fecal pellets are also studied for process-related data, such as grazing rates and carbon sequestration potential. Here, flow cytometric analyses of fecal pellet contents revealed intact phytoplankton with photophysiological characteristics mirroring those of free-living cells. Mapping the cytometrically derived properties of cells inside fecal pellets onto vertical profiles from free-living cells revealed the potential to estimate depth specific grazing by individual zooplankton. An experiment conducted at sea confirmed that the photophysiological characteristics of free-living phytoplankton from multiple depths, consumed by zooplankton, and excreted within fecal pellets are retained for at least 24 hours after grazing is initiated. These results have implications for high resolution modeling of individual or group specific zooplankton grazing dynamics that are critical for accurately linking zooplankton grazing in the surface ocean with the mesopelagic and deep ocean food webs and carbon export. Scientific Significance StatementFecal pellet forensics have provided significant contributions to the study of zooplankton grazing and the marine carbon cycle. Gaps in knowledge about these processes remain, and continued investigations into fecal pellet contents and their fate are important for assessing connections between the surface and deeper ocean ecosystems. We describe a study conducted in the North Atlantic in the Spring of 2021 using flow cytometry to investigate fecal pellets contents. Observations that intact phytoplankton within the pellets had similar photophysiological properties to the free-living community led to a series of sample collections and experiments which provided a path forward for determining depth specific grazing by zooplankton community members. Phytoplankton survival after passing through zooplankton guts and being packaged into fecal pellets, with their potential for release far below the surface mixed layer, support prior observations of healthy phytoplankton communities at depth and validate this mechanism for the rapid transport of freshly fixed carbon to deep ocean systems. The results should be of interest to plankton ecologists and carbon cycle scientists connecting surface and deep ocean ecosystems as application of this approach at a broader scale will provide opportunities for high resolution modeling of individual and group specific zooplankton behaviors.

Food webs trace the flow of organic matter and energy among producers and consumers; for pelagic marine food webs, network complexity directly influences the amount and form of carbon exported to the deep ocean via the biological pump. Here we present a synoptic view of mixed layer food web dynamics observed during the late summer 2018 EXport Processes in the Ocean from Remote Sensing (EXPORTS) field campaign in the subarctic Northeast Pacific at the long-running time-series site, Ocean Station Papa. Carbon biomass reservoirs of phytoplankton, microzooplankton, and bacterioplankton, were approximately equal while mesozooplankton biomass was 70% lower. Live organisms composed [~]40% of the total particulate organic carbon within the mixed layer: the remainder was attributed to detritus. Rates of carbon transfer among reservoirs indicated production and assimilation rates were well balanced by losses, leaving little organic carbon available for export. The slight positive net community production rate generated organic carbon that was exported from the system in the form of food web byproducts, such as large fecal pellets generated by mesozooplankton. This characteristically regenerative food web had relatively slow turnover times with small-magnitude transfers of carbon relative to standing stocks that occurred amidst a high background concentration of detrital particles and dissolved organic matter. The concurrent estimation of food web components and rates revealed that separated processes dominated the transfer of carbon within the food web compared to those that contributed to export. Plain Language SummaryThe biological carbon pump drives a downward flux of organic matter from the sunlit surface ocean to the vast ocean interior. Ecological interactions in the surface ocean directly affect the amount and type of carbon that is exported to the deep ocean. In this study, we present a synthesis of the late summer mixed layer food web in the Northeast Pacific that was extensively characterized during the 2018 EXport Processes in the Ocean from Remote Sensing (EXPORTS) field campaign. We found the majority of carbon was recycled within the mixed layer by microbes through multiple transfers between producers and consumers. Larger organisms, mesozooplankton and salps, only consumed a small amount of carbon but through the formation of sinking fecal pellets were the main mechanism of transporting carbon out of the system. The study highlights the need to concurrently study microbial and large organism dynamics to develop a predictive understanding of the fate of organic carbon in the oceans. Key PointsO_LIThe microbial loop dominated carbon flow in the late summer mixed layer food web of the North Pacific, most net production was respired leaving little carbon available for export. C_LIO_LIActive production and consumption of organic carbon occurred amid a high background of detrital particulate organic carbon (58% of total) with slow turnover time, 66 d. C_LIO_LIMesozooplankton which had relatively minor carbon consumption rates created the majority of export production due to efficient repackaging of consumed material. C_LI

Gravitational particle sinking is the main mechanism for carbon export in the biological carbon pump. However, the export dynamics of the particle-associated protist community are not fully understood. We used 18S rRNA gene metabarcoding to characterise the exported protist community within sinking particles and bathypelagic surficial sediments in oligotrophic subtropical and high-nutrient, low-chlorophyll subantarctic waters. Sinking particles were collected with formalin-fixed and preservative-free particle interceptor traps (fixed and live traps, respectively) to identify the community involved in particle export (fixed) and protist loss from remineralisation (live). We paired this with community analysis of the upper and lower water column (mixed layer and below mixed layer to mesopelagic, respectively) to compare the relative sources of exported protists. Amplicon sequences variants (ASVs) from upper water column samples accounted for 2 to 4-fold higher proportion of reads and ASV rich-ness compared to lower water column samples in fixed trap and sediment samples, suggesting low influence of the suspended protist community from the lower water column on export. We further traced the export patterns of upper water column protist taxa by analysing the change in taxa relative abundance across the mixed layer to mesopelagic depths. Export patterns differed between taxa, which is similarly suggested by taxa-specific loss of ASV richness between fixed and live traps, but remained the same across biogeochemically-contrasting water masses. This could imply that the drivers for protist loss during export are related to characteristics consistent across environmental conditions, such as specific microbial interactions or inherent cell properties.

Diatom blooms dominate nutrient-rich ecosystems. Less is known about the ecology and bloom dynamics of diatom populations in oligotrophic ecosystems. Here, we investigated seasonal succession of planktonic diatoms in the Gulf of Aqaba (GoA) at the northern Red Sea. The GoA is a subtropical ecosystem alternating between stratified, oligotrophic profiles during summer, and deeply mixed, mesotrophic during winter. Diatom density and diversity were lower during the stratified season, dominated by pennate species, and increased at mid-winter as nitrate exceeded [~]0.5 {micro}mol L-1. Diatom density lagged after total phytoplankton and entailed a transition to centric-diatom dominance, suggesting both higher nutrient requirements for diatom growth and ecophysiological differences between morphotypes. Ephemeral blooms were detected at the mixing-to-stratification transition. Under milder conditions, mixing was shallow and diatoms reached [~]98 individuals. mL-1. Small-centric Thalassiosiraceae and several pennates dominated. However, during the following colder year, mixing depth reached [~]700 m. Consequently, nutrient concentrations were higher and diatoms reached [~]390 individuals. mL-1. This enabled emergence of chain-forming species (namely Chaetoceros and Leptocylindrus) along small-centric and pennates, and high spore abundance was detected. Restratification led to rapid bloom decline. These results illustrate diatom community succession and bloom development as a function of nutrient availability in subtropical ecosystems.

Marine microorganisms drive the biogeochemical processes that sustain ocean ecosystems, such as primary production, nutrient cycling, and the transfer of carbon and energy to higher trophic levels. Single-celled eukaryotic organisms (microbial eukaryotes or protists) represent a multifaceted group that contribute to food web dynamics as primary producers, consumers, parasites, and nutrient remineralizers. The Northern Gulf of Mexico is a productive, river-influenced, semi-enclosed ecosystem with strong economic ties. To gain detailed insight into Gulf-based microbial communities, we present an 18S rRNA gene metabarcoding survey across 12 stations from the Louisiana coast to offshore Northern Gulf encompassing the surface to over 2,000 m. Together, water mass, the ratio of dissolved inorganic carbon to total alkalinity, distance to the coast, and depth structured protistan species composition; a secondary signal was associated with the edge of a Loop Current eddy. At the Mississippi River-Gulf interface, diatoms dominated the upper water column depths, with dinoflagellates, parasitic Syndiniales, and rhizaria making up the majority of the offshore communities throughout the entire water column. Shifts in species composition with Northern Gulf environmental gradients reflect varied trophic strategies and have implications for carbon transfer efficiency and food web structure. These results establish a baseline characterization of microeukaryotic biodiversity across coastal-to-offshore and surface to deep-sea gradients that provide critical context for future assessments of Northern Gulf ecosystem resilience.

Microeukaryotes are important components of sinking particles contributing to carbon export from the surface to deep oceans. The knowledge of the sinking microeukaryotic communities and their dynamics is limited. We applied 18S rDNA metabarcoding method to investigate the microeukaryotic communities in sinking and suspended particles distinguished by marine snow catchers (MSC) during spring in the Oyashio region. Sinking particles displayed distinct communities and lower diversity than did suspended particles. The community compositions of the sinking particles varied with depth, suggesting that microeukaryotes were selectively removed through disaggregation or decomposition during settling. Prymnesiophyceae and diatoms were effectively removed, as indicated by their decreased abundance in the sinking particles at increasing depths. Conversely, phototrophic dinoflagellates maintained a higher abundance in the sinking particles across depths, indicating resistance to disaggregation and decomposition. Heterotrophic dinoflagellates and Spirotrichea were enriched in sinking particles and marine stramenopiles (MAST) groups were enriched in suspended particles. Sinking heterotrophic protist communities in the deep layers were similar to those in the surface layers, whereas they differed from the suspended ones in the same layer. Therefore, heterotrophic protists in surface layers were transported to deeper layers. Overall, our results demonstrate the functional differences among microeukaryotes in the biological carbon pump.