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The vertical migrations of pelagic organisms play a crucial role in shaping marine ecosystems and influencing global biogeochemical cycles. They also form the foundation of what might be the largest daily biomass movement on Earth. Surprisingly, among this diverse group of organisms, some single-cell protists can transit depths exceeding 50 m without employing flagella or cilia. How these non-motile cells perform large migrations remains unknown. It has been previously proposed that this capability might rely on the cell's ability to regulate its internal density relative to seawater. Here, using the dinoflagellate algae Pyrocystis noctiluca as a model system, we discover a rapid cell inflation event post cell division, during which a single plankton cell expands its volume 6-fold in less than 10 min. We demonstrate this rapid cellular inflation is the primary mechanism of density control. This self-regulated cellular inflation selectively imports fluid less dense than surrounding seawater and can thus effectively sling-shot a cell and reverse sedimentation within minutes. To accommodate its dramatic cellular expansion, Pyrocystis noctiluca possesses a unique reticulated cytoplasmic architecture that enables a rapid increase in overall cell volume without diluting its cytoplasmic content. We further present a generalized mathematical framework that unifies cell-cycle-driven density regulation, stratified ecology, and associated cell behavior in the open ocean. Our study unveils an ingenious strategy employed by a non-motile plankton to evade the gravitational sedimentation trap, highlighting how precise control of cell size and cell density can enable long-distance migration in the open ocean. • Pyrocystis noctiluca uses cellular inflation for vertical migration in the ocean • Calcium-triggered inflation rapidly alters cell density relative to seawater • A reticulated cytoplasmic network enables 6-fold volume increase without dilution • Mathematical model shows how inflation helps cells escape "gravitational traps" Larson et al. use field work, lab experiments, and theory to demonstrate that the non-motile dinoflagellate Pyrocystis noctiluca uses rapid cellular inflation to travel tens of meters in the ocean. This mechanism, enabled by a complex cytoplasmic organization, allows the cell to alter its density relative to seawater and reverse sedimentation. [ABSTRACT FROM AUTHOR]

Benthic (seafloor) remineralization of organic material determines the fate of carbon in the ocean and its sequestration. Bottom water temperature and labile carbon supply to the seafloor are expected to increase in a warming Arctic and correspondingly, benthic remineralization rates. We provide some of the first experimental data on the response of sediment oxygen demand (SOD), an established proxy for benthic remineralization, to increased temperature and/or food supply across a range of Arctic conditions and regimes. Each factor significantly increased SOD rates (with different degrees of variability); however the largest increases were seen with both factors combined (50% to ten-fold increases), consistently across the four seasons and the spatial gradient covering shelf to deep basin included in our study. This ability of the Arctic benthos to process increased pulses of carbon suggests that increased sedimented carbon under warming conditions is likely to be utilized and processed, not accumulated, impacting carbon storage and decreasing the Arctic's role as a global carbon sink. [ABSTRACT FROM AUTHOR]

Photosynthesis by phytoplankton reduces partial pressure CO2 of at the surface of the ocean and is therefore a potential means of action for a marine CO2 removal technology. Here we study how glacial rock flour may influence photosynthesis in the open ocean. Glacial rock flour is a fine-grained silicate mineral from the bedrock grinded by the Greenland Ice Sheet and enters the ocean via fjords and coastal waters. It is therefore a natural source of nutrients and trace metals to the ocean. It is easily accessible in large quantities and could be a suitable source for large-scale CO2 removal. The impact of suspended glacial rock flour was analyzed through 14 incubation experiments with natural phytoplankton communities sampled in the subtropical Atlantic. A significant increase in photosynthesis was found in 12 experiments where variable fluorescence Fv/Fm increased 12% and the average concentration of chlorophyll a increased significantly in comparison with control treatments during a 6-day period. Incubations with glacial rock flour showed a significant uptake of phosphorus whereas the average concentrations of silicate and dissolved inorganic nitrogen increased. Nutrient changes could be explained by increasing phytoplankton and microbial biomass, remineralization of organic matter, and weathering (mobilization) of glacial rock flour. These short time experiments indicated that trace metals from glacial rock flour stimulated phytoplankton growth. Thus, glacial rock flour has the potential to increase photosynthesis and phytoplankton growth, and therefore may be a potential means of action for marine CO2 removal. [ABSTRACT FROM AUTHOR]

Abstract Benthic (seafloor) remineralization of organic material determines the fate of carbon in the ocean and its sequestration. Bottom water temperature and labile carbon supply to the seafloor are expected to increase in a warming Arctic and correspondingly, benthic remineralization rates. We provide some of the first experimental data on the response of sediment oxygen demand (SOD), an established proxy for benthic remineralization, to increased temperature and/or food supply across a range of Arctic conditions and regimes. Each factor significantly increased SOD rates (with different degrees of variability); however the largest increases were seen with both factors combined (50% to ten-fold increases), consistently across the four seasons and the spatial gradient covering shelf to deep basin included in our study. This ability of the Arctic benthos to process increased pulses of carbon suggests that increased sedimented carbon under warming conditions is likely to be utilized and processed, not accumulated, impacting carbon storage and decreasing the Arctic’s role as a global carbon sink.

Zooplankton play a key role in the cycling of carbon in aquatic ecosystems, yet their production of carbon-rich fecal pellets, which sink to depth and can fuel benthic community metabolism, is rarely quantified in estuaries. We measured fecal pellet carbon (FPC) production by the whole near-surface mesozooplankton community in the York River sub-estuary of Chesapeake Bay. Zooplankton biomass and taxonomic composition were measured with monthly paired day/night net tows. Live animal experiments were used to quantify FPC production rates of the whole community and dominant individual taxa. Zooplankton biomass increased in surface waters at night (2- to 29-fold) due to diel vertical migration, especially by Acartia spp. copepods. Biomass and diversity were seasonally low in the winter and high in the summer and often dominated by Acartia copepods. Whole community FPC production rates were higher (3- to 65-fold) at night than during the day, with the 0.5–1 mm size class contributing 2–26% to FPC production in the day versus 40–70% at night. An increase in the relative contribution of larger size fractions to total FPC production occurred at night due to diel vertical migration of larger animals into surface waters. Community FPC production was highest in fall due to increased diversity and abundance of larger animals producing larger fecal pellets, and lowest in summer likely due to top-down control of abundant crustacean taxa by gelatinous predators. This study indicates that zooplankton FPC production in estuaries can surpass that in oceanic systems and suggests that fecal pellet export is important in benthic-pelagic coupling in estuaries. [ABSTRACT FROM AUTHOR]

The ocean is the largest carbon reservoir on Earth and serves as a significant sink for atmospheric CO2. Of the known mechanisms for ocean CO2 absorption, the biological pump is considered the most important for carbon sequestration due to its economic and efficient advantages. Using microalgae to capture CO2 has several advantages, including a short cultivation period, high carbon sequestration efficiency, and a low investment cost. This article provides an overview of the methods used to screen algae species in microalgal carbon sequestration technology, as well as the expected effects of carbon sequestration. This article analyzes the appropriate cultivation conditions for algae, including the type of photobioreactor, light intensity, CO2 concentration, and nutritional requirements. The article contends that achieving accurate control of multiple environmental factors during microalgal cultivation and optimizing the internal structure of the cultivation device are key components for improving microalgal biomass production and CO2 fixation ability. The results of life-cycle assessments (LCA) indicate that the production of high-value compounds using microalgae is capable of achieving negative carbon emissions. Finally, this paper evaluates the practicality and economic feasibility of microalgae carbon sequestration and also discusses the future challenges and prospects for the development of this technology. [ABSTRACT FROM AUTHOR]

The ocean's organic carbon export is a key control on atmospheric pCO2 and stimulating this export could potentially mitigate climate change. We use a data‐constrained model to calculate the sensitivity of atmospheric pCO2 to local changes in export using an adjoint approach. A perpetual enhancement of the biological pump's export by 0.1 PgC/yr could achieve a roughly 1% reduction in pCO2 at average sensitivity. The sensitivity varies roughly 5‐fold across different ocean regions and is proportional to the difference between the mean sequestration time τseq of regenerated carbon and the response time τpre of performed carbon, which is the reduction in the preformed carbon inventory per unit increase in local export production. Air‐sea CO2 disequilibrium modulates the geographic pattern of τpre, causing particularly high sensitivities (2–3 times the global mean) in the Antarctic Divergence region of the Southern Ocean. Plain Language Summary: Atmospheric CO2 levels could be reduced by stimulating plankton in the ocean to produce more organic carbon that then sinks to depth (is exported) at a higher rate. The resulting carbon deficit in surface waters drives atmospheric carbon into the ocean. The efficacy of this process depends on how long the exported carbon stays isolated from the surface and how quickly the surface deficit can be filled. Here we investigate how sensitive atmospheric CO2 levels are to a given enhancement in carbon export and where such enhancements would be most effective. We show that the sensitivity is determined by the difference between the time for which exported carbon stays sequestered at depth and the time with which the rest of the carbon in the ocean responds to the additional export rate. High sequestration times are found where the organic carbon is exported into old deep waters, while fast response times are found where it is easy for gas exchange to inject carbon into the ocean and where the additionally exported carbon is less likely to resurface and escape into the atmosphere. These factors result in the Southern Ocean and the tropics having greatest sensitivity. Key Points: pCO2 sensitivity to carbon export perturbations is governed by sequestration time, preformed response time, and atmospheric turnover timeHigh sensitivity occurs in the tropics and Southern Ocean where sequestration time is long and preformed response time is shortThe preformed response time is modulated by air‐sea CO2 disequilibrium and shortest in the Atlantic and Southern Ocean [ABSTRACT FROM AUTHOR]

The demand for marine living resources is increasing at an unprecedented scale because of the need for continuous food provision to the world’s population. The potential of already exploited fish stocks to meet this demand is limited. Therefore, mesopelagic fish have recently become attractive potential targets for fisheries because of their vast conjectured biomass. However, the role of mesopelagic fish in marine ecosystems is poorly understood. Before developing commercial exploitation plans, the relationship between mesopelagic fish and other groups in the marine food web and biogeochemical cycles should be analyzed quantitatively. In this study, we coupled a one-dimensional biogeochemical model (North Atlantic Generic Ecosystem Model) with a higher-trophic-level food web model (Ecopath with Ecosim) for the Sargasso Sea in the North Atlantic to investigate changes in carbon export and trophodynamics under two mesopelagic fish harvesting scenarios. The coupled model represented the marine food web from plankton to fish and mammals, vertical carbon export dynamics, and their interaction with fisheries. The results showed that when mesopelagic fish were not harvested, they contributed approximately 6% of the total carbon export in the surface waters, but up to 40% of the total carbon export below 400 m. Harvesting mesopelagic fish altered the energy transfers within the food web as well as to fisheries. The ecological footprint of fisheries increased significantly. Due to declining competition in the food web, epipelagic fish increased to exert elevated grazing pressure on phytoplankton; hence, phytoplankton-mediated carbon export decreased. The total carbon export decreased by 14% due to the decreases in mesopelagic fish- and phytoplanktonmediated carbon exports. The simulated increase in zooplankton- and nonmesopelagic fish-mediated carbon exports (up to 92% and 96%, respectively) did not compensate for the total decrease in carbon exports under harvesting scenarios. The findings of this study highlighted that mesopelagic fish not only have a direct control on carbon dynamics by their metabolic releases and diel vertical migration, but also strong indirect controls through prey-predator interactions within the food web. Therefore, the implications of harvesting mesopelagic fish should be carefully considered from a holistic perspective. [ABSTRACT FROM AUTHOR]

Diatoms, dinoflagellates, and coccolithophores are dominant groups of marine eukaryotic phytoplankton that are collectively responsible for the majority of primary production in the ocean.1 These phytoplankton contain additional intracellular membranes around their chloroplasts, which are derived from ancestral engulfment of red microalgae by unicellular heterotrophic eukaryotes that led to secondary and tertiary endosymbiosis.2 However, the selectable evolutionary advantage of these membranes and the physiological significance for extant phytoplankton remain poorly understood. Since intracellular digestive vacuoles are ubiquitously acidified by V-type H+-ATPase (VHA),3 proton pumps were proposed to acidify the microenvironment around secondary chloroplasts to promote the dehydration of dissolved inorganic carbon (DIC) into CO2, thus enhancing photosynthesis.4,5 We report that VHA is localized around the chloroplasts of centric diatoms and that VHA significantly contributes to their photosynthesis across a wide range of oceanic irradiances. Similar results in a pennate diatom, dinoflagellate, and coccolithophore, but not green or red microalgae, imply the co-option of phagocytic VHA activity into a carbon-concentrating mechanism (CCM) is common to secondary endosymbiotic phytoplankton. Furthermore, analogous mechanisms in extant photosymbiotic marine invertebrates6,7,8 provide functional evidence for an adaptive advantage throughout the transition from endosymbiosis to symbiogenesis. Based on the contribution of diatoms to ocean biogeochemical cycles, VHA-mediated enhancement of photosynthesis contributes at least 3.5 Gtons of fixed carbon per year (or 7% of primary production in the ocean), providing an example of a symbiosis-derived evolutionary innovation with global environmental implications.

Photosynthesis by phytoplankton reduces partial pressure of CO2 at the surface of the ocean and is therefore a potential means of action for a marine CO2 removal technology. Here we study how glacial rock flour may influence photosynthesis in the open ocean. Glacial rock flour is a fine-grained silicate mineral from the bedrock grinded by the Greenland Ice Sheet and enters the ocean via fjords and coastal waters. It is therefore a natural source of nutrients and trace metals to the ocean. It is easily accessible in large quantities and could be a suitable source for large-scale CO2 removal. The impact of suspended glacial rock flour was analyzed through 14 incubation experiments with natural phytoplankton communities sampled in the subtropical Atlantic. A significant increase in photosynthesis was found in 12 experiments where variable fluorescence Fv/Fm increased 12% and the average concentration of chlorophyll a increased significantly in comparison with control treatments during a 6-day period. Incubations with glacial rock flour showed a significant uptake of phosphorus whereas the average concentrations of silicate and dissolved inorganic nitrogen increased. Nutrient changes could be explained by increasing phytoplankton and microbial biomass, remineralization of organic matter, and weathering (mobilization) of glacial rock flour. These short time experiments indicated that trace metals from glacial rock flour stimulated phytoplankton growth. Thus, glacial rock flour has the potential to increase photosynthesis and phytoplankton growth, and therefore may be a potential means of action for marine CO2 removal.

Through biological activity, marine dissolved inorganic carbon (DIC) is transformed into different types of biogenic carbon available for export to the ocean interior, including particulate organic carbon (POC), dissolved organic carbon (DOC), and particulate inorganic carbon (PIC). Each biogenic carbon pool has a different export efficiency that impacts the vertical ocean carbon gradient and drives natural air-sea carbon dioxide gas (CO2) exchange. In the Southern Ocean (SO), which presently accounts for ~40% of the anthropogenic ocean carbon sink, it is unclear how the production of each biogenic carbon pool contributes to the contemporary air-sea CO2 exchange. Based on 107 independent observations of the seasonal cycle from 63 biogeochemical profiling floats, we provide the basin-scale estimate of distinct biogenic carbon pool production. We find significant meridional variability with enhanced POC production in the subantarctic and polar Antarctic sectors and enhanced DOC production in the subtropical and sea-ice-dominated sectors. PIC production peaks between 47°S and 57°S near the great calcite belt. Relative to an abiotic SO, organic carbon production enhances CO2 uptake by 2.80 ± 0.28 Pg C y-1, while PIC production diminishes CO2 uptake by 0.27 ± 0.21 Pg C y-1. Without organic carbon production, the SO would be a CO2 source to the atmosphere. Our findings emphasize the importance of DOC and PIC production, in addition to the well-recognized role of POC production, in shaping the influence of carbon export on air-sea CO2 exchange.

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Abstract The ocean's organic carbon export is a key control on atmospheric pCO2 and stimulating this export could potentially mitigate climate change. We use a data‐constrained model to calculate the sensitivity of atmospheric pCO2 to local changes in export using an adjoint approach. A perpetual enhancement of the biological pump's export by 0.1 PgC/yr could achieve a roughly 1% reduction in pCO2 at average sensitivity. The sensitivity varies roughly 5‐fold across different ocean regions and is proportional to the difference between the mean sequestration time τseq of regenerated carbon and the response time τpre of performed carbon, which is the reduction in the preformed carbon inventory per unit increase in local export production. Air‐sea CO2 disequilibrium modulates the geographic pattern of τpre, causing particularly high sensitivities (2–3 times the global mean) in the Antarctic Divergence region of the Southern Ocean.

The demand for marine living resources is increasing at an unprecedented scale because of the need for continuous food provision to the world’s population. The potential of already exploited fish stocks to meet this demand is limited. Therefore, mesopelagic fish have recently become attractive potential targets for fisheries because of their vast conjectured biomass. However, the role of mesopelagic fish in marine ecosystems is poorly understood. Before developing commercial exploitation plans, the relationship between mesopelagic fish and other groups in the marine food web and biogeochemical cycles should be analyzed quantitatively. In this study, we coupled a one-dimensional biogeochemical model (North Atlantic Generic Ecosystem Model) with a higher-trophic-level food web model (Ecopath with Ecosim) for the Sargasso Sea in the North Atlantic to investigate changes in carbon export and trophodynamics under two mesopelagic fish harvesting scenarios. The coupled model represented the marine food web from plankton to fish and mammals, vertical carbon export dynamics, and their interaction with fisheries. The results showed that when mesopelagic fish were not harvested, they contributed approximately 6% of the total carbon export in the surface waters, but up to 40% of the total carbon export below 400 m. Harvesting mesopelagic fish altered the energy transfers within the food web as well as to fisheries. The ecological footprint of fisheries increased significantly. Due to declining competition in the food web, epipelagic fish increased to exert elevated grazing pressure on phytoplankton; hence, phytoplankton-mediated carbon export decreased. The total carbon export decreased by 14% due to the decreases in mesopelagic fish- and phytoplankton-mediated carbon exports. The simulated increase in zooplankton- and non-mesopelagic fish-mediated carbon exports (up to 92% and 96%, respectively) did not compensate for the total decrease in carbon exports under harvesting scenarios. The findings of this study highlighted that mesopelagic fish not only have a direct control on carbon dynamics by their metabolic releases and diel vertical migration, but also strong indirect controls through prey-predator interactions within the food web. Therefore, the implications of harvesting mesopelagic fish should be carefully considered from a holistic perspective.

Export production, which is defined as the export of organic matter fixed by photosynthesis, is crucial for sustaining oceanic carbon uptake. The export route in the open ocean is the sinking of biogenic particles through the bottom of the euphotic layer. In contrast, the export routes in the shelf seas are the sinking of biogenic particles to the sediment and the horizontal transport of biogenic particles across the boundary of the shelf seas to the open ocean. The biogenic particles in the shelf seas are supported bymultisource nutrients including riverine and oceanic ones. Their exports depend on the hydrodynamic conditions and biogeochemical processes responsible for different sources of nutrients. Here, a unique physical-biological coupled model with a tracking approach is applied to evaluate the export production supported by multisource dissolved inorganic nitrogen (DIN) over the East China Sea. The total export production is 6.83 kmol N s-1 (=17.16 Tg C yr-1), which is slightly lower than the reported atmospheric CO2 absorption. Approximately 80% of particulate organic nitrogen (PON) is exported via off-shelf transport, and the remaining 20% is buried in the sediment. The PON supported by DIN from rivers accounts for 8% of export production, with an e-ratio (export production/primary production) of 0.09. In comparison, that from the Kuroshio accounts for 64%, with an e-ratio of 0.22. This suggests that offshore areas here are more efficient in exporting local production than nearshore ones, largely supported by oceanic nutrients. [ABSTRACT FROM AUTHOR]

Pelagic tunicates (salps, pyrosomes) and fishes generate jelly falls and/or fecal pellets that sink roughly 10 times faster than bulk oceanic detritus, but their impacts on biogeochemical cycles in the ocean interior are poorly understood. Using a coupled physical‐biogeochemical model, we find that fast‐sinking detritus decreased global net primary production and surface export, but increased deep sequestration and transfer efficiency in much of the extratropics and upwelling zones. Fast‐sinking detritus generally decreased total suboxic and hypoxic volumes, reducing a "large oxygen minimum zone (OMZ)" bias common in global biogeochemical models. Newly aerobic regions at OMZ edges exhibited reduced transfer efficiencies in contrast with global tendencies. Reductions in water column denitrification resulting from improved OMZs improved simulated nitrate deficits relative to phosphate. The carbon flux to the benthos increased by 11% with fast‐sinking detritus from fishes and pelagic tunicates, yet simulated benthic fluxes remained on the lower end of observation‐based estimates. Plain Language Summary: Marine ecosystems play a critical role in the global carbon cycle through the food web regulation of air‐sea carbon fluxes and the transfer of particulate matter from the upper oceans to depth. Recent evidence has suggested that the detritus from fishes and gelatinous zooplankton (GZ), specifically the pelagic tunicates such as salps and pyrosomes, may have a disproportionate impact on the ocean's biological pump due to them sinking approximately 10 times faster than bulk detritus. These fluxes result in increased sequestration of particulate carbon and nutrients into the deep oceans, but their impact on biogeochemical cycles at depth is poorly understood. Here, we investigated the sensitivity of deep ocean carbon, oxygen, and nutrient cycles to fast‐sinking detritus from tunicates and fishes. We found that the fast‐sinking detritus decreased surface productivity and export, as well as the size of ocean oxygen minimum zones (OMZs). Also, we examined whether observational evidence of seafloor oxygen consumption could support the increased detrital fluxes (and respiration) at depth, and found that even with the increased oxygen consumption, the modeled values were still below the observations. This suggests that these processes could be realistically incorporated into future generations of Earth System Models. Key Points: We incorporated fast‐sinking detritus from pelagic tunicates and fishes into a modified version of the ocean biogeochemical model COBALTThe fast‐sinking detritus increased carbon sequestration and transfer efficiency to depth, but decreased surface productivity and exportFast‐sinking detritus decreased the size of oxygen minimum zones (OMZs) and water column denitrification, a common model bias [ABSTRACT FROM AUTHOR]

All else equal, if the ocean's "biological [carbon] pump" strengthens, the dissolved oxygen (O2) content of the ocean interior declines. Confidence is now high that the ocean interior as a whole contained less oxygen during the ice ages. This is strong evidence that the ocean's biological pump stored more carbon in the ocean interior during the ice ages, providing the core of an explanation for the lower atmospheric carbon dioxide (CO2) concentrations of the ice ages. Vollmer et al. (2022, https://doi.org/10.1029/2021PA004339) combine proxies for the oxygen and nutrient content of bottom waters to show that the ocean nutrient reservoir was more completely harnessed by the biological pump during the Last Glacial Maximum, with an increase in the proportion of dissolved nutrients in the ocean interior that were "regenerated" (transported as sinking organic matter from the ocean surface to the interior) rather than "preformed" (transported to the interior as dissolved nutrients by ocean circulation). This points to changes in the Southern Ocean, the dominant source of preformed nutrients in the modern ocean, with an apparent additional contribution from a decline in the preformed nutrient content of North Atlantic‐formed interior water. Vollmer et al. also find a lack of LGM‐to‐Holocene difference in the preformed 13C/12C ratio of dissolved inorganic carbon. This finding may allow future studies to resolve which of the proposed Southern Ocean mechanisms was most responsible for enhanced ocean CO2 storage during the ice ages: (a) coupled changes in ocean circulation and biological productivity, or (b) physical limitations on air‐sea gas exchange. Plain Language Summary: Recent studies have sealed the case that the concentration of oxygen was reduced in the deep ocean during the ice ages. Vollmer et al. (2022, https://doi.org/10.1029/2021PA004339) combine the oxygen results with data on deep ocean nutrient concentrations. They find that, relative to modern, more of the ice age deep ocean's nutrient reservoir arrived as sinking organic matter from surface waters, leading to more storage of carbon dioxide in the deep ocean. Moreover, they calculate that the entire observed drawdown in atmospheric carbon dioxide levels during ice ages can be explained by this strengthening of the ocean's "biological carbon pump." The findings bring the scientific community an important step closer to explaining why and how ice ages occur and end. The Southern Ocean, the ocean region around Antarctica, must have played a major role. However, it remains unclear whether the carbon dioxide was trapped in the Southern Ocean by changes in its circulation and biology or by limitations on air‐sea gas exchange across the Southern Ocean surface, such as might have occurred due to sea ice cover. Vollmer et al.'s reconstruction of the carbon isotopes in Southern Ocean surface waters may help to answer this question. Key Points: Proxies indicate less dissolved O2 in the ocean interior during ice ages than during interglacialsThis suggests an increase in CO2 storage by the ocean's biological pump adequate to explain the lower atmospheric CO2 of the ice agesReconstructed nutrient use implicates the Southern Ocean but does not yet resolve the roles of circulation, biology, and gas exchange [ABSTRACT FROM AUTHOR]

Understanding particle cycling processes in the ocean is critical for predicting the response of the biological carbon pump to external perturbations. Here, measurements of particulate organic carbon (POC) concentration in two size fractions (1–51 and >51 μm) from GEOTRACES Pacific meridional transect GP15 are combined with a POC cycling model to estimate rates of POC production, (dis)aggregation, sinking, remineralization, and vertical transport mediated by migrating zooplankton, in the euphotic zone (EZ) and upper mesopelagic zone (UMZ) of distinct environments. We find coherent variations in POC cycling parameters and fluxes throughout the transect. Thus, the settling speed of POC in the >51 μm fraction increased with depth in the UMZ, presumably due to higher particle densities at depth. The settling flux of total POC (>1 μm) out of the EZ was positively correlated with primary production integrated over the EZ; the highest export occurred in the subarctic gyre while the lowest occurred in the subtropical gyres. The ratio of POC settling flux to integrated primary production was low (<5%) along GP15, which suggests an efficient recycling of POC in the EZ in all trophic regimes. Specific rates of POC remineralization did not show clear variations with temperature or dissolved oxygen concentration, that is, POC recycling was apparently controlled by other factors such as microbial colonization and substrate lability. Particle cohesiveness, as approximated by the second‐order rate constant for particle aggregation, was negatively correlated with trophic regime: particles appeared more cohesive in low‐productivity regions than in high‐productivity regions. Key Points: Settling speed of particulate organic carbon (POC) in the large size fraction increased with depth in the upper mesopelagic zoneSettling flux of POC out of the euphotic zone was correlated with primary production, but the export efficiency was usually less than 5%Particles appeared to be more cohesive in low‐productivity regions than in high‐productivity regions [ABSTRACT FROM AUTHOR]

Estuarine and coastal environments are important transport pathways and regional sinks for anthropogenic pollutants. In this study, the occurrence and transport of the continuously released organophosphate esters (OPEs) was investigated together with physical and biochemical parameters throughout the water column in the Changjiang (Yangtze) River estuary during the summer. Total dissolved and particulate OPEs showed great spatial heterogeneity, with mean concentrations of 550 ± 280 ng/L in the estuary, 110 ± 270 ng/L in the front/upwelling zone, and 410 ± 450 ng/L in the continental shelf. OPE concentrations in the estuarine bottom waters were high due to massive terrestrial/sediment inputs. In contrast, the "surface enrichment and depth depletion" of OPEs in the continental shelf was closely related to seasonal stratification. Reduced OPE concentrations were observed in the frontal/upwelling zone due to isopycnal heaving. Frontal activity and upwelling induced phytoplankton blooms in the coastal regions, which jointly contributed to elevated OPEs beneath surface water with high phytoplankton aggregation. The OPEs mainly originated from wastewater treatment plant discharges, industrial pollution and consumer products. These OPEs generally posed a low ecological risk to aquatic lives, but their long‐term effects cannot be ignored due to their continuous high production, usage and release. Plain Language Summary: The environmental behavior and fate of anthropogenic contaminants, such as the continuously released organophosphate esters (OPEs), is complex in estuaries and coastal environments. In this study, the occurrence and transport of OPEs in the Changjiang River estuary was associated with different oceanographic processes, including fronts, upwelling and impacts of the biological pump. Dissolved and particulate OPEs varied greatly from the estuary to the coastal region. High OPE concentrations were found in turbid bottom waters of estuary, likely caused by massive sediment inputs. Due to summertime stratification, the OPEs showed high levels in surface seawater, but generally low concentrations in middle/bottom layers of the continental shelf. As for the transition zone between the estuary and continental shelf where fronts/upwelling coexisted, relatively low OPE concentrations were present through the water column, because of the heaving of (cleaner) bottom water masses. Relatively higher OPE concentrations were present just below the algae‐rich surface water, probably caused by the settling of biogenic particles with OPEs attached. The observed OPEs were mainly from wastewater treatment plant discharges, industrial pollution and consumer products. The ecological risks posed by OPEs to aquatic life cannot be ignored due to the continuous exposure. Key Points: Sediment inputs increased organophosphate ester (OPE) concentrations in turbid bottom watersThe biological pump strongly affected OPEs in frontal/upwelling and continental shelf regionsDischarge from wastewater treatment plants contributed more than 50% of OPEs in the Changjiang River estuary [ABSTRACT FROM AUTHOR]

Abstract Pelagic tunicates (salps, pyrosomes) and fishes generate jelly falls and/or fecal pellets that sink roughly 10 times faster than bulk oceanic detritus, but their impacts on biogeochemical cycles in the ocean interior are poorly understood. Using a coupled physical‐biogeochemical model, we find that fast‐sinking detritus decreased global net primary production and surface export, but increased deep sequestration and transfer efficiency in much of the extratropics and upwelling zones. Fast‐sinking detritus generally decreased total suboxic and hypoxic volumes, reducing a “large oxygen minimum zone (OMZ)” bias common in global biogeochemical models. Newly aerobic regions at OMZ edges exhibited reduced transfer efficiencies in contrast with global tendencies. Reductions in water column denitrification resulting from improved OMZs improved simulated nitrate deficits relative to phosphate. The carbon flux to the benthos increased by 11% with fast‐sinking detritus from fishes and pelagic tunicates, yet simulated benthic fluxes remained on the lower end of observation‐based estimates.

Fecal pellets (FPs) are generated by various species and have gained attention as contributors to the biological carbon pump. Metazoans and protozoans are known as FP and minipellet‐producers, respectively. Herein, we discovered fecal pellet‐like dinoflagellates (FLDs) in the seasonal sea ice zone in the Southern Ocean. The size and form of these FLDs were similar to those of zooplankton oval FPs. However, due to their appearance, they have been misclassified as FPs rather than dinoflagellates, leading to potential oversight of their role in the carbon cycle. Thus, we aimed to identify FLD cells at the species level and examine the impact of FLDs on flux estimation of particulate organic carbon (POC). Our findings are as follows: first, FLD cells were identified as Gyrodinium rubrum and Gyrodinium heterogrammum through 18S rRNA gene sequencing. Second, FLDs can potentially excrete larger FPs than minipellets. Third, the sinking rate of FLDs is higher than that of other protozoa and dinoflagellate cysts. Finally, a maximum of 12 mgC m−2 day−1 of the POC flux can be attributed to FLDs (representing 32% of POC flux). These results suggest that FLDs are important drivers not only for the microbial loop but also for the biological carbon pump. In future projections of carbon sequestration, the contribution of metazoans to carbon export must be considered, but not that of FLDs. Their unknown physiological and ecological characteristics, especially including the responses to climate changes, must be urgently investigated for future projections of carbon sequestration in the Southern Ocean. Plain Language Summary: Zooplankton feed on phytoplankton cells and excrete fecal pellets (FPs). FPs have gained attention as carbon carriers from the surface to the deep ocean because of their high carbon contents and sinking rate. Previous studies have sampled FPs using sediment traps and estimated carbon fluxes through microscopic observations to better understand the ocean carbon cycle. Here, we report the discovery of a dinoflagellate that closely resembles FP and was captured using a drifting sediment trap. This dinoflagellate has been named "fecal‐pellet‐like dinoflagellate (FLD)." It is believed that FLD cells have been misclassified as FPs given their appearance. Our findings indicated that a maximum of 12 mgC m−2 day−1 (representing 32% of the particulate organic carbon flux at 50 m depths during the summer productive season) may be attributed to FLD carbon flux. This result suggests that FLD cells serve as potential contributors for carbon export, contrary to the knowledge that heterotrophic dinoflagellates had been known as one of the dominant microbial loop components. Studies on FLDs are essential to understand the Southern Ocean carbon cycle. Key Points: Fecal pellet‐like dinoflagellates (FLDs) were observed among drifting sediment trap samples in the East Antarctic ice zoneFLD cells were identified as Gyrodinium rubrum and Gyrodinium heterogrammumFLDs determined as potential players for carbon export that were overlooked because of their appearance [ABSTRACT FROM AUTHOR]

The ocean biological pump is the mechanism by which carbon and nutrients are transported to depth. As such, the biological pump is critical in the partitioning of carbon dioxide between the ocean and atmosphere, and the rate at which that carbon can be sequestered through burial in marine sediments. How the structure and function of planktic ecosystems in the ocean govern the strength and efficiency of the biological pump and its resilience to disruption are poorly understood. The aftermath of the impact at the Cretaceous/Palaeogene (K/Pg) boundary provides an ideal opportunity to address these questions as both the biological pump and marine plankton size and diversity were fundamentally disrupted. The excellent fossil record of planktic foraminifera as indicators of pelagic-biotic recovery combined with carbon isotope records tracing biological pump behaviour, show that the recovery of ecological traits (diversity, size and photosymbiosis) occurred much later (approx. 4.3 Ma) than biological pump recovery (approx. 1.8 Ma). We interpret this decoupling of diversity and the biological pump as an indication that ecosystem function had sufficiently recovered to drive an effective biological pump, at least regionally in the South Atlantic.

This document is an editorial published in the journal Frontiers in Marine Science. It discusses the important roles of zooplankton and nekton organisms in marine ecosystems and their impact on the biological pump, which is responsible for the downward export of carbon and nutrients into the ocean's interior. The editorial highlights the diverse roles of zooplankton and nekton in particle dynamics, from consumption and fragmentation to the active transport of organic matter. It also presents several studies that investigate the seasonal impact of copepods on carbon export, the distribution of macrozooplankton and nekton across polar fronts, the influence of diversity on the biogeochemical role of zooplankton, and the challenges of estimating carbon flux contribution by different taxa. The editorial emphasizes the need for size- and weight-specific parameterizations and the adoption of FAIR principles for data publication to enhance our understanding of the role of zooplankton and nekton in the biological pump. [Extracted from the article]

We examined the flux and composition of sinking particles collected at a water depth of 800 m in the northwest Pacific from November 2017 to August 2018 to assess the impact of dust deposition on organic carbon export. The fluxes of total particulate matter and particulate organic carbon averaged over the study period were 88 ± 63 mg m-2 d-1 and 9.0 ± 5.8 mg m−2 d−1, respectively. Biogenic particles accounted for 82% of the sinking particles, on average. There were two notable pulses in the particle fluxes of both biogenic and lithogenic material in February and May 2018. These flux peaks were decoupled from net primary production in the surface waters but coincided with intervals of high rates of atmospheric dust deposition. The biogenic component of the two peaks was dominated by two different phytoplankton communities, which may have influenced carbon export efficiency. Correlations between the sinking particle flux and the lithogenic flux are found at several locations in the northwest Pacific, implying that East Asian dust deposition has a prevalent influence on the biological pump. Attention should be paid to the effects of changes in the continental dust supply to the oceans on oceanic carbon export.

Multiyear estimates of organic matter (OM) export based primarily on oxygen and dissolved inorganic carbon surface layer budgets applied basin‐wide for the Pacific, Atlantic, and S. Indian Oceans yield an inter‐basin range from 1 to 3 mol C/m2/yr with a global mean of 2.0 mol C/m2/yr (8.5 Gt C/yr). OM export rates per area in the Pacific and Atlantic oceans are twice than that in the Indian Ocean. The supply of nutrients from the Southern Ocean can potentially support ∼70% of the observed OM export in the ocean based on observed surface current velocities and PO4 distributions. Horizontal flux of PO4 and dissolved organic phosphorous in the surface layer can support ∼50%, 20%, and 15% of observed OM export in the Pacific, S. Indian and Atlantic oceans, respectively, with the remainder being supplied vertically from the subsurface. Potential utilization of unused surface PO4 in the subtropical gyre yields ∼0.1 mol C/m2/yr increase in OM export in the Pacific and Atlantic oceans but a ∼0.8 mol C/m2/yr increase in the S. Indian ocean suggesting that stronger nutrient limitation contributes to lower export rates observed in the Indian ocean. Plain Language Summary: A small fraction of the organic matter (OM) produced photosynthetically by phytoplankton in the surface ocean is transferred (exported) to deeper portions of the ocean. The rate at which the OM is exported to the subsurface ocean has a significant impact on the ocean's concentrations of dissolved carbon dioxide and oxygen gases. New estimates of the OM export rate per unit area for the Atlantic, Pacific, and Indian oceans are presented based on surface layer oxygen and carbon dioxide budgets. The OM export rate is similar for the Atlantic and Pacific oceans but twice the rate in the Indian ocean. Most of the dissolved nutrients needed by phytoplankton to produce OM originate on the surface of the Southern Ocean (south of 50°S) and are transported via circulation to all the ocean basins where they are consumed by phytoplankton. Between ∼15 and 50% of the nutrients are supplied via surface currents, whereas the remainder is supplied via mixing from the subsurface layers of the ocean. Key Points: Oxygen and inorganic carbon budgets yield organic matter (OM) export rates for the major ocean basins that range from 1 to 3 mol C/m2/yrDissolved nutrients originating in the Southern Ocean can support ∼70% of the global OM exportSurface transport of dissolved nutrients support 15%–50% of export in the ocean basins with the rest supplied vertically from depth [ABSTRACT FROM AUTHOR]

We use a nutrient‐ratio budget method to investigate the relative importance of different nutrient source and sink terms at time‐series Station ALOHA and Bermuda Atlantic Time‐series Study (BATS) in the North Pacific and North Atlantic subtropical gyres, respectively. At mean state conditions over annual and multi‐year time scales, vertical phosphate (PO43– ${\mathrm{P}\mathrm{O}}_{4}^{3\mbox{--}}$) supply from the subsurface accounts for ∼60% of the total phosphorus supply at both sites. Dissolved organic matter transport and zooplankton excretion are more important phosphorous export pathways than sinking particles at Station ALOHA and BATS. The nutrient‐ratio budget approach provides quantitative, observation‐based constraints on nutrient sources and sinks in the surface ocean, which helps improve our understanding of the biological carbon pump in oligotrophic oceans. Plain Language Summary: In this study, we explore the cycling of nutrients that support primary production in the surface ocean and its subsequent export to depth using observed elemental ratios of nitrogen to phosphorus for various nutrient sources and sinks. We use nutrient observations from long‐term oceanographic time‐series studies at Station ALOHA near Hawaii and the Bermuda Atlantic Time‐series Study near Bermuda. We assume that both stations are under conditions of steady state in which nutrient concentrations are not changing over long time periods, and therefore, that the nitrogen‐to‐phosphorus ratio between inputs and outputs should be balanced. We apply a mathematical model to estimate the relative contribution of each input and output term. Our results suggest that nutrient input is driven primarily by the vertical transport of subsurface water at both study sites. Nutrient output (loss) is driven by the gravitational sinking of large particles, the downward mixing of dissolved constituents, and the active transport of migrant animals. The loss due to the latter two processes is more important in magnitude. Our simple methodology provides quantitative, observational constraints of nutrient sources and sinks to the upper ocean, contributing improved understanding of the biological carbon pump in the oligotrophic subtropical ocean. Key Points: A nitrogen‐to‐phosphorus ratio budget method is used to quantify nutrient sources and sinks at two subtropical ocean study sitesVertical phosphate supply is the dominant source of phosphorus to the surface of the North Pacific and the North Atlantic study siteDissolved organic phosphorus transport and zooplankton excretion are more important than sinking particles as nutrient sinks [ABSTRACT FROM AUTHOR]

Photosynthesis in the surface ocean converts atmospheric CO2 into organic particles, with the fraction sinking to depth representing a major part of the ocean's biological pump. Although sinking particles are known to be altered by attached‐bacteria during transit, most prior organic geochemical data indicated only minor replacement of plankton‐derived particles by bacterial material. We exploit bacteria‐specific biomarkers (d‐amino acids) in a multi‐year sediment trap in the Pacific Ocean (1,200 m) and suggest a different view. Major d‐amino acids were consistently measured at abundance demonstrating widespread accumulation of bacterial material in sinking particles. Bacterial detritus was estimated to account for up to 19% of particulate organic carbon and up to 36% of particulate nitrogen, much higher than cell count‐based values. The bacterial relative contribution increased with decreasing export production. Our results indicate that bacterial material constitutes an underappreciated component of the biological pump, a role expected to rise as the ocean warms. Plain Language Summary: Phytoplankton photosynthesis in the surface ocean plays a critical role in stabilizing atmospheric CO2. It converts CO2 into organic particles that sink and are reworked by colonizing bacteria. Bacteria respire most particles back to CO2 while also transforming some into their cell components. Although the involvement of bacteria can replace the plankton‐derived particles to bacterial material, most past organic geochemical data have suggested that the deep‐sea particles are still comprised mainly of plankton remnants. This renders the contribution of bacterial material to total particle export an unresolved and yet important question, because the source and composition of particles are important to their fate in the ocean. Here, we analyzed bacteria‐specific molecules in deep‐sea sinking particles and found that bacterial organic matter actually made up a large fraction of the particles. In addition, the relative contribution of bacterial material to the sinking particles increased as the total carbon export decreased. This has important implications for the future ocean carbon cycle, because modeling work predicts a scenario of lower carbon export to the deep sea as the ocean warms. In this context, our findings imply a greater importance of bacteria in marine organic matter export and sequestration in a warming ocean. Key Points: We exploit d‐amino acid biomarkers in multi‐year deep‐sea sediment trap time series to evaluate bacterial contribution to total exportBacterial detritus accounts for up to 19 ± 8% of sinking POC and up to 36 ± 14% of PN, making up a large unrecognized part of biological pumpThe relative contribution of bacterial detritus to sinking particles increases with decreased export production [ABSTRACT FROM AUTHOR]

Increasing Asian dust fluxes, associated with late Cenozoic cooling and intensified glaciations, are conventionally thought to drive iron fertilization of phytoplankton productivity in the North Pacific, contributing to ocean carbon storage and drawdown of atmospheric CO2. During the early Pleistocene glaciations, however, productivity remained low despite higher Asian dust fluxes, only displaying glacial stage increases after the mid-Pleistocene climate transition (~800 ka B.P.). We solve this paradox by analyzing an Asian dust sequence, spanning the last 3.6 My, from the Tarim Basin, identifying a major switch in the iron composition of the dust at ~800 ka, associated with expansion of Tibetan glaciers and enhanced production of freshly ground rock minerals. This compositional shift in the Asian dust was recorded synchronously in the downwind, deep sea sediments of the central North Pacific. The switch from desert dust, containing stable, highly oxidized iron, to glacial dust, richer in reactive reduced iron, coincided with increased populations of silica-producing phytoplankton in the equatorial North Pacific and increased primary productivity in more northerly locations, such as the South China Sea. We calculate that potentially bioavailable Fe2+ flux to the North Pacific was more than doubled after the switch to glacially-sourced dust. These findings indicate a positive feedback between Tibetan glaciations, glaciogenic production of dust with enhanced iron bioavailability, and changes in North Pacific iron fertilization. Notably, this strengthened link between climate and eolian dust coincided with the mid-Pleistocene transition to increased storage of C in the glacial North Pacific and more intense northern hemisphere glaciations. [ABSTRACT FROM AUTHOR]

The ocean’s biological pump connects the surface ocean, where light-driven photosynthetic processes fix dissolved carbon dioxide (CO2), to the ocean’s mesopelagic zone (approx. 200 –1000 meters) and beyond. It is a process that depletes the ocean’s surface of CO2 relative to the CO2 in deep water through mechanisms such as the sinking of organic material to the deep Ocean. The Atlantic Ocean features high productivity of the macroalgae genus Sargassum floating on the ocean’s surface in the Sargasso Sea, and in recent years giant blooms of the brown algae Sargassum have been observed stretching from the west coast of Africa to the Gulf of Mexico, the largest macroalgae bloom that has ever been recorded. The sinking of macroalgae from surface waters to the seafloor is considered to be an important carbon sink, but one that is little understood. With the logistical challenges of accessing the deep sea, the record of Sargassum appearing on the seafloor remains limited.This project utilized an archived exploratory dataset that is freely available to the public in order to make novel discoveries in previously unexplored areas. The following report documents the presence and distribution of Sargassum falls in the deep sea during six dives conducted by NOAA Ship Okeanos Explorer’s ROV Deep Discoverer off the Southeastern United States, in the Gulf of Mexico, and in the Caribbean. Sargassum was observed on each of the dives, in numbers ranging from 6 to 30 observations per dive, with Sargassum being observed an average of every 171 linear meters. This suggests that Sargassum does make its way to the deep sea, in potentially significant amounts.

Through biological activity, marine dissolved inorganic carbon (DIC) is transformed into different types of biogenic carbon available for export to the ocean interior, including particulate organic carbon (POC), dissolved organic carbon (DOC), and particulate inorganic carbon (PIC). Each biogenic carbon pool has a different export efficiency that impacts the vertical ocean carbon gradient and drives natural air-sea carbon dioxide gas (CO2) exchange. In the Southern Ocean (SO), which presently accounts for ~40% of the anthropogenic ocean carbon sink, it is unclear how the production of each biogenic carbon pool contributes to the contemporary air-sea CO2 exchange. Based on 107 independent observations of the seasonal cycle from 63 biogeochemical profiling floats, we provide the basin-scale estimate of distinct biogenic carbon pool production. We find significant meridional variability with enhanced POC production in the subantarctic and polar Antarctic sectors and enhanced DOC production in the subtropical and sea-ice-dominated sectors. PIC production peaks between 47°S and 57°S near the "great calcite belt." Relative to an abiotic SO, organic carbon production enhances CO2 uptake by 2.80 ± 0.28 Pg C y-1, while PIC production diminishes CO2 uptake by 0.27 ± 0.21 Pg C y-1. Without organic carbon production, the SO would be a CO2 source to the atmosphere. Our findings emphasize the importance of DOC and PIC production, in addition to the well-recognized role of POC production, in shaping the influence of carbon export on air-sea CO2 exchange. [ABSTRACT FROM AUTHOR]

Whereas the vertical transport of biomass from productive surface waters to the deep ocean (the biological pump) is a critical component of the global carbon cycle, its magnitude and variability is poorly understood. Global-scale estimates of ocean carbon export vary widely, ranging from ∼5 to ∼20 Gt C y – 1 due to uncertainties in methods and unclear definitions. Satellite-derived properties such as phytoplankton biomass, sea surface temperature, and light attenuation at depth provide information about the oceanic ecosystem with unprecedented coverage and resolution in time and space. These products have been the basis of an intense effort over several decades to constrain different biogeochemical production rates and fluxes in the ocean. One critical challenge in this effort has been to estimate the magnitude of the biological pump from satellite-derived properties by establishing how much of the primary production is exported out of the euphotic zone, a flux that is called export production. Here we present a review of existing algorithms for estimating export production from satellite-derived properties, available in-situ datasets that can be used for testing the algorithms, and earlier evaluations of the proposed algorithms. The satellite-derived products used in the algorithm evaluation are all based largely on the Ocean Colour Climate Change Initiative (OC-CCI) products, and carbon products derived from them. The different resources are combined in a meta-analysis.

Abstract We use a nutrient‐ratio budget method to investigate the relative importance of different nutrient source and sink terms at time‐series Station ALOHA and Bermuda Atlantic Time‐series Study (BATS) in the North Pacific and North Atlantic subtropical gyres, respectively. At mean state conditions over annual and multi‐year time scales, vertical phosphate (PO43–) supply from the subsurface accounts for ∼60% of the total phosphorus supply at both sites. Dissolved organic matter transport and zooplankton excretion are more important phosphorous export pathways than sinking particles at Station ALOHA and BATS. The nutrient‐ratio budget approach provides quantitative, observation‐based constraints on nutrient sources and sinks in the surface ocean, which helps improve our understanding of the biological carbon pump in oligotrophic oceans.

Understanding the effect of hydrodynamics on aggregate size and structure is key to predicting mass transport in the aquatic environment. Aggregation theory of particles is well established but our knowledge of deformation processes, biological bonding forces, and their effects on fragmentation of aquatic aggregates is still limited. To better comprehend fragmentation processes and adhesion forces we implemented breakup experiments with diatom and microplastic aggregates made in the laboratory. We captured a substantial number of events showing deformation and subsequent fragmentation of these aggregates in an oscillatory shear flow. Polystyrene and polyethylene aggregates showed distinct fragmentation strengths and provided comparative upper and lower limits to the biological bonding strength of the diatom aggregates. Additionally, we employed a force balance model to evaluate attractive interactions within clusters of particles using the Lagrangian stress history and morphology. We found that the fractal structures of aggregates led to a power law of breakup strength with size and that time-integrated stress governed the overall fragmentation process. We also found that the weakening of the aggregates through deformation with shear exposure enabled their disaggregation at very low shear rates typical of the ocean environment.

Abstract Photosynthesis in the surface ocean converts atmospheric CO2 into organic particles, with the fraction sinking to depth representing a major part of the ocean's biological pump. Although sinking particles are known to be altered by attached‐bacteria during transit, most prior organic geochemical data indicated only minor replacement of plankton‐derived particles by bacterial material. We exploit bacteria‐specific biomarkers (d‐amino acids) in a multi‐year sediment trap in the Pacific Ocean (1,200 m) and suggest a different view. Major d‐amino acids were consistently measured at abundance demonstrating widespread accumulation of bacterial material in sinking particles. Bacterial detritus was estimated to account for up to 19% of particulate organic carbon and up to 36% of particulate nitrogen, much higher than cell count‐based values. The bacterial relative contribution increased with decreasing export production. Our results indicate that bacterial material constitutes an underappreciated component of the biological pump, a role expected to rise as the ocean warms.

Net community production (NCP) is defined as the difference between gross primary production (GPP) and total community respiration (R). NCP indicates the balance between the production and consumption of community organic carbon, therefore making it a key parameter for evaluating the efficiency of carbon sequestration using the biological pump (BP). It is difficult to quantify NCP directly via satellite, because there are complex processes in community production and respiration. We reviewed previous research on satellite-based NCP and classified the methods into two primary categories: empirical methods and semi-analytical methods. The former category was established based on numerical relationships between NCP and satellite-based proxies, while the latter was developed by utilizing mechanistic analysis to establish quantitative expressions linking NCP to such proxies. Although satellite-based calculations of NCP have been attempted, they still suffer from significant uncertainties. Future research should focus on the precise calculation of satellite-based NCP by investigating the underlying processes and mechanisms that regulate NCP, developing regional models, and increasing the resolution of satellite sensors, as well as applying satellite lidar and coordinated multi-sensor observation technology.

Zooplankton are important consumers of sinking particles in the ocean's twilight zone. However, the impact of different taxa depends on their feeding mode. In contrast to typical suspension-feeding zooplankton, flux-feeding taxa preferentially consume rapidly sinking particles that would otherwise penetrate into the deep ocean. To quantify the potential impact of two flux-feeding zooplankton taxa [Aulosphaeridae (Rhizaria), and Limacina helicina (euthecosome pteropod)] and the total suspension-feeding zooplankton community, we measured depth-stratified abundances of these organisms during six cruises in the California Current Ecosystem. Using allometric-scaling relationships, we computed the percentage of carbon flux intercepted by flux feeders and suspension feeders. These estimates were compared to direct measurements of carbon flux attenuation (CFA) made using drifting sediment traps and 238U-234Th disequilibrium. We found that CFA in the shallow twilight zone typically ranged from 500 to 1000 μmol organic C flux remineralized per 10-m vertical depth bin. This equated to approximately 6-10% of carbon flux remineralized/10 m. The two flux-feeding taxa considered in this study could account for a substantial proportion of this flux near the base of the euphotic zone. The mean flux attenuation attributable to Aulosphaeridae was 0.69%/10 m (median = 0.21%/10 m, interquartile range = 0.04-0.81%) at their depth of maximum abundance (~100 m), which would equate to ~10% of total flux attenuation in this depth range. The maximum flux attenuation attributable to Aulosphaeridae reached 4.2%/10 m when these protists were most abundant. L. helicina, meanwhile, could intercept 0.45-1.6% of carbon flux/10 m, which was slightly greater (on average) than the Aulosphaeridae. In contrast, suspension-feeding zooplankton in the mesopelagic (including copepods, euphausiids, appendicularians, and ostracods) had combined clearance rates of 2-81 L m-3 day-1 (mean of 19.6 L m-3 day-1). This implies a substantial impact on slowly sinking particles, but a negligible impact on the presumably rapidly sinking fecal pellets that comprised the majority of the material collected in sediment traps. Our results highlight the need for a greater research focus on the many taxa that potentially act as flux feeders in the oceanic twilight zone.

The cycling of marine particulate matter is critical for sequestering carbon in the deep ocean and in marine sediments. Biogenic minerals such as calcium carbonate (CaCO3) and opal add density to more buoyant organic material, facilitating particle sinking and export. Here, we compile and analyze a global data set of particulate organic carbon (POC), particulate inorganic carbon (PIC, or CaCO3), and biogenic silica (bSi, or opal) concentrations collected using large volume pumps (LVPs). We analyze the distribution of all three biogenic phases in the small (1–53 μm) and large (>53 μm) size classes. Over the entire water column 76% of POC exists in the small size fraction. Similarly, the small size class contains 82% of PIC, indicating the importance of small‐sized coccolithophores to the PIC budget of the ocean. In contrast, 50% of bSi exists in the large size fraction, reflecting the larger size of diatoms and radiolarians compared with coccolithophores. We use PIC:POC and bSi:POC ratios in the upper ocean to document a consistent signal of shallow mineral dissolution, likely linked to biologically mediated processes. Sediment trap PIC:POC and bSi:POC are elevated with respect to LVP samples and increase strongly with depth, indicating the concentration of mineral phases and/or a deficit of POC in large sinking particles. We suggest that future sampling campaigns pair LVPs with sediment traps to capture the full particulate field, especially the large aggregates that contribute to mineral‐rich deep ocean fluxes, and may be missed by LVPs. Plain Language Summary: In the surface ocean, organisms produce minerals along with the organic carbon that fuels most marine life. A portion of this material sinks to the deep ocean, which is driven by the formation of large aggregates and the ballasting effect of dense biominerals. Here we investigate a global compilation of particulate material in the ocean and compare it to a recent compilation of sinking material collected by sediment traps. We find that large, sedimenting particles contain much less organic carbon relative to the amount of biominerals, compared to smaller particles collected on filters. This result has implications for both the processing of organic carbon and biominerals in the ocean, and the delivery of minerals and organic carbon to the deep ocean and to the seafloor. Key Points: Particle data from large‐volume pumps was compiled globallyParticles collected with pumps are enriched in organic carbon compared with sediment trapsRapidly sinking particles appear efficient at delivering minerals to the deep ocean and the seafloor [ABSTRACT FROM AUTHOR]

Ocean alkalinity plays a fundamental role in the apportionment of CO2 between the atmosphere and the ocean. The primary driver of the ocean's vertical alkalinity distribution is the formation of calcium carbonate (CaCO3) by organisms at the ocean surface and its dissolution at depth. This so‐called "CaCO3 counterpump" is poorly constrained, however, both in terms of how much CaCO3 is exported from the surface ocean, and at what depth it dissolves. Here, we created a steady‐state model of global ocean alkalinity using Ocean Circulation Inverse Model transport, biogeochemical cycling, and field‐tested calcite and aragonite dissolution kinetics. We find that limiting CaCO3 dissolution to below the aragonite and calcite saturation horizons cannot explain excess alkalinity in the upper ocean, and that models allowing dissolution above the saturation horizons best match observations. Linking dissolution to organic matter respiration, or imposing a constant dissolution rate both produce good model fits. Our best performing models require export between 1.1 and 1.8 Gt PIC y−1 (from 73 m), but all converge to 1.0 Gt PIC y−1 export at 279 m, indicating that both high‐ and low‐export scenarios can match observations, as long as high export is coupled to high dissolution in the upper ocean. These results demonstrate that dissolution is not a simple function of seawater CaCO3 saturation (Ω) and calcite or aragonite solubility, and that other mechanisms, likely related to the biology and ecology of calcifiers, must drive significant dissolution throughout the water column. Plain Language Summary: The acid neutralizing capacity, or alkalinity, in the ocean surface affects how much CO2 the ocean can absorb from the atmosphere. Alkalinity is lost from the surface ocean when marine organisms make calcium carbonate (CaCO3) shells; in contrast, alkalinity is returned to seawater when shells dissolve. Both processes are not well quantified. Here, we created a model to constrain how much CaCO3 sinks out of the surface ocean, and at what depth it dissolves. We find that CaCO3 must dissolve in the upper ocean where calcite and aragonite—the two most common forms of CaCO3—are thermodynamically oversaturated. We also find that a range of CaCO3 exports—from 1.1 to 1.8 Gt PIC y−1—can match observations, as long as high export is coupled to high dissolution in the upper ocean. These results suggest that CaCO3 dissolution is not only controlled by seawater chemistry, and that other mechanisms, likely related to the biology and ecology of organisms that make CaCO3 shells, are important drivers of CaCO3 dissolution in the ocean. Key Points: Restricting calcium carbonate (CaCO3) dissolution to below saturation horizons cannot reproduce excess alkalinity observations in the upper oceanExports between 1.1 and 1.8 Gt PIC y−1 (from 73 m) can match GLODAP observations, but models converge to 1.0 Gt PIC y−1 export at 279 mIn addition to seawater CaCO3 saturation and mineral solubility, other mechanisms drive CaCO3 dissolution throughout the water column [ABSTRACT FROM AUTHOR]

[Display omitted] The goal of achieving carbon neutrality in the next 30–40 years is approaching worldwide consensus and requires coordinated efforts to combat the increasing threat of climate change. Two main sets of actions have been proposed to address this grand goal. One is to reduce anthropogenic CO 2 emissions to the atmosphere, and the other is to increase carbon sinks or negative emissions, i.e., removing CO 2 from the atmosphere. Here we advocate eco-engineering approaches for ocean negative carbon emission (ONCE), aiming to enhance carbon sinks in the marine environment. An international program is being established to promote coordinated efforts in developing ONCE-relevant strategies and methodologies, taking into consideration ecological/biogeochemical processes and mechanisms related to different forms of carbon (inorganic/organic, biotic/abiotic, particulate/dissolved) for sequestration. We focus on marine ecosystem-based approaches and pay special attention to mechanisms that require transformative research, including those elucidating interactions between the biological pump (BP), the microbial carbon pump (MCP), and microbially induced carbonate precipitation (MICP). Eutrophic estuaries, hypoxic and anoxic waters, coral reef ecosystems, as well as aquaculture areas are particularly considered in the context of efforts to increase their capacity as carbon sinks. ONCE approaches are thus expected to be beneficial for both carbon sequestration and alleviation of environmental stresses. [ABSTRACT FROM AUTHOR]

210Bi (t1/2 = 5.01 d) is theoretically a radionuclide for tracing the particle cycle over a timescale of hours to days. However, it has been rarely investigated in marine environments due to its very short half‐life and low activity. Here, 210Bi and 210Pb were examined in the water column on the shelf/slope of the northern South China Sea (SCS), as well as their atmospheric deposition. In rainwater, the 210Bi/210Pb ratio averaged 0.54 ± 0.28, indicating the influence of atmospheric deposition on the disequilibrium between 210Bi and 210Pb in surface seawater. On the shelf, 210Bi/210Pb averaged 0.73 ± 0.10 in the euphotic zone and 1.25 ± 0.10 below, supporting a quick removal of 210Bi from the euphotic zone and regeneration in the twilight zone. On the slope, deficits in 210Bi (210Bi/210Pb of 0.81 ± 0.07) were also observed in the productive low euphotic zone. The concurrence of 210Bi deficits and higher particulate organic carbon (POC) concentrations implied that POC largely dominates the deficit and excess of 210Bi. Based on a simple model, the removal fluxes of 210Bi at the euphotic base were 728 ± 73 dpm m−2 d−1 and 216 ± 89 dpm m−2 d−1 on the shelf and slope. The residence time of particulate 210Bi was 14 ± 2 d. The 210Bi‐derived export flux of POC was 1.7 ± 0.7 mmol‐C m−2 d−1 out of the euphotic zone over the slope. These results lay the foundation for 210Bi/210Pb to quantify the sinking and remineralization of particulate organic matter in coastal seas. Key Points: For the first time, vertical profiles of 210Bi were examined in natural seawaterDeficit 210Bi in the euphotic zone and excess 210Bi in the upper twilight zone were observed210Bi‐210Pb is a new tracer of particle dynamics on a short timescale in the sea [ABSTRACT FROM AUTHOR]

This contribution provides an introduction to a sequence of five papers (CCE I- CCE V) that describe the impact of the Warm Anomaly of 2014–15 and El Niño 2015–16 on the pelagic food web of the southern California Current Ecosystem. These contributions analyze the influence of these two warm water perturbations on satellite-based measures of ocean fronts, export efficiency out of the euphotic zone, copepod egg production, mesozooplankton community structure, and a synthesis of primary production, mesozooplankton grazing, and gravitational fluxes of organic carbon.

This contribution provides an introduction to a sequence of five papers (CCE I- CCE V) that describe the impact of the Warm Anomaly of 2014–15 and El Niño 2015–16 on the pelagic food web of the southern California Current Ecosystem. These contributions analyze the influence of these two warm water perturbations on satellite-based measures of ocean fronts, export efficiency out of the euphotic zone, copepod egg production, mesozooplankton community structure, and a synthesis of primary production, mesozooplankton grazing, and gravitational fluxes of organic carbon.

Abstract 210Bi (t1/2 = 5.01 d) is theoretically a radionuclide for tracing the particle cycle over a timescale of hours to days. However, it has been rarely investigated in marine environments due to its very short half‐life and low activity. Here, 210Bi and 210Pb were examined in the water column on the shelf/slope of the northern South China Sea (SCS), as well as their atmospheric deposition. In rainwater, the 210Bi/210Pb ratio averaged 0.54 ± 0.28, indicating the influence of atmospheric deposition on the disequilibrium between 210Bi and 210Pb in surface seawater. On the shelf, 210Bi/210Pb averaged 0.73 ± 0.10 in the euphotic zone and 1.25 ± 0.10 below, supporting a quick removal of 210Bi from the euphotic zone and regeneration in the twilight zone. On the slope, deficits in 210Bi (210Bi/210Pb of 0.81 ± 0.07) were also observed in the productive low euphotic zone. The concurrence of 210Bi deficits and higher particulate organic carbon (POC) concentrations implied that POC largely dominates the deficit and excess of 210Bi. Based on a simple model, the removal fluxes of 210Bi at the euphotic base were 728 ± 73 dpm m−2 d−1 and 216 ± 89 dpm m−2 d−1 on the shelf and slope. The residence time of particulate 210Bi was 14 ± 2 d. The 210Bi‐derived export flux of POC was 1.7 ± 0.7 mmol‐C m−2 d−1 out of the euphotic zone over the slope. These results lay the foundation for 210Bi/210Pb to quantify the sinking and remineralization of particulate organic matter in coastal seas.

The ocean holds vast quantities of carbon that it continually exchanges with the atmosphere through the air-sea interface. Because of its enormous size and relatively rapid exchange of carbon with the atmosphere, the ocean controls atmospheric CO2 concentration and thereby Earth's climate on timescales of tens to thousands of years. This review examines the basic functions of the ocean's carbon cycle, demonstrating that the ocean carbon inventory is determined primarily by the mass of the ocean, by the chemical speciation of CO2 in seawater, and by the action of the solubility and biological pumps that draw carbon into the ocean's deeper layers, where it can be sequestered for decades to millennia. The ocean also plays a critical role in moderating the impacts of climate change by absorbing an amount of carbon equivalent to about 25% of anthropogenic CO2 emissions over the past several decades. However, this also leads to ocean acidification and reduces the chemical buffering capacity of the ocean and its future ability to take up CO2. This review closes with a look at the uncertain future of the ocean carbon cycle and the scientific challenges that this uncertainty brings. [ABSTRACT FROM AUTHOR]