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The transfer of organic carbon from the upper to the deep ocean by particulate export flux is the starting point for the long-term storage of photosynthetically fixed carbon. This ‘biological carbon pump’ is a critical component of the global carbon cycle, reducing atmospheric CO 2 levels by ~200 ppm relative to a world without export flux. This carbon flux also fuels the productivity of the mesopelagic zone, including important fisheries. Here we show that, despite its importance for understanding future ocean carbon cycling, Earth system models disagree on the projected response of the global export flux to climate change, with estimates ranging from −41% to +1.8%. Fundamental constraints to understanding export flux arise because a myriad of interconnected processes make the biological carbon pump challenging to both observe and model. Our synthesis prioritizes the processes likely to be most important to include in modern-day estimates (particle fragmentation and zooplankton vertical migration) and future projections (phytoplankton and particle size spectra and temperature-dependent remineralization) of export. We also identify the observations required to achieve more robust characterization, and hence improved model parameterization, of export flux and thus reduce uncertainties in current and future estimates in the overall cycling of carbon in the ocean. A synthesis of recent work on marine carbon export fluxes finds that many processes that are key to understanding the effects of a warming climate on ocean carbon cycling are missing from current climate models.

The efficiency of the ocean's biological carbon pump (BCPeff – here the product of particle export and transfer efficiencies) plays a key role in the air–sea partitioning of CO2. Despite its importance in the global carbon cycle, the biological processes that control BCPeff are poorly known. We investigate the potential role that zooplankton play in the biological carbon pump using both in situ observations and model output. Observed and modelled estimates of fast, slow, and total sinking fluxes are presented from three oceanic sites: the Atlantic sector of the Southern Ocean, the temperate North Atlantic, and the equatorial Pacific oxygen minimum zone (OMZ). We find that observed particle export efficiency is inversely related to primary production likely due to zooplankton grazing, in direct contrast to the model estimates. The model and observations show strongest agreement in remineralization coefficients and BCPeff at the OMZ site where zooplankton processing of particles in the mesopelagic zone is thought to be low. As the model has limited representation of zooplankton-mediated remineralization processes, we suggest that these results point to the importance of zooplankton in setting BCPeff, including particle grazing and fragmentation, and the effect of diel vertical migration. We suggest that improving parameterizations of zooplankton processes may increase the fidelity of biogeochemical model estimates of the biological carbon pump. Future changes in climate such as the expansion of OMZs may decrease the role of zooplankton in the biological carbon pump globally, hence increasing its efficiency.

The Southern Ocean plays a critical role in regulating global climate as a major sink for atmospheric carbon dioxide (CO2), and in global ocean biogeochemistry by supplying nutrients to the global thermocline, thereby influencing global primary production and carbon export. Biogeochemical processes within the Southern Ocean regulate regional primary production and biological carbon uptake, primarily through iron supply, and support ecosystem functioning over a range of spatial and temporal scales. Here we assimilate existing knowledge and present new data to examine the biogeochemical cycles of iron, carbon and major nutrients, their key drivers and their responses to, and roles in, contemporary climate and environmental change. Projected increases in iron supply, coupled with increases in light availability to phytoplankton through increased near-surface stratification and longer ice-free periods, are very likely to increase primary production and carbon export around Antarctica. Biological carbon uptake is likely to increase for the Southern Ocean as a whole, whilst there is greater uncertainty around projections of primary production in the Sub-Antarctic and basin-wide changes in phytoplankton species composition, as well as their biogeochemical consequences. Phytoplankton, zooplankton, higher trophic level organisms and microbial communities are strongly influenced by Southern Ocean biogeochemistry, in particular through nutrient supply and ocean acidification. In turn, these organisms exert important controls on biogeochemistry through carbon storage and export, nutrient recycling and redistribution, and benthic-pelagic coupling. The key processes described in this paper are summarised in the graphical abstract. Climate-mediated changes in Southern Ocean biogeochemistry over the coming decades are very likely to impact primary production, sea-air CO2 exchange and ecosystem functioning within and beyond this vast and critically important ocean region.

Climate change is causing persistent, widespread, and significant impacts on marine ecosystems which are predicted to interact and intensify. Overfishing and associated habitat degradation have put many fish populations and marine ecosystems at risk and is making the ocean more vulnerable to climate change and less capable of buffering against its effects. In this Perspective, we review how overfishing is disrupting the important role of marine vertebrates in the ocean carbon cycle, causing disturbance and damage to the carbon-rich seabed, and contributing to rising greenhouse gas emissions through fuel use. We discuss how implementing good fisheries management can reduce or remove many of the impacts associated with overfishing, including fish stock collapse, destruction of seabed habitats, provision of harmful subsidies and accompanying socio-economic impacts. Managing overfishing is one of the most effective strategies in protecting ocean carbon stores and can make an important contribution to climate mitigation and adaptation.

Detritivores need to upgrade their food to increase its nutritional value. One method is to fragment detritus promoting the colonization of nutrient‐rich microbes, which consumers then ingest along with the detritus; so‐called microbial gardening. Observations and numerical models of the detritus‐dominated ocean mesopelagic zone have suggested microbial gardening by zooplankton is a fundamental process in the ocean carbon cycle leading to increased respiration of carbon‐rich detritus. However, no experimental evidence exists to demonstrate that microbial respiration rates are higher on recently fragmented sinking detrital particles. Using aquaria‐reared Antarctic krill fecal pellets, we showed fragmentation increased microbial particulate organic carbon (POC) turnover by 1.9×, but only on brown fecal pellets, formed from the consumption of other pellets. Microbial POC turnover on un‐ and fragmented green fecal pellets, formed from consuming fresh phytoplankton, was equal. Thus, POC content, fragmentation, and potentially nutritional value together drive POC turnover rates. Mesopelagic microbial gardening could be a risky strategy, as the dominant detrital food source is settling particles; even though fragmentation decreases particle size and sinking rate, it is unlikely that an organism would remain with the particle long enough to nutritionally benefit from attached microbes. We propose “communal gardening” occurs whereby additional mesopelagic organisms nearby or below the site of fragmentation consume the particle and the colonized microbes. To determine how fragmentation impacts the remineralization of sinking carbon‐rich detritus and to parameterize microbial gardening in mesopelagic carbon models, three key metrics from further controlled experiments and observations are needed; how particle composition (here, pellet color/krill diet) impacts the response of microbes to the fragmentation of particles; the nutritional benefit to zooplankton from ingesting microbes after fragmentation along with identification of which essential nutrients are being targeted; how both these factors vary between physical (shear) and biological particle fragmentation. We present results from the first experiment on ocean mesopelagic zone microbial gardening. Marine detritivores would benefit from fragmenting only the most detrital particles as only these resulted in increases in microbial activity (and thus essential nutrients for detritivores). As microbial gardening is an important carbon feedback in the ocean, carbon cycling models need to parameterize microbial activity using not just the size of the particle, but also nutrient content, to simulate this process.

The biological carbon pump is crucial for the export of particulate organic matter in the ocean. Recent studies on marine microbes have shown the profound influence of bacteria and archaea as regulators of particulate organic matter export. The biological carbon pump (BCP) in the Southern Ocean is driven by phytoplankton productivity and is a significant organic matter sink. However, the role of particle-attached (PA) and free-living (FL) prokaryotes (bacteria and archaea) and their diversity in influencing the efficiency of the BCP is still unclear. To investigate this, we analyzed the metagenomes linked to suspended and sinking marine particles from the Sub-Antarctic Southern Ocean Time Series (SOTS) by deploying a Marine Snow Catcher (MSC), obtaining suspended and sinking particulate material, determining organic carbon and nitrogen flux, and constructing metagenome-assembled genomes (MAGs). The suspended and sinking particle-pools were dominated by bacteria with the potential to degrade organic carbon. Bacterial communities associated with the sinking fraction had more genes related to the degradation of complex organic carbon than those in the suspended fraction. Archaea had the potential to drive nitrogen metabolism via nitrite and ammonia oxidation, altering organic nitrogen concentration. The data revealed several pathways for chemoautotrophy and the secretion of recalcitrant dissolved organic carbon (RDOC) from CO 2 , with bacteria and archaea potentially sequestering particulate organic matter (POM) via the production of RDOC. These findings provide insights into the diversity and function of prokaryotes in suspended and sinking particles and their role in organic carbon/nitrogen export in the Southern Ocean. IMPORTANCE The biological carbon pump is crucial for the export of particulate organic matter in the ocean. Recent studies on marine microbes have shown the profound influence of bacteria and archaea as regulators of particulate organic matter export. Yet, despite the importance of the Southern Ocean as a carbon sink, we lack comparable insights regarding microbial contributions. This study provides the first insights regarding prokaryotic contributions to particulate organic matter export in the Southern Ocean. We reveal evidence that prokaryotic communities in suspended and sinking particle fractions harbor widespread genomic potential for mediating particulate organic matter export. The results substantially enhance our understanding of the role played by microorganisms in regulating particulate organic matter export in suspended and sinking marine fractions in the Southern Ocean.

Marine ecosystems regulate atmospheric carbon dioxide levels by transporting and storing photosynthetically fixed carbon in the ocean’s interior. In particular, the subantarctic and polar frontal zone of the Southern Ocean is a significant region for physically-driven carbon uptake due to mode water formation, although it is under-studied concerning biologically-mediated uptake. Regional differences in iron concentrations lead to variable carbon export from the base of the euphotic zone. Contrary to our understanding of export globally, where high productivity results in high export, naturally iron-fertilized regions exhibit low carbon export relative to their surface productivity, while HNLC (High Nutrient, Low Chlorophyll) waters emerge as a significant area for carbon export. Zooplankton, an integral part of the oceanic food web, play an important role in establishing these main carbon export regimes. In this mini review, we explore this role further by focusing on the impact of grazing and the production of fecal pellets on the carbon flux. The data coverage in the subantarctic region will be assessed by comparing two case studies - the iron-replete Kerguelen Plateau and the HNLC region south of Australia. We then discuss challenges in evaluating the contributions of zooplankton to carbon flux, namely gaps in seasonal coverage of sampling campaigns, the use of non-standardized and biased methods and under-sampling of the mesopelagic zone, an important area of carbon remineralization. More integrated approaches are necessary to improve present estimates of zooplankton-mediated carbon export in the Southern Ocean.

Ecological feedbacks are fundamental features of the Earth system, affecting physical processes and chemical cycles. Our understanding of the interactions underlying these feedbacks at different spatial and temporal scales and the extent to which feedbacks affect Earth system functioning remains limited. Climate change and other anthropogenic pressures are already negatively affecting ecological processes in marine, freshwater, and terrestrial ecosystems. These will most likely be amplified in the coming decades under our current warming and socioeconomic pathways. The knock‐on impacts on ecological feedbacks have the potential to cause rapid perturbations to the Earth system, and may significantly impact the structure and functioning of ecosystems. Yet, the role of our planet's diverse ecological feedbacks in Earth system processes and the impacts of perturbations are major knowledge gaps. Here we review and synthesize current understanding of ecological feedbacks and how they affect physical and chemical processes. We then consider the implications of ecological feedbacks for analyses of anthropogenically‐driven change, development of scientific understanding and models, and provision of scientific advice for policymakers. Finally, we identify three priority future research areas for the rapid assessment and integration of ecological feedbacks in Earth system science: (a) including ecological feedbacks in assessments of global change and Earth system models, (b) incorporating ecological feedbacks across scales, and (c) producing projections suitable for policy advice. Overall, this review presents an urgent call to the scientific community for the rapid development of understanding of ecological feedbacks and integrated ecosystem—Earth system research. Organisms in the ocean, lakes, rivers, and on land, interact together, affecting each other and modifying their physical and chemical environments. These ecological interactions form feedback loops, where a change in one part of an ecosystem has knock‐on effects that lead to further changes that enhance or reduce the change. Whilst we know these ecological feedbacks are important, we have limited understanding of how they work within and between ecosystems and over larger scales across the world. Ecological feedbacks are also likely to be disrupted due to ongoing climate change and destructive human activities (e.g., deforestation, pollution). Major gaps in our understanding of ecological feedbacks make it difficult to predict how ecosystems are affected by change and the larger scale impacts. Here, we explore what is currently known about ecological feedbacks, how they affect the physical environment and chemical processes, how they may be affected by human‐driven environmental changes, and what this means for advising policy makers. We also highlight three priority areas of future research, and the need for rapid development of understanding of ecological feedbacks and their role in globally important processes. The role of our planet's diverse ecological feedbacks in Earth system processes is a major knowledge gap We review current knowledge on ecological feedbacks within ecosystems, and between ecological, physical, and biogeochemical processes Research priorities involve integrating ecological feedbacks in models, mapping feedbacks across scales, and refining projections for policy

Marine ecosystems and their associated biodiversity sustain life on Earth and hold intrinsic value. Critical marine ecosystem services include maintenance of global oxygen and carbon cycles, production of food and energy, and sustenance of human wellbeing. However marine ecosystems are swiftly being degraded due to the unsustainable use of marine environments and a rapidly changing climate. The fundamental challenge for the future is therefore to safeguard marine ecosystem biodiversity, function, and adaptive capacity whilst continuing to provide vital resources for the global population. Here, we use foresighting/hindcasting to consider two plausible futures towards 2030: a business-as-usual trajectory (i.e. continuation of current trends), and a more sustainable but technically achievable future in line with the UN Sustainable Development Goals. We identify key drivers that differentiate these alternative futures and use these to develop an action pathway towards the desirable, more sustainable future. Key to achieving the more sustainable future will be establishing integrative (i.e. across jurisdictions and sectors), adaptive management that supports equitable and sustainable stewardship of marine environments. Conserving marine ecosystems will require recalibrating our social, financial, and industrial relationships with the marine environment. While a sustainable future requires long-term planning and commitment beyond 2030, immediate action is needed to avoid tipping points and avert trajectories of ecosystem decline. By acting now to optimise management and protection of marine ecosystems, building upon existing technologies, and conserving the remaining biodiversity, we can create the best opportunity for a sustainable future in 2030 and beyond.