The biological carbon pump is the process by which CO 2 is transformed to organic carbon via photosynthesis, exported through sinking particles, and finally sequestered in the deep ocean. While the intensity of the pump correlates with plankton community composition, the underlying ecosystem structure driving the process remains largely uncharacterised. Here we use environmental and metagenomic data gathered during the Tara Oceans expedition to improve our understanding of carbon export in the oligotrophic ocean. We show that specific plankton communities, from the surface and deep chlorophyll maximum, correlate with carbon export at 150 m and highlight unexpected taxa such as Radiolaria, alveolate parasites, as well as Synechococcus and their phages, as lineages most strongly associated with carbon export in the subtropical, nutrient-depleted, oligotrophic ocean. Additionally, we show that the relative abundance of just a few bacterial and viral genes can predict most of the variability in carbon export in these regions. Guidi et al. Page 2 Nature. Author manuscript; available in PMC 2016 September 22. Europe PMC Funders Author Manuscripts Europe PMC Funders Author ManuscriptsMarine planktonic photosynthetic organisms are responsible for approximately fifty percent of Earth's primary production and fuel the global ocean biological carbon pump 1 . The intensity of the pump is correlated to plankton community composition 2,3 , and controlled by the relative rates of primary production and carbon remineralisation 4 . About 10% of this newly produced organic carbon in the surface ocean is exported through gravitational sinking of particles. Finally, after multiple transformations, a fraction of the exported material reaches the deep ocean where it is sequestered over thousand-year timescales 5 .Like most biological systems, marine ecosystems in the sunlit upper layer of the ocean (denoted the euphotic zone) are complex 6,7 , characterised by a wide range of biotic and abiotic interactions [8][9][10] and in constant balance between carbon production, transfer to higher trophic levels, remineralisation, and export to the deep layers 11 . The marine ecosystem structure and its taxonomic and functional composition likely evolved to comply with this loss of energy by modifying organism turnover times and by the establishment of complex feedbacks between them 6 and the substrates they can exploit for metabolism 12 .Decades of groundbreaking research have focused on identifying independently the key players involved in the biological carbon pump. Among autotrophs, diatoms are commonly attributed to being important in carbon flux because of their large size and fast sinking rates 13-15 while small autotrophic picoplankton may contribute directly through subduction of surface water 16 or indirectly by aggregating with larger settling particles or consumption by organisms at higher trophic levels 17 . Among heterotrophs, zooplankton such as crustaceans impact carbon flux via production of fast-sinking fecal pellets...

SummaryThe ocean is home to myriad small planktonic organisms that underpin the functioning of marine ecosystems. However, their spatial patterns of diversity and the underlying drivers remain poorly known, precluding projections of their responses to global changes. Here we investigate the latitudinal gradients and global predictors of plankton diversity across archaea, bacteria, eukaryotes, and major virus clades using both molecular and imaging data from Tara Oceans. We show a decline of diversity for most planktonic groups toward the poles, mainly driven by decreasing ocean temperatures. Projections into the future suggest that severe warming of the surface ocean by the end of the 21st century could lead to tropicalization of the diversity of most planktonic groups in temperate and polar regions. These changes may have multiple consequences for marine ecosystem functioning and services and are expected to be particularly significant in key areas for carbon sequestration, fisheries, and marine conservation.Video Abstract

The structure, robustness, and dynamics of ocean plankton ecosystems remain poorly understood due to sampling, analysis, and computational limitations. The Tara Oceans consortium organizes expeditions to help fill this gap at the global level.

Carbon uptake by marine phytoplankton, and its export as organic matter to the ocean interior (i.e., the ''biological pump''), lowers the partial pressure of carbon dioxide (pCO 2) in the upper ocean and facilitates the diffusive drawdown of atmospheric CO 2. Conversely, precipitation of calcium carbonate by marine planktonic calcifiers such as coccolithophorids increases pCO 2 and promotes its outgassing (i.e., the ''alkalinity pump''). Over the past Ϸ100 million years, these two carbon fluxes have been modulated by the relative abundance of diatoms and coccolithophores, resulting in biological feedback on atmospheric CO 2 and Earth's climate; yet, the processes determining the relative distribution of these two phytoplankton taxa remain poorly understood. We analyzed phytoplankton community composition in the Atlantic Ocean and show that the distribution of diatoms and coccolithophorids is correlated with the nutricline depth, a proxy of nutrient supply to the upper mixed layer of the ocean. Using this analysis in conjunction with a coupled atmosphere-ocean intermediate complexity model, we predict a dramatic reduction in the nutrient supply to the euphotic layer in the coming century as a result of increased thermal stratification. Our findings indicate that, by altering phytoplankton community composition, this causal relationship may lead to a decreased efficiency of the biological pump in sequestering atmospheric CO 2, implying a positive feedback in the climate system. These results provide a mechanistic basis for understanding the connection between upper ocean dynamics, the calcium carbonate-to-organic C production ratio and atmospheric pCO 2 variations on time scales ranging from seasonal cycles to geological transitions.coccolithophorids ͉ diatoms ͉ stratification ͉ climate change

[1] We use an inverse method to estimate the global-scale pattern of the air-sea flux of natural CO 2 , i.e., the component of the CO 2 flux due to the natural carbon cycle that already existed in preindustrial times, on the basis of ocean interior observations of dissolved inorganic carbon (DIC) and other tracers, from which we estimate DC gasex , i.e., the component of the observed DIC that is due to the gas exchange of natural CO 2 . We employ a suite of 10 different Ocean General Circulation Models (OGCMs) to quantify the error arising from uncertainties in the modeled transport required to link the interior ocean observations to the surface fluxes. The results from the contributing OGCMs are weighted using a model skill score based on a comparison of each model's simulated natural radiocarbon with observations. We find a pattern of air-sea flux of natural CO 2 characterized by outgassing in the Southern Ocean between 44°S and 59°S, vigorous uptake at midlatitudes of both hemispheres, and strong outgassing in the tropics. In the Northern Hemisphere and the tropics, the inverse estimates generally agree closely with the natural CO 2 flux results from forward simulations of coupled OGCM-biogeochemistry models undertaken as part of the second phase of the Ocean Carbon Model Intercomparison Project (OCMIP-2). The OCMIP-2 simulations find far less air-sea exchange than the inversion south of 20°S, but more recent forward OGCM studies are in better agreement with the inverse estimates in the Southern Hemisphere. The strong source and sink pattern south of 20°S was not apparent in an earlier inversion study, because the choice of region boundaries led to a partial cancellation of the sources and sinks. We show that the inversely estimated flux pattern is clearly traceable to gradients in the observed DC gasex , and that it is relatively insensitive to the choice of OGCM or potential biases in DC gasex . Our inverse estimates imply a southward interhemispheric transport of 0.31 ± 0.02 Pg C yr À1