Predicting the ocean’s role in the global carbon cycle requires an understanding of the stoichiometric coupling between carbon and growth-limiting elements in biogeochemical processes. A recent addition to such knowledge is that the carbon/nitrogen ratio of inorganic consumption and release of dissolved organic matter may increase in a high-CO2 world1. This will, however, yield a negative feedback on atmospheric CO2 only if the extra organic material escapes mineralization within the photic zone. Here we show, in the context of an Arctic pelagic ecosystem, how the fate and effects of added degradable organic carbon depend critically on the state of the microbial food web. When bacterial growth rate was limited by mineral nutrients, extra organic carbon accumulated in the system. When bacteria were limited by organic carbon, however, addition of labile dissolved organic carbon reduced phytoplankton biomass and activity and also the rate at which total organic carbon accumulated, explained as the result of stimulated bacterial competition for mineral nutrients. This counterintuitive ‘more organic carbon gives less organic carbon’ effect was particularly pronounced in diatom-dominated systems where the carbon/mineral nutrient ratio in phytoplankton production was high. Our results highlight how descriptions of present and future states of the oceanic carbon cycle require detailed understanding of the stoichiometric coupling between carbon and growth-limiting mineral nutrients in both autotrophic and heterotrophic processes.
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This work was financed by the Research Council of Norway through the International Polar Year project 175939/S30 ‘PAME-Nor’ (IPY activity ID no. 71), with additional support from the strategic institution project 158936/I10 ‘Patterns in microbial diversity’, Bjerknes Centre of Climate Research Centre of Excellence Project 146003/V30, project 178441/S40 ‘Interact’ and project 184860/S30 ‘MERCLIM’. Support was received also from Norsk Hydro Produksjon AS project number 5404889, and from the Svalbard Science Forum as ‘Arktisstipend’. We thank Kings Bay A/S and the staff at Ny Ålesund for help with logistics.
The file contains Supplementary Data, Supplementary Methods and Supplementary Discussion, Supplementary Figures S1-S7, Supplementary Table 1 and additional references. (PDF 1758 kb)
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Thingstad, T., Bellerby, R., Bratbak, G. et al. Counterintuitive carbon-to-nutrient coupling in an Arctic pelagic ecosystem. Nature 455, 387–390 (2008). https://doi.org/10.1038/nature07235
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