Elevated pCO2 Increases Respiration of DOC by Natural Bacterioplankton

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The size of the oceanic dissolved organic carbon (DOC) and atmospheric carbon dioxide (CO₂) reservoirs are comparable at ~700 PgC (Hansell and Carlson 2015) such that small changes in DOC removal or production have significant implications with regards to CO₂ exchange between the ocean and atmosphere. Heterotrophic bacterioplankton are the dominant consumers of DOC in the ocean. DOC that escapes or resists consumption by bacterioplankton can accumulate in the surface ocean and be transported to depth by physical processes (vertical export), where it is removed from exchange with the atmosphere on a variety of timescales. Therefore, factors that affect the ability of heterotrophic bacterioplankton to consume DOC (i.e. DOC bioavailability) can impact the rate of DOC accumulation and the role of DOC in vertical export. Once DOC is consumed, the growth efficiency of heterotrophic bacterioplankton communities dictates how much organic carbon is retained within marine food webs versus how much is respired to CO₂. The present study investigated the direct effects of elevated pCO₂ on the consumption of DOC and the efficiency of its use by marine bacterioplankton. Elevated pCO₂ enhanced both the rate and magnitude of DOC consumption by bacterioplankton and decreased bacterial growth efficiencies relative to responses under low and ambient pCO₂ conditions. These results suggest that short-term perturbations of elevated pCO₂ may increase both DOC bioavailability and bacterial respiration, ultimately decreasing the effectiveness of DOC as a sink of carbon in the ocean and enhancing the respiratory loss of DOC to CO₂.