Elevated pCO2 enhances bacterioplankton removal of organic carbon

Abstract

Factors that affect the removal of organic carbon by heterotrophic bacterioplankton can impact the rate and magnitude of organic carbon loss in the ocean through the conversion of a portion of consumed organic carbon to CO2. Through enhanced rates of consumption, surface bacterioplankton communities can also reduce the amount of dissolved organic carbon (DOC) available for export from the surface ocean. The present study investigated the direct effects of elevated pCO2 on bacterioplankton removal of several forms of DOC ranging from glucose to complex phytoplankton exudate and lysate, and naturally occurring DOC. Elevated pCO2 (1000–1500 ppm) enhanced both the rate and magnitude of organic carbon removal by bacterioplankton communities compared to low (pre-industrial and ambient) pCO2 (250 –~400 ppm). The increased removal was largely due to enhanced respiration, rather than enhanced production of bacterioplankton biomass. The results suggest that elevated pCO2 can increase DOC consumption and decrease bacterioplankton growth efficiency, ultimately decreasing the amount of DOC available for vertical export and increasing the production of CO2 in the surface ocean.

Elevated pCO2 alters marine heterotrophic bacterial community composition and metabolic potential in response to a pulse of phytoplankton organic matter

“…() showed that elevated p CO 2 stimulated transcript abundance of respiratory proton pumps that aid in translocating protons across the cell membrane, suggesting that bacteria upregulate respiratory proton pumps to export protons that invade the cell as a result of high external p CO 2 . Consistent with our results from seawater culture experiments (James et al, ), Bunse et al . () suggest that upregulating proton pumps may decrease bacterial growth efficiency.…”

“…In the presence of phytoplankton communities, Bunse et al (2016) showed that elevated pCO 2 stimulated transcript abundance of respiratory proton pumps that aid in translocating protons across the cell membrane, suggesting that bacteria upregulate respiratory proton pumps to export protons that invade the cell as a result of high external pCO 2 . Consistent with our results from seawater culture experiments (James et al, 2017), Bunse et al (2016) suggest that upregulating proton pumps may decrease bacterial growth efficiency. Although this study offers insight to the physiological and phylogenetic responses of bacteria to changes in pCO 2 , predicting how elevated pCO 2 will affect bacteria-mediated carbon cycling requires measurements of organic carbon removal and bacterial growth dynamics.…”

“…Grey arrows (A) indicate time points when metagenomic DNA samples were obtained. Data modified from James et al, .…”

“…levels. We were unable to measure p CO 2 at sea but applied the same flow rate and bubbling time that resulted in effective p CO 2 adjustment previously (James et al, ). Using ship‐board measurements of total alkalinity (∼2340 μmol kg −1 ) and assuming successful alteration of p CO 2 to 1000 p.p.m., we were able to estimate pH in the elevated p CO 2 incubations ‐ this calculation indicates that at most, external pH values may have dropped from ∼8.1 to ∼7.7.…”

“…1000 p.p.m.) in short‐term experiments enhanced bacterial removal of organic carbon and increased bacterial respiration from the start of the experiments to stationary growth phase (James et al, ). This result was observed for natural bacterial communities growing on a range of organic carbon compounds, from glucose to more complex phytoplankton products and naturally occurring DOC.…”