Elevated pCO2 Decreases the Role of DOC in Vertical Export

Seawater culture experiments were designed to assess the direct effect of pCO₂ on bacterioplankton use of dissolved organic carbon (DOC) with the ultimate goal of understanding the flow of carbon through microbial communities under variable pCO₂ conditions. Treatments included elevated (≥ 1000 ppm), ambient (400 ppm), and pre-industrial (250 ppm) pCO₂. Experiments were conducted in the Santa Barbara Channel where surface waters are naturally exposed to variable pCO₂ fluctuations via frequent upwelling events. Results demonstrate a clear response in which elevated pCO₂ leads to greater consumption of myriad DOC (including model compounds, phytoplankton-derived DOC, and naturally occurring DOC) by natural bacterioplankton communities. Elevated pCO₂ thus directly increased the consumption of labile and naturally occurring DOC by bacterioplankton in the Santa Barbara Channel. These results suggest that short-term perturbations of elevated pCO­₂ may increase the bioavailability of DOC and ultimately, decrease the role of DOC in vertical export. Direct measurements of bacterial biomass carbon will enable a discussion of the fate of the consumed DOC, whether this material is retained within bacterial biomass or quickly lost via respiration to CO₂. These results have implications regarding DOC processing, accumulation and export potential.