Impact of Photochemistry on Carbon Cycling in the Sea

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Physical and biological processes are historically considered to be the most important factors affecting the carbon cycle in the ocean. Specific processes include air-sea CO2 exchange, surface mixing, venting of deep waters, carbon fixation, respiration, calcium carbonate formation and sedimentation. Recent studies now suggest that light-initiated (photochemical) processes also strongly impact carbon cycling at the sea surface, particularly with respect to the photodegradation of dissolved organic carbon (DOC) and the production of bacterial substrates. The evidence for the importance of photochemical oxidation of aged, biologically refractory DOC includes: 1) irradiation of sterile-filtered seawater results in high production rates of COz, CO and biologically utilizable substrates, 2) low productivity central gyres appear to be net heterotrophic (i.e., bacterial C utilization rates often exceed primary productivity rates), 3) bacterial C utilization rates are estimated to be about the same as the production rate of biologically labile substrates from DOM photodegradation in open oceanic surface waters, and 4) a significant fraction of the carbon assimilated by open ocean bacteria in surfaces waters is isotonically old. Despite this strong evidence the quantitative importance of photochemistry in the oceanic carbon cycle is still unknown. Studies will focus on: (1) the photochemical production of CO2 and CO from marine DOM, (2) the photochemical loss of DOC, and (3) the microbial uptake and remineralization of photooxidized DOM. Results will assess the impact of photochemistry on carbon fluxes and organic carbon mass balances in the water column, including primary and bacterial productivities, microbial respiration and, at the BATS station, particulate carbon fluxes. We expect Bacterial utilization of photooxidized, aged DOM may largely explain the apparent discrepancy between high bacterial C utilization rates and low primary productivity rates that have been frequently observed for low productivity waters. Another important goal of this study is to develop algorithms, based on action spectra, DOM absorbance and surface irradiance, to predict photochemical rates in surface waters. These algorithms in combination with remotely sensed DOM fluorescence to Predict CO2 and CO photochemical production and DOC photodegradation rates over large areas of the ocean. Previous successes in studying photochemical processes in a variety of marine environments gives a strong foundation for assessing the importance of these processes in the oceanic carbon cycle.