The biological pump is a process whereby CO(2) in the upper ocean is fixed by primary producers and transported to the deep ocean as sinking biogenic particles or as dissolved organic matter. The fate of most of this exported material is remineralization to CO(2), which accumulates in deep waters until it is eventually ventilated again at the sea surface. However, a proportion of the fixed carbon is not mineralized but is instead stored for millennia as recalcitrant dissolved organic matter. The processes and mechanisms involved in the generation of this large carbon reservoir are poorly understood. Here, we propose the microbial carbon pump as a conceptual framework to address this important, multifaceted biogeochemical problem.

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Most of the oceanic reservoir of dissolved organic matter (DOM) is of marine origin and is resistant to microbial oxidation, but little is known about the mechanisms of its formation. In a laboratory study, natural assemblages of marine bacteria rapidly (in <48 hours) utilized labile compounds (glucose, glutamate) and produced refractory DOM that persisted for more than a year. Only 10 to 15% of the bacterially derived DOM was identified as hydrolyzable amino acids and sugars, a feature consistent with marine DOM. These results suggest that microbial processes alter the molecular structure of DOM, making it resistant to further degradation and thereby preserving fixed carbon in the ocean.

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[1] Molecular level characterizations of dissolved lignin were conducted in Mississippi River plume waters to study the impact of various removal mechanisms (photooxidation, microbial degradation, and flocculation) on dissolved organic material (DOM) concentrations and compositions. Prior to analysis, dissolved (<0.2-mm pore size) samples were size fractionated by ultrafiltration into high molecular weight (HMW; >1 kDalton) and low molecular weight (LMW; <1 kDalton) components. At salinities <25 psu, flocculation and microbial degradation were the primary factors affecting lignin concentrations. At salinities >25 psu, photooxidation was a dominant factor influencing lignin compositions and concentrations. Diagnostic indicators of photooxidation include a sharp decrease in the percentage of lignin in the HMW size fraction, changes in ratios of syringyl to vanillyl phenols, and increases in LMW acid:aldehyde ratios for both vanillyl and syringyl phenols. A 10-day incubation experiment with plume water indicated rates of microbial degradation of dissolved lignin that were $30% of photooxidation rates in surface waters. These results highlight the importance of microbial as well as photochemical processes in the cycling of terrigenous DOM in coastal waters. Neither flocculation nor microbial degradation significantly altered lignin composition, suggesting that composition is primarily determined by source and photochemical transformation. Overall, high removal rates indicate the potential importance of terrigenous DOM as a carbon and nutrient source in the coastal ocean. Strong correlations between absorption coefficients at 350 nm and dissolved lignin demonstrate the potential for using absorption to trace terrigenous DOM in coastal environments with significant riverine input. Citation: Hernes, P. J., and R. Benner, Photochemical and microbial degradation of dissolved lignin phenols: Implications for the fate of terrigenous dissolved organic matter in marine environments,

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