The relative significance of microbial and photochemical oxidation for the fate of terrigenous dissolved organic matter in the ocean

Terrigenous organic matter represents a huge source of reduced carbon to the marine environment. The annual discharge of organic carbon by rivers that serve as critical links in accumulating and transporting terrigenous organic matter is sufficient to replace the entire oceanic reservoir of dissolved organic carbon (DOC) on a time scale of ~3000 y. Despite this large input, DOC in the deep ocean has an apparent age of > 6000 y, and isotopic and biomarker measurements indicate little terrigenous organic matter in seawater. The question on the fate of terrigenous organic matter in the ocean has therefore been a major issue in biogeochemistry during the past decade. Riverine DOC reveals conservative mixing behaviour in estuaries. Thus, other than physical processes, such as flocculation, precipitation and particulate adsorption must be responsible for the intense and rapid removal of terrigenous DOC from seawater. The main objective of the present research is to determine the relative significance of microbial and photochemical processes in the remineralization of terrigenous, dissolved organic mattter (DOM). The proposed research aims to describe the initial signature of terrigenous and marine DOM using specific chemical and optical properties of the bulk DOM and of different molecular weight DOM fractions. A second research task will be to follow irradiation- induced changes of these chemical and optical signatures of terrigenous DOM and compare these to marine DOM. Furthermore, the relative significance of direct and indirect photo-oxidation as compared to microbial oxidation as removal mechanisms of terrigenous DOM will be evaluated by establishing bio-assay experiments. Bacterioplankton growth on terrigenous vs. marine DOM that has been exposed to surface solar radiation and kept in the dark, respectively, will be followed in order to determine the impact of photochemical transformations on the biological availability of DOM. Contrasting river systems will be sampled in South Carolina in order to cover a range of terrigenous DOM pools characterized by varying concentrations and compositions. In contrast to riverine DOM, DOM originating from different marine phytoplankton cultures will be considered as marine DOM.