The impact of biogeochemical processes on the composition of dissolved organic matter in marine subsurface sediments

Organic matter in marine sediments is one of the largest pools of reduced carbon on Earth. It had been known as recalcitrant carbon pool with scarce living biomasses. The findings of microbes at several kilometers below seafloor suggest there is a substantial amount of living biomasses in the subsurface sediments under extreme conditions, for example, decreasing labile biomolecules as carbon sources, electron acceptors, and thermal conditions at deeper layers. Those microbes might drive a slow but extensive carbon flow and play an important role in the carbon cycle in deep Earth. It is not known how the microbes could survive on the recalcitrant organic matter (ROM) leftover . The gap of knowledge is especially in the initial step of degradation process: how the ROM is degraded to assimilable substrates. Dissolved organic matter (DOM) encompasses the analytical window for the initial degradation of ROM. However, the DOM pool is complex and comprises tens of thousands of formulae. Therefore, this study focused on firstly the method of DOM characterization, secondly the DOM cycling and degradation process in subseafloor sediment. In further, this study aims to answer the questions: how the degradation patterns of organic matter change with environmental condition in the subsurface sediments, which are highly stratified by redox conditions and thermal gradients. In summary, this dissertation addressed the interactions between microbes and DOM in stratified anaerobic sediments by combinations of EEMs and FT-ICR MS. It is indicated that the metabolism of microbes is sensitive and adaptive to environmental conditions, which ultimately influence the carbon and nitrogen flow in subsurface sediments due to the change of microbial metabolisms and abiotic processes.