Molecular-isotopic studies of carbon turnover and microbial processes in an active subduction zone (Nan TroSEIZE)
Final Report Abstract
This project investigated carbon turnover and related microbial processes in up to 867 m deep subsurface sediments of the Nankai subduction zone off Japan, using data and samples that resulted from IODP Expeditions 314-316 and 322 of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). Our research was driven by the question, how fluid flow and elevated temperatures in the active plate boundary system influence the carbon and energy supply to the deep biosphere. Previously, an increase of organic substrates with depth and temperature had been observed at other sites in the Nankai Trough (ODP Leg 201). With our study, we aimed to understand whether elevated in situ temperatures either stimulate the degradation of recalcitrant organic matter and microbial activity, or go along with a decline in substrate utilization. Our research approach employed qualitative, quantitative and stable carbon isotopic investigations of intact membrane lipids (IPLs) and lowmolecular weight metabolites such as methane and acetate, as well as microbial cell counts and other data that were provided by our collaborators. While cell counts and IPLs provide information on the size and composition of live microbial communities in the deeply buried sediments, the presence and isotopic composition of water-soluble metabolites in pore-waters reflect metabolic pathways that are active in situ. Moreover the composition of bulk dissolved organic matter (DOM) was characterized by 3D-fluorescence spectroscopy coupled with parallel factor analysis. This method provides information on the presence of various protein-like and humic-like DOM components in pore-waters, which helps to characterize DOM with respect to the presence of bioavailable compounds and the contribution of marine and terrestrial sources. In addition, stable isotope probing experiments were employed to track the turnover of 13C-labelled substrates in selected sediment samples. We found that temperature has a strong effect on the size and composition of the microbial communities in the subseafloor. Cell concentrations dropped sharply and the lipid type in cell membranes changed markedly at sediment depths, where in situ temperatures exceeded 40°C due to geothermal heating. Concurrently, potential microbial substrates such as acetate accumulated in the pore-waters and the protein-like fraction of the DOM pool increased, while methane concentrations rose. SIP experiments support the conclusion that substrate utilization is related to temperature. During incubation of sediments at in situ temperatures and redox conditions, complete turnover of 13C-labelled glucose was only observed under sulfate reducing conditions at ~37°C but not under methanogenic conditions at higher temperatures. In incubation experiments, methanogenic activity was not observed. Nevertheless, the natural isotopic composition of pore-water methane indicates the importance of biological methanogenesis as source of subseafloor methane, in particular at Site C0011, but is also consistent with the mixing of biogenic and thermogenic hydrocarbon gases from deeper sources. The apparent reduction in microbial activity, community size and composition at temperatures >~40°C may be related to energy limitation caused by the increasing demand of maintenance energy with increasing temperature. At Site C0012, the potential energy gain of microbial communities increases at depth, where sulfate is diffusing from basement fluids Into the overlying methanogenic sediment, and a stimulation of microbial activity is observed in pore-water constituents and elevated IPL concentrations though temperatures are ~60°C. Our findings suggest that the elevated temperatures and hydrogeological complexity of the accretionary complex impact the turnover of carbon in the incoming sediments of the Shikoku Basin. The up-dip migration of hot fluids seems to influence the size and composition of microbial communities as well as the mineralization of organic matter via the heating of sediments. Moreover, the migration of fluids through the basement supports the energy gain of microbial communities in the overlying sediments through the supply of electron acceptors.
