Interactions between microbes and dissolved organic matter in marine subsurface sediment
Sedimentary organic matter is the major reservoir of organic carbon in the global carbon cycle and plays an important role in geobiological element cycles. The microbial ecosystems in marine sediments may comprise substantial fractions of Earth’s living, which is also known as deep biosphere. Through the alteration of sedimentary dissolved organic matter, release of CH4, CO2 and formation of refractory dissolved organic carbon, the deep biosphere might significantly impact atmospheric CO2 and O2 levels and the global carbon cycle on long time scales. Two typical zones in subsurface sediment are defined as sulfate-reducing zone and methanogenic zone according to the redox condition. More specifically, the sulfate-reducer and methanogen are typical ‘workers’ in the two zones modulated by redox condition. They are assumed to get energy in a different way from the aerobic microbes in the upper layers. This project aims at finding the typical mechanism of anaerobic degradation in different redox condition by characterization of dissolved organic matter, in further, figuring out the carbon metabolism of microbes (sulfate-reducer and methanogen) under the special condition in deep biosphere where only complex recalcitrant organic residue and limited electron reducer are provided. To open the door of DOM biogeochemistry and identify the complex organic residue, this study uses a new ‘key’: ultrahigh resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS). This technique resolves thousands of individual molecular species in complex organic mixtures based on precise mass determinations and enables the characterization of the molecular composition of hitherto “uncharacterizable” dissolved organic matter. Other methods, eg. Two-dimensional Nuclear Magnetic Resonance (NMR) spectroscopy and 3-Dimension Fluorescence Spectroscopy (EEMs), low molecular organic compounds, e.g., volatile alcohols, amines, are also used to monitoring changes in organic matter. The expected result would elucidate the changes in molecular composition, metabolic pathways, and carbon metabolism of microbes under sulfate-reducing zone and methanogenic zone.