BGC-03 Biogeochemistry of DOM
Microbially mediated DOM distribution in cold seep sediments of the South China Sea
Xiaoxia Chen* , State Key Laboratory of Marine Environmental Science and College of Ocean and Earth Sciences, Xiamen University, Xiamen, China
Ruanhong Cai, State Key Laboratory of Marine Environmental Science and College of Ocean and Earth Sciences, Xiamen University, Xiamen, China
Xiaocun Cen, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, China
Chen He, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, China
Quan Shi, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, China
Nianzhi Jiao, State Key Laboratory of Marine Environmental Science and College of Ocean and Earth Sciences, Xiamen University, Xiamen, China

Dissolved organic matter (DOM) in marine sediments provides carbon substrates and energy sources for microorganisms that drive benthic biogeochemical processes and in turn microbes can modify and influence the quantity and quality of DOM. The interactions between microbes and DOM in the sediments are complex and poorly understood. Here we determined the molecular information of water-soluble DOM collected from two sediment cores of the South China Sea using ultra-high-resolution mass spectrometry (FT-ICR MS) and high-throughput sequencing, accompanied with bulk geochemical analyses to investigate the vertical profile of DOM composition in surface sediments and the link between DOM and microbial composition in cold seep sediments, as well as the preservation of sediment-derived RDOM in overlying and bottom waters. Results revealed that sediment-derived DOM and microbial communities are clearly stratified by depth. The bacterial communities in the surface sediment mainly Alphaproteobacteria, and Gammaproteobacteria, while the deeper sediment mainly Anaerolineae and Dehalococcoidia. Nitrososphaeria was predominant archaeal community in the surface sediments, while Bathyarchaeia prevailed in the deeper layers. Co-occurrence relationships revealed an intimate link between DOM composition and the microbial communities residing in sediment. The decrease in H/C of DOM in surface sediments (0-6 cm) was accompanied by an increase in CRAM, probably strongly influenced by the microbial mineralization of fresh organic matter, resulting in more inert DOM. The prevalence of Nitrososphaeria in surface sediments may support the ammonia oxidation process in surface sediments and affect the DOM fraction, and Nitrososphaeria may be associated with the accumulation of aromatic substances. The DOM of the subsurface (6-38 cm) may be utilized by anaerobic heterotrophic microbes such as Bathyarchaeia, allowing the recalcitrant DOM to be possibly slightly depleted. The higher relative abundance of CRAM in the overlying water compared to the sediment suggests that sediment-derived DOM may be able to be converted to be more recalcitrant by microorganisms in the water column. Considering sediment-derived DOM as a significant source of water column, the role played by microbe in the biogeochemical cycling of sedimentary DOM warrants further study.