Microbial Studies On Core SH2B In The Gas Hydrate Area Of Shenhu In The South China Sea

It is significative to investigate the microbial structure,the variation with depth,and the biogeochemical processes they participated in the formation and dissolution of the gas hydrate in marine sediments.Although gas hydrate samples were collected by drilling program from Shenhu area of the northern South China Sea(SCS)in 2007,we know little about microbial structure in hydrate-bearing sediments in this area.Six surface sediment samples(the top of core SH2B,SH3B and SH7B with hydrate and core SH1B,SH5B and SH5C without hydrate)and a full-length drilling core SH2B(238 meters)were analyzed for microbial diversity,phospholipid fatty acids(PLFA),and glycerol dibiphytanyl glycerol tetraether(GDGT).The distribution of microbial structure at surface sediments and the variation with depth were discussed.The conclusions are as following:1.Microbial diversity in Shenhu area was much lower than other areas,such as in Dongsha cold seep sediments and in a gravity piston core collected from Shenhu area.Sulfate reduction reaction was strong above sulfate-methane interface(SMI),and weakened with depth.Anaerobic Methane Oxidation Group(ANME)-1 was the dominant archaea,which indicating strongly anaerobic oxidation of methane(AOM)reaction occurred between SMI to 71.95 mbsf.Crenarchaeol,a specific lipid identified in the membranes of various ammonia-oxidizing archaea,which may represents a signature lipid for the Thaumarchaeota was the dominant GDGT at SMI depth.Microbial diversity was a little higher at the top of gas hydrate occurrence zone(GHOZ)than at the bottom.GDGTs decreased linearly at GHOZ.Variations of stable carbon isotope values of PLFA and total organic carbon reflected that bacteria produce PLFA are depleted with respect to the carbon source by up to-16.1‰ at GHOZ.Microbial communities were unique below GHOZ.2.Microbial structure corresponded with the transformation of the methane from dissolved to hydrated,and then to gaseous.3.Methanogens were detected at different depths indicating that hydrate source methane can be produced by microorganisms.4.Thaumarchaeota,the third proposal archaeal phylum which contributes significantly to the global relevance for N-and C-cycle was first detected abundantly in gas hydrate sediments at Shenhu drilling core.These microorganisms may participate in an undiscovered process in the formation or dissolution of gas hydrate.The thermophiles DNA detected in the core,probably associated with the past volcanic and hydrothermal activities as potential geomicrobiological evidence reflecting novel records of geologic thermal events in the Pleistocene to Miocene period in the SCS.Over all,microbial structure variation with depth indicated that methane produced at the bottom,upward and consumed by AOM at specific depth,and exhausted at SMI in Shenhu drilling core.These results provided new insight for exploring the gas hydrate in the SCS in the future.