Characterization of bacterial and archaeal community structure in deep subsurface sediments in the Shenhu area, northern South China Sea

Gas hydrate-bearing sediments provide an extreme and unique habitat for bacteria and archaea, but the relationship between the microbial community and gas hydrate formation is poorly known. In the Shenhu area of the northern South China Sea, it was inferred that the gas source is mixed biogenic-thermogenic, and contradictions exist between relatively low total organic carbon content and high saturation of gas hydrates. The synthesized geophysical and geological evidence showed that deep thermogenic fluid could migrate upward through migrating pathways. In this study, samples were collected from Shenhu area from 224 to 271 mbsf (meters below the seafloor), which covered gas hydrate-bearing sediments (GHBS), free gas-bearing sediments (FGBS), and non gas hydrate-bearing sediments (NGHBS), and 16S rRNA gene sequencing was used to characterize bacterial and archaeal community structure. The results revealed significant differences of microbial community structure among the three sediment types as indicated by alpha (richness and diversity indices) and beta (Bray-Curtis based non-metric multidimensional scaling) diversity analyses as well as community composition analyses. In bacterial community, abundant genera included Pseudomonas, Halomonas, Alcanivorax, Thiobacillus, Marinobacter, Rhodococcus, and Dietzia, some of which were identified as putative hydrocarbon degraders. Most abundant archaeal genera were potential methanogenic archaea, and the dominant genera between GHBS and FGBS differed. Multiple types of methanogenesis were noted, and the hydrogenotrophic type was inferred to be predominant. Deviations of temperature of methanogens from the geothermal gradient of the study area again suggested the migration of organic matters. Based on the characterization of bacterial and archaeal community structure in the deep subsurface sediments, we proposed a geological model for the gas hydrate formation and accumulation of the gas hydrate reservoir, emphasizing the possibility of microbially-mediated secondary microbial gas formation from deep thermogenic fluid. This study characterized bacterial and archaeal community structure in deep subsurface sediments, which can provide insight into the potential roles of bacteria and archaea including hydrocarbon degradation and methanogenesis in gas hydrate formation and help reevaluate the contribution of deep thermogenic fluid to gas hydrate formation in the Shenhu area of the northern continental slope of the South China Sea.