Environment-Dependent Distribution of the SedimentnifH-Harboring Microbiota in the Northern South China Sea

Abstract

ABSTRACTThe South China Sea (SCS), the largest marginal sea in the Western Pacific Ocean, is a huge oligotrophic water body with very limited influx of nitrogenous nutrients. This suggests that sediment microbial N2fixation plays an important role in the production of bioavailable nitrogen. To test the molecular underpinning of this hypothesis, the diversity, abundance, biogeographical distribution, and community structure of the sediment diazotrophic microbiota were investigated at 12 sampling sites, including estuarine, coastal, offshore, deep-sea, and methane hydrate reservoirs or their prospective areas by targetingnifHand some other functional biomarker genes. Diverse and novelnifHsequences were obtained, significantly extending the evolutionary complexity of extantnifHgenes. Statistical analyses indicate that sedimentin situtemperature is the most significant environmental factor influencing the abundance, community structure, and spatial distribution of the sedimentnifH-harboring microbial assemblages in the northern SCS (nSCS). The significantly positive correlation of the sediment pore water NH4+concentration with thenifHgene abundance suggests that the nSCS sedimentnifH-harboring microbiota is active in N2fixation and NH4+production. Several other environmental factors, including sediment pore water PO43−concentration, sediment organic carbon, nitrogen and phosphorus levels, etc., are also important in influencing the community structure, spatial distribution, or abundance of thenifH-harboring microbial assemblages. We also confirmed that thenifHgenes encoded by archaeal diazotrophs in the ANME-2c subgroup occur exclusively in the deep-sea methane seep areas, providing for the possibility to develop ANME-2cnifHgenes as a diagnostic tool for deep-sea methane hydrate reservoir discovery.