Deep mud-volcano biosphere in the Nankai accretionary wedge

Sampling

Submarine mud-volcanoes are globally distributed in the plate convergent margins1,2. The mud-volcanism transports deep-sourced fluids and hydrocarbons to the seafloor3, which upward supply often supports chemosynthetic benthic life and microbial communities that play significant ecological roles in biogeochemical carbon cycle4,5. Yet, geochemical and microbiological characteristics of the deep mud-volcano subsurface remain largely unknown. Here, we studied an active submarine mud-volcano in the Nankai Trough by drilling down to 200 meters from the summit. Cell count and molecular analyses indicate that relatively small (102~105 cells/cm3) but phylogenetically diverse microbial populations are present throughout the cored sediment. In the pore water, carbon isotopic compositions of bicarbonate and acetate are notably 13C-enriched up to the maximum value of +40 and -25, respectively. High concentrations of hydrogen (up to 1,800 nM) are observed, suggesting a thermodynamically feasible condition for microbial acetogenesis via CO2 reduction. Radiotracer experiments consistently showed that acetogenesis activities are generally 2-3 orders of magnitude higher than those of methanogenesis. These data indicate that the deep biosphere in the submarine mud-volcano is characterized by tectonic and sedimentolgoical regimes of the oceanic accretionary wedge, and hence different from the stratified sedimentary biosphere.

Method steps

1. Pipeline used: https://www.ebi.ac.uk/metagenomics/pipelines/4.1