Microbial community structure of dark and light layers in the Quaternary hemipelagic sediments of the Japan Sea

The deep marine subsurface harbor a remarkable number of microbes where cells may live on geologic timescales. It has been known that specific microbial lineages often become predominant (e.g., Atribacteria and Dehalococcoidia¹) in the microbial community of subseafloor sediment through the selection processes by the modern environmental stresses such as energy limitation. In contrast, a part of microbial communities reflects paleoceanographic conditions at the time of deposition even in the deeper part of subsurface sediment². However, it is not clear to which extent deep subsurface microbial community shifts in response to post-burial geochemical conditions, or whether they are merely subsisting for thousands to millions of years after isolation from the surface. We targeted the microbial communities from sediments of the Japan Sea, focusing on dark and light subsurface sediment layers. The deeper part of the sediment in the Japan Sea is characterized by centimeter- to meter-scale alternations of dark and light layers, which were formed in association with the past oxygen depletion associated with glacial-interglacial cycles. We sequenced the 16S rRNA genes of DNA samples extracted from 118 samples spanning from 1.4 to 257.5 meters below the seafloor. The Class of JS1 (Caldatribacteriota) dominated the entire microbial community throughout the whole samples, and Dehalococcoidia (Chloroflexi) was the second most abundant Class across all sites. We will further discuss the relationship between the microbial community structures and the dark- and light-colored layers.

1) Starnawski et al. (2017) PNAS 114(11), 2940-2945.

2) Orsi et al. (2017) Sci. Rep. 7(1), 1-12.