We here report on specific chemical and microbial compositions observed at the Archaean hydrothermal site in the Southern Mariana backarc spreading center, which produces two remarkably different hydrothermal fluids. One was black smoker hydrothermal fluid at 340 °C containing a low concentration of methane with a relatively high carbon isotope ratio (d13C of methane relative to VPDB), -7.8 per mil, indicating that methane originated from a magmatic source; thus, this is a fairly ordinary hydrothermal fluid for basalt-hosted hydrothermal activity. In contrast, the other fluid was clear smoker hydrothermal fluid at 117 °C containing a high concentration of methane with a very low carbon isotope ratio (-49.7 per mil). The host rock of the Archaean hydrothermal site is basalt, and therefore Fischer-Tropsch-type reactions resulting from serpentinization of mantle rocks are not feasible as a source for the "light" carbon in methane. Additionally, the carbon isotope ratio of carbon dioxide demonstrated that the sources of CO2 and CH4 were not organic materials. The remaining possibility, biogenic methane, was confirmed by cultivation of hyperthermophilic hydrogen-oxidizing methanogens from both rocks covering the vent of clear smoker hydrothermal fluids and in situ cultivation systems. Although the dominance and abundance of methanogens were very low in this ecosystem, the number was consistent with the relationship between hydrogen concentration and methanogen abundance (Takai et al., 2015). These results suggest that phase separation led to concentration of hydrogen and subsequent persistence of a hyperthermophilic subsurface lithoautotrophic microbial ecosystem in both serpentine-hosted and basalt-hosted hydrothermal systems. This type of ecosystem may occur in similar settings elsewhere on modern earth; in addition, similar communities may have existed in other types of deep-sea hydrothermal systems in the geological past.

Supplement to: Toki, Tomohiro; Hamamoto, Arito; Tawata, Miki; Miyazaki, Junichi; Nakamura, Kentaro; Abe, Mariko; Takai, Ken; Sano, Yuji; Takahata, Naoto; Tsunogai, Urumu; Ishibashi, Jun-ichiro (2016): Methanogens in H2-rich hydrothermal fluids resulting from phase separation in a sediment-starved, basalt-hosted hydrothermal system. Chemical Geology, 447, 208-218