The GC-MS analysis of the gas sample detected methanethiol as a major organosulfur product (Fig. ). Throughout the 14-day reaction, the amount of methanethiol increased steadily up to 109 μmol (Fig. ), which corresponded to 7.9% of the initial amount of H2S. Other identified gas-phase products include CO, methane (CH4), dimethyl sulfide (CH3SCH3), and trace amounts of carbonyl sulfide (OCS) and CS2 (Fig. and SI Appendix, Figs. and). Dimethyl disulfide (CH3SSCH3) was also seen, but it was likely formed through methanethiol oxidation by atmospheric oxygen (O2) at the timing of gas sampling and/or sample injection into the GC. In the aqueous solution, formate (HCOO -) was observed as the dominant dissolved product (SI Appendix, Fig. and). Formate exhibited the highest concentration at the shortest reaction duration (86 mmol L -1 after the 3-day reaction) and subsequently reduced the value (Fig. ). Considering the gradual depletion of H2 during the experiment (Fig. ), formate was expected to rapidly attain thermodynamic equilibrium with H2 and CO2 for its formation and decomposition (CO2 + H2 ⇌ HCOO -+ H + ) under the examined condition.