CH4-metabolizing (producing or consuming CH4) archaea and methylotrophic bacteria are predominant in the chimneys, suggesting that CH4 plays an important role in the biogeochemical cycling of carbon in this extreme environment. Dissolved inorganic carbon concentration (ΣCO2 = CO2(aq), HCO3-, CO3 2-) is very low in the high pH, anaerobic LCHF chimney fluids (4 lmol/kg on average), and mantle-derived CO2 is removed in the subsurface through conversion to hydrocarbons or carbonate precipitation. In addition, other carbon species such as formate and acetate (1–35 lmol/L) are important sources of carbon to sustain microbial activity in this system.