Hyperthermophilic methanogens are often H2 limited in hot subseafloor environments and their survival may be due in part to physiological adaptations to low H2 conditions and interspecies H2 transfer. The hyperthermophilic methanogen Methanocaldococcus jannaschii was grown in monoculture at high (80-83 μM) and low (15-27 μM) aqueous H2 concentrations and in co-culture with the hyperthermophilic H2 producer Thermococcus paralvinellae. The purpose was to measure changes in growth and CH4 production kinetics, CH4 fractionation, and gene expression in M. jannaschii with changes in H2 flux. Growth and cell-specific CH4 production rates of M. jannaschii decreased with decreasing H2 availability and decreased further in co-culture. However, cell yield (cells produced per mole of CH4 produced) increased six-fold when M. jannaschii was grown in co-culture relative to monoculture. Relative to high H2concentrations, isotopic fractionation of CO2 to CH4 (ϵCO2-CH4) was 16‰ larger for cultures grown at low H2 concentrations and 45‰ and 56‰ larger for M. jannaschii growth in co-culture on maltose and formate, respectively. Gene expression analyses showed H2-dependent methylene-tetrahydromethanopterin (H4MPT) dehydrogenase expression decreased and coenzyme F420-dependent methylene-H4MPT dehydrogenase expression increased with decreasing H2 availability and in co-culture growth. In co-culture, gene expression decreased for membrane-bound ATP synthase and hydrogenase. The results suggest that H2 availability significantly affects the CH4 and biomass production and CH4 fractionation by hyperthermophilic methanogens in their native habitats.