Growth Kinetics, Carbon Isotope Fractionation, and Gene Expression in the Hyperthermophile Methanocaldococcus jannaschii during Hydrogen-Limited Growth and Interspecies Hydrogen Transfer

Hyperthermophilic methanogens are often H limited in hot subseafloor environments, and their survival may be due in part to physiological adaptations to low H conditions and interspecies H transfer. The hyperthermophilic methanogen was grown in monoculture at high (80 to 83 μM) and low (15 to 27 μM) aqueous H concentrations and in coculture with the hyperthermophilic H producer The purpose was to measure changes in growth and CH production kinetics, CH fractionation, and gene expression in with changes in H flux. Growth and cell-specific CH production rates of decreased with decreasing H availability and decreased further in coculture. However, cell yield (cells produced per mole of CH produced) increased 6-fold when was grown in coculture rather than monoculture. Relative to high H concentrations, isotopic fractionation of CO to CH (ε ) was 16‰ larger for cultures grown at low H concentrations and 45‰ and 56‰ larger for growth in coculture on maltose and formate, respectively. Gene expression analyses showed H -dependent methylene-tetrahydromethanopterin (H MPT) dehydrogenase expression decreased and coenzyme F -dependent methylene-H MPT dehydrogenase expression increased with decreasing H availability and in coculture growth. In coculture, gene expression decreased for membrane-bound ATP synthase and hydrogenase. The results suggest that H availability significantly affects the CH and biomass production and CH fractionation by hyperthermophilic methanogens in their native habitats. Hyperthermophilic methanogens and H -producing heterotrophs are collocated in high-temperature subseafloor environments, such as petroleum reservoirs, mid-ocean ridge flanks, and hydrothermal vents. Abiotic flux of H can be very low in these environments, and there is a gap in our knowledge about the origin of CH in these habitats. In the hyperthermophile , growth yields increased as H flux, growth rates, and CH production rates decreased. The same trend was observed increasingly with interspecies H transfer between and the hyperthermophilic H producer With decreasing H availability, isotopic fractionation of carbon during methanogenesis increased, resulting in isotopically more negative CH with a concomitant decrease in H -dependent methylene-tetrahydromethanopterin dehydrogenase gene expression and increase in F -dependent methylene-tetrahydromethanopterin dehydrogenase gene expression. The significance of our research is in understanding the nature of hyperthermophilic inte