Growth Kinetics, Carbon Isotope Fractionation, and Gene Expression in the HyperthermophileMethanocaldococcus jannaschiiduring Hydrogen-Limited Growth and Interspecies Hydrogen Transfer
ABSTRACTHyperthermophilic methanogens are often H2limited in hot subseafloor environments, and their survival may be due in part to physiological adaptations to low H2conditions and interspecies H2transfer. The hyperthermophilic methanogenMethanocaldococcus jannaschiiwas grown in monoculture at high (80 to 83 μM) and low (15 to 27 μM) aqueous H2concentrations and in coculture with the hyperthermophilic H2producerThermococcus paralvinellae. The purpose was to measure changes in growth and CH4production kinetics, CH4fractionation, and gene expression inM. jannaschiiwith changes in H2flux. Growth and cell-specific CH4production rates ofM. jannaschiidecreased with decreasing H2availability and decreased further in coculture. However, cell yield (cells produced per mole of CH4produced) increased 6-fold whenM. jannaschiiwas grown in coculture rather than monoculture. Relative to high H2concentrations, isotopic fractionation of CO2to CH4(εCO2-CH4) was 16‰ larger for cultures grown at low H2concentrations and 45‰ and 56‰ larger forM. jannaschiigrowth in coculture 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 H2availability and in coculture growth. In coculture, gene expression decreased for membrane-bound ATP synthase and hydrogenase. The results suggest that H2availability significantly affects the CH4and biomass production and CH4fractionation by hyperthermophilic methanogens in their native habitats.IMPORTANCEHyperthermophilic methanogens and H2-producing heterotrophs are collocated in high-temperature subseafloor environments, such as petroleum reservoirs, mid-ocean ridge flanks, and hydrothermal vents. Abiotic flux of H2can be very low in these environments, and there is a gap in our knowledge about the origin of CH4in these habitats. In the hyperthermophileMethanocaldococcus jannaschii, growth yields increased as H2flux, growth rates, and CH4production rates decreased. The same trend was observed increasingly with interspecies H2transfer betweenM. jannaschiiand the hyperthermophilic H2producerThermococcus paralvinellae. With decreasing H2availability, isotopic fractionation of carbon during methanogenesis increased, resulting in isotopically more negative CH4with a concomitant decrease in H2-dependent methylene-tetrahydromethanopterin dehydrogenase gene expression and increase in F420-dependent methylene-tetrahydromethanopterin dehydrogenase gene expression. The significance of our research is in understanding the nature of hyperthermophilic interspecies H2transfer and identifying biogeochemical and molecular markers for assessing the physiological state of methanogens and possible source of CH4in natural environments.