The thermoacidophilic methanotroph Methylacidiphilum fumariolicum SolV oxidizes subatmospheric H2 with a high-affinity, membrane-associated [NiFe] hydrogenase

The trace amounts (0.53 ppmv) of atmospheric hydrogen gas (H-2) can be utilized by microorganisms to persist during dormancy. This process is catalyzed by certain Actinobacteria, Acidobacteria, and Chloroflexi, and is estimated to convert 75 x 10(12) g H-2 annually, which is half of the total atmospheric H-2. This rapid atmospheric H-2 turnover is hypothesized to be catalyzed by high-affinity [NiFe] hydrogenases. However, apparent high-affinity H-2 oxidation has only been shown in whole cells, rather than for the purified enzyme. Here, we show that the membrane-associated hydrogenase from the thermoacidophilic methanotroph Methylacidiphilum fumariolicum SolV possesses a high apparent affinity (K-m(app) = 140 nM) for H-2 and that methanotrophs can oxidize subatmospheric H-2. Our findings add to the evidence that the group 1h [NiFe] hydrogenase is accountable for atmospheric H-2 oxidation and that it therefore could be a strong controlling factor in the global H-2 cycle. We show that the isolated enzyme possesses a lower affinity (K-m = 300 nM) for H-2 than the membrane-associated enzyme. Hence, the membrane association seems essential for a high affinity for H-2. The enzyme is extremely thermostable and remains folded up to 95 degrees C. Strain SolV is the only known organism in which the group 1h [NiFe] hydrogenase is responsible for rapid growth on H-2 as sole energy source as well as oxidation of subatmospheric H-2. The ability to conserve energy from H-2 could increase fitness of verrucomicrobial methanotrophs in geothermal ecosystems with varying CH4 fluxes. We propose that H-2 oxidation can enhance growth of methanotrophs in aerated methane-driven ecosystems. Group 1h [NiFe] hydrogenases could therefore contribute to mitigation of global warming, since CH4 is an important and extremely potent greenhouse gas.