M. thermoautotrophicum was grown on a defined mineral salts medium under strictly anaerobic conditions with H2 and CO2 as the sole energy and C sources, resp. The cultivation medium was optimized with respect to non-org. components, including Se(IV), W(VI), N, Ni(II), Fe(II), Co(II), and Mo(VI). S2- concn. in the medium was maintained const. using an online regulatory system by the addn. of 0.5M Na2S. A max. supply rate of 0.6 vvm of a mixt. of 80% H2 and 20% CO2 was achieved for the gaseous substrates. Under these conditions, a sp. growth rate of 0.30/h and a cell concn. of 4.8 g cell dry wt. (DW)/L, representing a 140% increase over previously published results, were obtained. The growth yield of 2.3 g DW/mol CH4 was similar to published values. However, the overall sp. productivity was enhanced from 11 to 24 mmol CH4/g DW-h, corresponding to an improvement of 120%. [on SciFinder (R)]