The aim of the present work was to investigate whether uncoupling of catabolism from anabolism, which was often obsd. in heterotrophic microorganisms under energy-sufficient growth conditions, also occurs in the autotrophic bacterium M. thermoautotrophicum. For this purpose, M. thermoautotrophicum was cultivated in continuous cultures that were limited by the trace element Fe. The influences of both diln. rate and Fe supply rate on the coupling between anabolism and catabolism were investigated. As compared to continuous cultures of M. thermoautotrophicum limited by the energy substrate H2, a 5-fold decrease in the biomass concn. and a 3-fold decrease in H2, CO2, and CH4 conversion rates were obsd. in Fe-limited cultures. However, the sp. substrate and product conversion rates increased as compared to the values detd. in energy-limited cultures. Thus, Fe limitation provoked an uncoupling of catabolism from anabolism. At a diln. rate of 0.096/h and at an Fe concn. of 17 mM in the feed, the sp. H2 consumption rate was 100% higher than the rate detd. under H2-limiting conditions, whereas at a diln. rate of 0.168/h, the values differed by only 5%. Uncoupling of catabolism from anabolism also increased dramatically when the Fe supply rate was lowered but the diln. rate was kept const. Thus, the extent of uncoupling is a function of both the diln. rate and the Fe supply rate. The sp. consumption rate of H2 increased in parallel with the partial pressure of H2 in the culture medium. This suggested that the catabolic activity of M. thermoautotrophicum was not stringently controlled at the enzymic level and can be considerably stimulated by the excess of H2 in the medium. Hypotheses as to the fate of the excess energy derived from uncoupled catabolism are discussed, but the physiol. reason for the partial uncoupling between catabolism and anabolism remains yet to be clarified. [on SciFinder (R)]