The aim of the present work was to investigate whether uncoupling of catabolism from anabolism, which was often observed in heterotrophic microorganisms under energy-sufficient growth conditions, also occurs in the autotrophic bacterium Methanobacterium thermoautotrophicum. For this purpose, M. thermoautotrophicum was cultivated in continuous cultures that were limited by the trace element iron. The influences of both dilution rate and iron supply rate on the coupling between anabolism and catabolism were investigated. As compared to continuous cultures of M. thermoautotrophicum limited by the energy substrate H-2,H- a 5-fold decrease in the biomass concentration and a 3-fold decrease in H-2, CO2, and CH4 conversion rates were observed in iron-limited cultures. However, the specific substrate and product conversion rates increased as compared to the values determined in energy-limited cultures. Thus, iron limitation provoked an uncoupling of catabolism from anabolism. At a dilution rate of 0.096 h(-1) and at an iron concentration of 17 mu M in the feed. the specific H-2 consumption rare was 100% higher than the rate determined under H-2-limiting conditions, whereas at a dilution rate of 0.168 h(-l), the values differed only by 5%. Uncoupling of catabolism from anabolism also increased dramatically when the iron supply rate was lowered but the dilution rate was kept constant. Thus, the extent of uncoupling is a function of both the dilution rate and the iron supply rate. It was found that the specific consumption rate of H-2 increased in parallel with the partial pressure of H-2 in the culture medium. This suggested that the catabolic activity of M. thermoautotrophicum was not stringently controlled at the enzymatic level and can be considerably stimulated by the excess of H-2 in the medium. Hypotheses as to the fate of the excess energy derived from uncoupled catabolism are discussed, but the physiological reason for the partial uncoupling between catabolism and anabolism remains yet to be clarified.