The coupling between catabolism and anabolism of Methanobacterium thermoautotrophicum in H-2- and iron-limited continuous cultures

The aim of the present work was to investigate whether uncoupling of catabo lism from anabolism, which was often observed in heterotrophic microorganis ms under energy-sufficient growth conditions, also occurs in the autotrophi c bacterium Methanobacterium thermoautotrophicum. For this purpose, M. ther moautotrophicum was cultivated in continuous cultures that were limited by the trace element iron. The influences of both dilution rate and iron suppl y 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 i ron-limited cultures. However, the specific substrate and product conversio n rates increased as compared to the values determined in energy-limited cu ltures. Thus, iron limitation provoked an uncoupling of catabolism from ana bolism. At a dilution rate of 0.096 h(-1) and at an iron concentration of 1 7 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 ra te 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 l owered but the dilution rate was kept constant. Thus, the extent of uncoupl ing is a function of both the dilution rate and the iron supply rate. It wa s found that the specific consumption rate of H-2 increased in parallel wit h the partial pressure of H-2 in the culture medium. This suggested that th e catabolic activity of M. thermoautotrophicum was not stringently controll ed 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 derive d from uncoupled catabolism are discussed, but the physiological reason for the partial uncoupling between catabolism and anabolism remains yet to be clarified. (C) 1999 Elsevier Science Inc. All rights reserved.