Adaptation of methane formation and enzyme contents during growth of Methanobacterium thermoautotrophicum (strain Delta H) in a fed-batch fermentor

During growth of Methanobacterium thermoautotrophicum in a fed-batch fermen ter, the cells are confronted with a steady decrease in the concentration o f the hydrogen energy supply. In order to investigate how the organism resp onds to these changes, cells collected during different growth phases were examined for their methanogenic properties. Cellular levels of the various methanogenic isoenzymes and functionally equivalent enzymes were also deter mined. Cells were found to maintain the rates of methanogenesis by lowering their affinity for hydrogen: the apparent K-m(H2) decreased in going from the exponential to the stationary phase. Simultaneously, the maximal specif ic methane production rate changed. Levels of H-2-dependent methenyl-tetrah ydromethanopterin dehydrogenase (H-2-MDH) and methyl coenzyme M reductase i soenzyme II (MCR II) decreased upon entry of the stationary phase. Cells gr own under conditions that favored MCR II expression had higher levels of MC R II and H2-MDH, whereas in cells grown under conditions favoring MCR I, le vels of MCR II were much lower and the cells had an increased affinity for hydrogen throughout the growth cycle. The use of thiosulfate as a medium re ductant was found to have a negative effect on levels of MCR II and H2-MDH. From these results it was concluded that M. thermoautotrophicum responds t o variations in hydrogen availability and other environmental conditions (p H, growth temperature, medium reductant) by altering its physiology. The ad aptation includes, among others, the differential expression of the MDH and MCR isoenzymes.