ENVIRONMENTAL AND MOLECULAR REGULATION OF METHANOGENESIS

Hydrogen plays a central role in regulating the anaerobic biodegradati on of organic materials to carbon dioxide and methane. At an intermedi ate stage, alcohols and fatty acids are fermented to acetate, CO2 and H-2. Methanogens consume this H-2, gaining energy by reducing CO2 and the CH3-moieties of methanol, methylamines and acetate to CH4, and gro wing by assimilating these same substrates into biomass. There are sev en biochemical steps in the H-2-dependent pathway of CO2 reduction to CH4 in Methanobacterium thermoautotrophicum, a very common inhabitant of anaerobic digesters, several of which can be catalyzed by more than one enzyme. The choice of which enzyme is synthesized and therefore u sed in methanogenesis is determined by the availability of H-2. With h igh H-2 availability, M. thermoautotrophicum cells grow rapidly but th eir overall growth yield (Y-CH4; biomass synthesized per mole of CH4 s ynthesized) is lower than for cells growing more slowly under H-2-limi ted conditions. Experiments are reported that document the relationshi ps between H-2 availability, alternative methane gene expression and g rowth yield, and that demonstrate H-2-dependent reversible switching b etween rapid, relatively inefficient growth and slower more efficient growth. This switch is controlled by the mixing rate of the impeller i n fed-batch fermenters sparged with CO2 and H-2. (C) 1997 IAWQ. publis hed by Elsevier Science Ltd.