Hydrogen plays a central role in regulating the anaerobic biodegradation of organic materials to carbon dioxide and methane. At an intermediate stage, alcohols and fatty acids are fermented to acetate, CO2 and H2. Methanogens consume this H2, gaining energy by reducing CO2 and the CH3-moieties of methanol, methylamines and acetate to CH4, and growing by assimilating these same substrates into biomass. There are seven biochemical steps in the H2-dependent pathway of CO2 reduction to CH4 in Methanobacterium thermoautotrophicum, a very common inhabitant of anaerobic digestors, several of which can be catalyzed by more than one enzyme. The choice of which enzyme is synthesized and therefore used in methanogenesis is determined by the availability of H2. With high H2 availability, M. thermoautotrophicum cells grow rapidly but their overall growth yield (YCH4; biomass synthesized per mole of CH4 synthesized) is lower than for cells growing more slowly under H2-limited conditions. Experiments are reported that document the relationships between H2 availability, alternative methane gene expression and growth yield, and that demonstrate H2-dependent reversible switching between rapid, relatively inefficient growth and slower more efficient growth. This switch is controlled by the mixing rate of the impeller in fed-batch fermentors sparged with CO2 and H2.

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