Growth of methanogens under defined hydrogen conditions.

Hydrogen (H(2)) is a primary electron donor for methanogenesis and its availability can have profound effects on gene expression and the physiology of energy conservation. The rigorous evaluation of the effects of hydrogen conditions requires the comparison of cultures that are grown under hydrogen limitation and hydrogen excess. The growth of methanogens under defined hydrogen conditions is complicated by the dynamics of hydrogen dissolution and its utilization by the cells. In batch culture, gassing and agitation conditions must be carefully calibrated, and even then variations in growth rate and cell density are hard to avoid. Using chemostats, continuous cultures can be achieved whose nutritional states are known, while growth rate and cell density are invariant. Cultures whose growth is limited by hydrogen can be compared to cultures whose growth is limited by some other nutrient and are therefore under hydrogen excess.