Abstract
Hydrogen production was studied in four species of methanogens (Methanothermobacter marburgensis, Methanosaeta thermophila, Methanosarcina barkeri, and Methanosaeta concilii) under conditions of low (sub-nanomolar) ambient hydrogen concentration using a specially designed culture apparatus. Transient hydrogen production was observed and quantified for each species studied. Methane was excluded as the electron source, as was all organic material added during growth of the cultures (acetate, yeast extract, peptone). Hydrogen production showed a strong temperature dependence, and production ceased at temperatures below the growth range of the organisms. Addition of polysulfides to the cultures greatly decreased hydrogen production. The addition of bromoethanesulfonic acid had little influence on hydrogen production. These experiments demonstrate that some methanogens produce excess reducing equivalents during growth and convert them to hydrogen when the ambient hydrogen concentration becomes low. The lack of sustained hydrogen production by the cultures in the presence of methane provides evidence against "reverse methanogenesis" as the mechanism for anaerobic methane oxidation.
Highlights
Hydrogen (H2) is an important intermediate in the microbially-dominated degradation of organic material in anoxic environments (Schink 1997; Wolin 1982)
Additional experiments were performed using three other methanogens (Methanosaeta thermophila, Methanosarcina barkeri, and Methanosaeta concilii, Fig. 2) to determine if H2 production is a general feature of methanogens in lowH2 environments
Due to the kinetic ability of Methanosaeta thermophila to consume acetate to low micromolar levels, it was difficult to ensure that the observed H2 was not generated during methanogenesis from acetate (Min and Zinder 1989)
Summary
Hydrogen (H2) is an important intermediate in the microbially-dominated degradation of organic material in anoxic environments (Schink 1997; Wolin 1982). The ambient H2 concentration is dynamically controlled and is generally indicative of the dominant terminal electron-accepting process. Different terminal electron-accepting processes occur in close proximity to each other, both spatially and temporally; both microbial transport and changes in environmental conditions can lead to changes in the redox condition for a given organism. For example, are able to reverse their metabolism and convert acetate to CO2 and H2 when H2 levels become too low for homoacetogenic growth (Lee and Zinder 1988). It has been proposed that methanogens are capable of reversing their metabolism under low H2 (i.e., sulfate-reducing conditions) to convert CH4 to CO2 and H2 syntrophically, a process referred to as reverse methanogenesis (Hoehler et al 1994)
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