Effect of Molecular Hydrogen on Acetate Degradation by Methanosarcina Barkeri 227
Acetate is the ultimate intermediate for over two-thirds of the methane produced by anaerobic digestors, and methanosarcinae are the predominant aceticlastic methanogens, especially in high-rate digestors. The remainder of the methane produced in anaerobic digestors (about one third) comes from the oxidation of hydrogen and the concomitant reduction of carbon dioxide to form methane. Hydrogen concentrations control many important catabolic reactions in anaerobic digestion, especially those involved in hydrogen production and degradation. Because hydrogen concentration is normally very low (several tenths of a micromole) and its turnover is very rapid, the entire pool size is degraded many times per second, and so minute changes in the production or degradation rate of hydrogen may be instantaneously translated into fluctuations in its concentration. Also, hydrogen concentration may not be uniform in the digestor. It is likely that there are microenvironments in anaerobic digestors having higher and lower concentrations than the average since hydrogen production is not uniform, and particles may account for a major portion of the hydrogen production. These fluctuations and the presence of microenvironments may be important in the inhibition of acetate degradation by Methanosarcina barkeri.
Acetate degradation by Methanosarcina barkeri (the type species), as well as other Methanosarcina species, is inhibited by elevated levels of molecular hydrogen. This inhibition appears to be a typical catabolite repression. Another kind of hydrogen inhibition has been noted also: pure cultures which were grown on hydrogen are not able to use acetate when inoculated into media containing acetate as the sole catabolic substrate.
We investigated catabolite repression of acetate degradation by growing Methanosarcina barkeri on acetate and providing various levels of hydrogen. Syntrophomonas wolfei produces hydrogen during growth on butyrate, but hydrogen can be produced only when its concentration is very low (near that found in digestors). M. barkeri 227 was unable to use hydrogen produced by this bacterium. We found that hydrogen at concentrations near those found in digestors neither inhibited nor stimulated acetate degradation by Methanosarcina barkeri 227. Under these conditions this organism did not appear to utilize hydrogen. At higher hydrogen concentrations M. barkeri 227 was able to use hydrogen, but as long as hydrogen concentration was elevated acetate degradation remained inhibited.
Another type of inhibition of acetate degradation was investigated. Hydrogen-grown cultures of Methanosarcina barkeri are unable to utilize acetate except after a prolonged lag-phase (several weeks) and sometimes not even then. We have found that, when M. barkeri 227 cells were grown on a combination of hydrogen and acetate that hydrogen was utilized first. When hydrogen was exhausted, acetate degradation commenced with no inhibition. If acetate was absent and these cultures were used to inoculate media with acetate as substrate, a prolonged inhibition normally resulted. We also found that at the beginning of or after the onset of this inhibition, the addition of small amounts of hydrogen relieved the inhibition, and that methanol or trimethylamine addition was less efficient at relieving inhibition.
KeywordsAnaerobic Digestor Hydrogen Production Hydrogen Concentration Catabolite Repression Methanosarcina Barkeri
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