Hydrogen metabolism during methanogenesis from acetate byMethanosarcina barkeri

Energy Conservation via Hydrogen Cycling in the Methanogenic Archaeon Methanosarcina barkeri

Energy conservation via hydrogen cycling, which generates proton motive force by intracellular H₂ production coupled to extracellular consumption, has been controversial since it was first proposed in 1981. It was hypothesized that the methanogenic archaeon Methanosarcina barkeri is capable of energy conservation via H₂ cycling, based on genetic data that suggest that H₂ is a preferred, but nonessential, intermediate in the electron transport chain of this organism. Here, we characterize a series of hydrogenase mutants to provide direct evidence of H₂ cycling. M. barkeri produces H₂ during growth on methanol, a phenotype that is lost upon mutation of the cytoplasmic hydrogenase encoded by frhADGB, although low levels of H₂, attributable to the Ech hydrogenase, accumulate during stationary phase. In contrast, mutations that conditionally inactivate the extracellular Vht hydrogenase are lethal when expression of the vhtGACD operon is repressed. Under these conditions, H₂ accumulates, with concomitant cessation of methane production and subsequent cell lysis, suggesting that the inability to recapture extracellular H₂ is responsible for the lethal phenotype. Consistent with this interpretation, double mutants that lack both Vht and Frh are viable. Thus, when intracellular hydrogen production is abrogated, loss of extracellular H₂ consumption is no longer lethal. The common occurrence of both intracellular and extracellular hydrogenases in anaerobic microorganisms suggests that this unusual mechanism of energy conservation may be widespread in nature.IMPORTANCE ATP is required by all living organisms to facilitate essential endergonic reactions required for growth and maintenance. Although synthesis of ATP by substrate-level phosphorylation is widespread and significant, most ATP is made via the enzyme ATP synthase, which is energized by transmembrane chemiosmotic gradients. Therefore, establishing this gradient across the membrane is of central importance to sustaining life. Experimental validation of H₂ cycling adds to a short list of mechanisms for generating a transmembrane electrochemical gradient that is likely to be widespread, especially among anaerobic microorganisms.

Methanosarcina spp. drive vinyl chloride dechlorination via interspecies hydrogen transfer

Two highly enriched cultures containing Dehalococcoides spp. were used to study the effect of aceticlastic methanogens on reductive vinyl chloride (VC) dechlorination. In terms of aceticlastic methanogens, one culture was dominated by Methanosaeta, while the other culture was dominated by Methanosarcina, as determined by fluorescence in situ hybridization. Cultures amended with 2-bromoethanesulfonate (BES), an efficient inhibitor of methanogens, exhibited slow VC dechlorination when grown on acetate and VC. Methanogenic cultures dominated by Methanosaeta had no impact on dechlorination rates, compared to BES-amended controls. In contrast, methanogenic cultures dominated by Methanosarcina displayed up to sevenfold-higher rates of VC dechlorination than their BES-amended counterparts. Methanosarcina-dominated cultures converted a higher percentage of [2-(14)C]acetate to (14)CO(2) when concomitant VC dechlorination took place, compared to nondechlorinating controls. Respiratory indices increased from 0.12 in nondechlorinating cultures to 0.51 in actively dechlorinating cultures. During VC dechlorination, aqueous hydrogen (H(2)) concentrations dropped to 0.3 to 0.5 nM. However, upon complete VC consumption, H(2) levels increased by a factor of 10 to 100, indicating active hydrogen production from acetate oxidation. This process was thermodynamically favorable by means of the extremely low H(2) levels during dechlorination. VC degradation in nonmethanogenic cultures was not inhibited by BES but was limited by the availability of H(2) as electron donor, in cultures both with and without BES. These findings all indicate that Methanosarcina (but not Methanosaeta), while cleaving acetate to methane, simultaneously oxidizes acetate to CO(2) plus H(2), driving hydrogenotrophic dehalorespiration of VC to ethene by Dehalococcoides.

The effect of redox potential changes on reductive dechlorination of pentachlorophenol and the degradation of acetate by a mixed, methanogenic culture

The effect of changes in redox potential on methanogenesis from acetate, and on the reductive dechlorination of pentachlorophenol (PCP), was evaluated using a computer-monitored and feedback-controlled bioreactor. PCP was transformed via 2,3,4, 5-tetrachlorophenol (2,3,4,5-TeCP) to 3,4,5-trichlorophenol (3,4, 5-TCP). In 6- to 12-d experiments, pH, acetate concentration, and temperature were held constant; the redox potential, defined here as the potential measured at a platinum electrode (EPt), was maintained at different set points, while transformation of multiple PCP additions was monitored. Without redox potential control, the value of EPt for the culture was approximately -0.26 V (vs. SHE). The value of EPt was elevated from -0.26 V for periods up to 10 h by computer-controlled addition of H2O2 or K3Fe(CN)6. Methanogenesis continued during a relatively mild shift of EPt to -0.2 V with H2O2, but was halted when EPt was raised to -0.1 V with either H2O2 or K3Fe(CN)6. Methanogenesis resumed when EPt returned to -0.26 V. During periods in which EPt was elevated significantly and methanogenesis stopped, transformation of PCP and 2,3,4,5-TeCP continued at progressively slower rates, but the rate of 2,3,4, 5-TeCP transformation was diminished to a greater extent. When a small volume of pure H2 was added to the reactor headspace, while EPt was maintained at -0.1 V, reductive dechlorination rates increased dramatically. Lower H2 concentrations during periods of oxidant addition, perhaps due to the effect of the oxidant on H2-producing bacteria, may contribute to decreased reductive dechlorination rates. Copyright 1999 John Wiley & Sons, Inc.

Characterization of a CO: heterodisulfide oxidoreductase system from acetate-grown Methanosarcina thermophila

During the methanogenic fermentation of acetate by Methanosarcina thermophila, the CO dehydrogenase complex cleaves acetyl coenzyme A and oxidizes the carbonyl group (or CO) to CO2, followed by electron transfer to coenzyme M (CoM)-S-S-coenzyme B (CoB) and reduction of this heterodisulfide to HS-CoM and HS-CoB (A. P. Clements, R. H. White, and J. G. Ferry, Arch. Microbiol. 159:296-300, 1993). The majority of heterodisulfide reductase activity was present in the soluble protein fraction after French pressure cell lysis. A CO:CoM-S-S-CoB oxidoreductase system from acetate-grown cells was reconstituted with purified CO dehydrogenase enzyme complex, ferredoxin, membranes, and partially purified heterodisulfide reductase. Coenzyme F420 (F420) was not required, and CO:F420 oxidoreductase activity was not detected in cell extracts. The membranes contained cytochrome b that was reduced with CO and oxidized with CoM-S-S-CoB. The results suggest that a novel CoM-S-S-CoB reducing system operates during acetate conversion to CH4 and CO2. In this system, ferredoxin transfers electrons from the CO dehydrogenase complex to membrane-bound electron carriers, including cytochrome b, that are required for electron transfer to the heterodisulfide reductase. The cytochrome b was purified from solubilized membrane proteins in a complex with six other polypeptides. The cytochrome was not reduced when the complex was incubated with H2 or CO, and H2 uptake hydrogenase activity was not detected; however, the addition of CO dehydrogenase enzyme complex and ferredoxin enabled the CO-dependent reduction of cytochrome b.