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CO and H2 have been implicated in methanogenesis from acetate, but it is unclear whether they are directly involved in methanogenesis or electron transfer in acetotrophic methanogens. We compared metabolism of H2 , CO, and formate by cultures of the thermophilic acetotrophic methanogensMethanosarcina thermophila TM-1 andMethanothrix sp. strain CALS-1.M. thermophila accumulated H2 to partial pressures of 40 to 70 Pa (1 Pa = 0.987 × 10-5 atm), as has been previously reported for this and otherMethanosarcina cultures. In contrast,Methanothrix sp. strain CALS-1 accumulated H2 to maximum partial pressures near 1 Pa. Growing cultures ofMethanothrix sp. strain CALS-1 initially accumulated CO, which reached partial pressures near 0.6 Pa (some CO came from the rubber stopper) during the middle of methanogenesis; this was followed by a decrease in CO partial pressures to less than 0.01 Pa by the end of methanogenesis. Accumulation or consumption of CO by cultures ofM. thermophila growing on acetate was not detected. Late-exponential-phase cultures ofMethanothrix sp. strain CALS-1, in which the CO partial pressure was decreased by flushing with N2 -CO2 , accumulated CO to 0.16 Pa, whereas cultures to which ca. 0.5 Pa of CO was added consumed CO until it reached this partial pressure. Cyanide (1 mM) blocked CO consumption but not production. High partial pressures of H2 (40 kPa) inhibited methanogenesis from acetate byM. thermophila but not byMethanothrix sp. strain CALS-1, and 2 kPa of CO was not inhibitory toM. thermophila but was inhibitory toMethanothrix sp. strain CALS-1. Levels of CO dehydrogenase, hydrogenase, and formate dehydrogenase inMethanothrix sp. strain CALS-1 were 9.1, 0.045, and 5.8 μmol of viologen reduced min-1 mg of protein-1 . These results suggest that CO plays a role inMethanothrix sp. strain CALS-1 similar to that of H2 inM. thermophila and are consistent with the conclusion that CO is an intermediate in a catabolic or anabolic pathway inMethanothrix sp. strain CALS-1; however, they could also be explained by passive equilibration of CO with a metabolic intermediate.

applied and environmental microbiology

Carbon Monoxide, Hydrogen, and Formate Metabolism during Methanogenesis from Acetate by Thermophilic Cultures of <i>Methanosarcina</i> and <i>Methanothrix</i> Strains

Carbon Monoxide, Hydrogen, and Formate Metabolism during Methanogenesis from Acetate by Thermophilic Cultures of Methanosarcina and Methanothrix Strains