A methanogenic bacterium, commonly seen in digested sludge and referred to as the "fat rod" or Methanobacterium soehngenii, has been enriched to a monoculture and is characterized. Cells are gramnegative, non-motile and appear as straight rods with flat ends. They form filaments which can grow to great lengths. The structure of the outer cell envelop is similar to Methanospirillum hungatii. The organism grows on a mineral salt medium with acetate as the only organic component. Acetate is the energy source, and methane is formed exclusively from the methyl group. Acetate and carbon dioxide act as sole carbon source and are assimilated in a molar ratio of about 1.9:1. The reducing equivalents necessary to build biomass from these two precursors are obtained from the total oxidation of some acetate. Hydrogen is not used for methane formation and is not needed for growth. Formate is cleaved into hydrogen and carbon dioxide. Coenzyme M was found to be present at levels of 0.35 nmol per mg of dry cells and F420 amounted to 0.55 microgram per mg protein. The mean generation time was 9 days at 33 degrees C.
A new genus of methanogenic bacteria is described, which was isolated from a mesophilic sewage digester. It is most probably the filamentous bacterium, earlier referred to as Methanobacterium soehngenii, "fat rod" or "acetate organism". The single non-motile, non-sporeforming cells are rod-shaped (0.8 x 2 µm) and are normally combined end to end in long filaments, surrounded by a sheath-like structure. The filaments form characteristic bundles. Methanothrix soehngenii decarboxylates acetate, yielding methane and carbon dioxide. Other methanogenic substrates (H 2 -C0 2 , formate, methanol, methylamines) are not used for growth or methane formation. Formate is split into hydrogen and carbon dioxide. The temperature optimum for growth and methane formation is 37" C and the optimal pH range is 7.4-7.8. Sulfide and ammonia serve as sulfur and nitrogen source respectively. Oxygen completely inhibits growth and methane formation, but the bacteria do not loose their viability when exposed to high oxygen concentrations. 100 mg/I vancomycin showed no inhibition of growth and methanogenesis. No growth and methane formation was observed in the presence of: 2-bromoethanesulfonic acid, viologen dyes, chloroform, and KCN. The bacterium has a growth yield on acetate of 1.1-1.4 g biomass per mo! acetate. The apparent "Ks" of the acetate conversion system to methane and carbon dioxide is 0.7 mmol/I. The DNA base composition is 51.9 mol % guanine plus cytosine. The name Methanothrix is proposed for this new genus of filamentous methane bacterium. The type species, Methanothrix soehngenii sp. nov., is named in honor of N. L. S6hngen.
Although a gas proportional counter is the most convenient method of measuring the radioactivity of fixed gases such as methane, it cannot be used when high nonradioactive concentrations of methane are present in the gas phase, due to quenching. If only methane and carbon dioxide are present in radioactive form in the gas phase, a liquid scintillation method for measuring these substances can be used. The procedure is described in detail, and the solubility of methane in liquid scintillation cocktails is determined.
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