Methanogenium marinum sp nov., a H-2-using methanogen from Skan Bay, Alaska, and kinetics of H-2 utilization

A methanogen, strain AK-1, was isolated from permanently cold marine sediments, 38- to 45-cm below the sediment surface at Skan Bay, Alaska. The cells were highly irregular, nonmotile coccoids (diameter, 1 to 1.2 mum), occurring singly. Cells grew by reducing CO2 with H-2 or formate as electron donor. Growth on formate was much slower than that on H-2. Acetate, methanol, ethanol, 1- or 2-propanol, 1- or 2-butanol and trimethylamine were not catabolized. The cells required acetate, thiamine, riboflavin, a high concentration of vitamin B-12, and peptones for growth; yeast extract stimulated growth but was not required. The cells grew fastest at 25degreesC (range 5degreesC to 25degreesC), at a pH of 6.0-6.6 (growth range, pH 5.5-7.5), and at a salinity of 0.25-1.25 M Na+. Cells of this and other H-2-using methanogens from saline environments metabolized H2 to a very low threshold pressure (less than 1 Pa) that was dependent on the methane partial pressure. We propose that the threshold pressure may be limited by the energetics of catabolism. The sequence of the 16S rDNA gene of strain AK-1 was most similar (98%) to the sequences of Methanogenium cariaci JR-1 and Methanogenium frigidum Ace-2. DNA-DNA hybridization between strain AK-1 and these two strains showed only 34.9% similarity to strain JR-1 and 56.5% similarity to strain Ace-2. These analyses indicated strain AK-1 should be classified as a new species within the genus Methanogenium. Phenotypic differences between strain AK-1 and these strains (including growth temperature, salinity range, pH range, and nutrient requirements) support this. Therefore, a new species, Methanogenium marinum, is proposed with strain AK-1 as type strain.