Phenotypic and phylogenetic characterization of dominant culturable methanogens isolated from ricefield soils

Populations of the four major trophic groups of methanogens were enumerated by most probable numbers (MPN) on selective media in a sample of 13 soils representative of major types of rice soils. Dominant strains were isolated and their phenotypic and phylogenetic characteristics were studied. MPN counts ranged from 10(2) to 10(6) g(-1) d.w. on H-2, from < 10 to 10(4) g(-1) d.w. on acetate, from <10 to 10(5) on methanol, and from 50 to 10(6) on formate. In most soils, counts of hydrogenotrophs were higher than counts of acetotrophs, partly because acetotrophs were aggregated sarcinae difficult to separate into individual cells. Methylotrophs other than acetotrophic sarcinae were not recorded. In most soils, rods enumerated on formate were 5-400 times less abundant than those enumerated on H-2, indicating that hydrogenotrophic-nonformatotrophic rods are probably dominant in ricefields. Dominant strains isolated comprised: 15 hydrogenotrophic-nonformatotrophic rods affiliated to Methanobacterium bryantii; three hydrogenotrophic-formatotrophic rods affiliated to Mb. formicicum; one hydrogenotrophic-formatotrophic rod not affiliated to a sequenced species; two sarcinae affiliated to Methanosarcina barkeri and Methanosarcina mazei; and one irregular coccus affiliated to Methanoculleus marisnigri - a species so far isolated from marine sediments only. Results from classical counts of methanogens and strains isolated from ricefields suggest the dominance of Methanobacterium spp. (mostly responsible for CH4 production from H-2/CO2) and Methanosarcina spp. (mostly responsible for CH4 production from acetate among culturable organisms. Both genera are probably ubiquitous, In particular, Mb. bryantii was isolated from 12 of the 13 soils. (C) 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V.