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Abstract
The population of filamentous acetate-utilizing methanogens in paddy field soils was 2.0×104 MPN/g dry soil in the submerged condition. They were able to form colonies in a deep agar medium, but not in a roll tube. Filamentous acetate-utilizing methanogens isolated from Kanagi, Japan (strain K-5) and Tsukuba, Japan (strain T-3) were divided into two types based on length of filaments. One type, strain K-5, formed a short chain which was dispersed easily by weak shaking. The other type, strain T-3, formed a long chain, which formed cotton-like flocs and was not dispersed by weak shaking. They had sheaths composed of a pair of adjacent membranes on the outside of the cell membranes. The 16S rRNA gene similarities of strain T-3 and K-5 to Methanosaeta concilii strain Opfikon were 100% and 99.5% respectively. Filamentous acetate-utilizing methanogens were also isolated from paddy field soils in various other regions of Japan. Our results suggest that Methanosaeta is universal in paddy soils and that it plays an important role in methane production from acetate.