The effect of microbial composition on the methanogenic degradation of cellulose was studied using two lines of anaerobic cellulose-fermenting methanogenic microbial cultures at two different temperatures: that at 15 degreesC being dominated by Methanosaeta and that at 30 degreesC by Methanosarcina. In both cultures, CH4 production and acetate consumption were completely inhibited by either 2-bromoethanesulfonate or chloroform, whereas H-2 consumption was only inhibited by chloroform, suggesting that homoacetogens utilized H-2 concomitantly with methanogens. Hydrogen was the intermediate that was consumed first, while acetate continued to accumulate. At 15 degreesC, acetoclastic methanogenesis smoothly followed H-2-dependent CH4 production. Fluorescence in situ hybridization showed that populations of Methanosaeta steadily increased with time from 5 to 25% of total cell counts. At 30 degreesC, two phases of CH4 production were obtained, with acetate consumed after the abrupt increase of Methanosarcina from 0 to 45% of total cell counts. Whereas populations of Methanosaeta were able to adapt after transfer from 15 to 30 degreesC, those of Methanosarcina were not, irrespective of during which phase the cultures were transferred from 30 degreesC to 15 degreesC. Our results thus show that the community structure of methanogens indeed affects the function of a cellulose-fermenting community with respect to temperature response.