Functional and structural response of a cellulose-degrading methanogenic microbial community to multiple aeration stress at two different temperatures

Two cellulose-fermenting methanogenic enrichment cultures originating from rice soil, one at 15 degreesC with Methanosaeta and the other at 30 degreesC with Methanosarcina as the dominant acetoclastic methanogen, both degraded cellulose anaerobically via propionate, acetate and H(2) to CH(4). The degradation was a two-stage process, with CH(4) production mainly from H(2)/CO(2) and accumulation of acetate and propionate during the first, and methanogenic consumption of acetate during the second stage. Aeration stress of 12, 24, 36 and 76 h duration was applied to these microbial communities during both stages of cellulose degradation. The longer the aeration stress, the stronger the inhibition of CH(4) production at both 30 degreesC and 15 degreesC. The 72 h stressed culture at 30 degreesC did not fully recover. Aeration stress at 30 degreesC exerted a more pronounced effect, but lasted for a shorter time than that at 15 degreesC. The aeration stress was especially effective during the second stage of fermentation, when consumption of acetate (and to a lesser extent propionate) was also increasingly inhibited as the duration of the stress increased. The patterns of CH4 production and metabolite accumulation were consistent with changes observed in the methanogenic archaeal community structure. Fluorescence in situ hybridization showed that the total microbial community at the beginning consisted of about 4% and 10% archaea, which increased to about 50% and 30% during the second stage of cellulose degradation at 30 degreesC and 15 degreesC respectively. Methanosarcina and Methanosaeta species became the dominant archaea at 30 degreesC and 15 degreesC respectively. The first round of aeration stress mainly reduced the non-Methanosarcina archaea (30 degreesC) and the non-Methanosaeta archaea (15 degreesC). Aeration stress also retarded the growth of Methanosarcina and Methanosaeta at 30 degreesC and 15 degreesC respectively. The longer the stress, the lower was the percentage of Methanosarcina cells to total microbial cells after the first stress at 30 degreesC. A later aeration stress decreased the population of Methanosarcina (at 30 degreesC) in relation to the duration of stress, so that non-Methanosarcina archaea became dominant. Hence, aeration stress affected the acetotrophic methanogens more than the hydrogenotrophic ones, thus explaining the metabolism of the intermediates of cellulose degradation under the different incubation conditions.