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 degrees C with Methanosarcina as the dominant acetoclastic methanogen, both degrade d cellulose anaerobically via propionate, acetate and H-2 to CH4. The degra dation was a two-stage process, with CH4 production mainly from H-2/CO2 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 stron ger the inhibition of CH4 production at both 30 degreesC and 15 degreesC. T he 72 h stressed culture at 30 degreesC did not fully recover. Aeration str ess at 30 degreesC exerted a more pronounced effect, but lasted for a short er time than that at 15 degreesC. The aeration stress was especially effect ive during the second stage of fermentation, when consumption of acetate (a nd to a lesser extent propionate) was also increasingly inhibited as the du ration of the stress increased. The patterns of CH4 production and metaboli te accumulation were consistent with changes observed in the methanogenic a rchaeal community structure. Fluorescence in situ hybridization showed that the total microbial community at the beginning consisted of about 4% and 1 0% archaea, which increased to about 50% and 30% during the second stage of cellulose degradation at 30 degreesC and 15 degreesC respectively. Methano sarcina and Methanosaeta species became the dominant archaea at 30 degreesC and 15 degreesC respectively. The first round of aeration stress mainly re duced the non-Methanosarcina archaea (30 degreesC) and the non-Methanosaeta archaea (15 degreesC). Aeration stress also retarded the growth of Methano sarcina and Methanosaeta at 30 degreesC and 15 degreesC respectively. The l onger the stress, the lower was the percentage of Methanosarcina cells to t otal microbial cells after the first stress at 30 degreesC. A later aeratio n stress decreased the population of Methanosarcina (at 30 degreesC) in rel ation 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 inte rmediates of cellulose degradation under the different incubation condition s.