How stable is stable? Function versus community composition

The microbial community dynamics of a functionally stable, well-mixed, meth anogenic reactor fed with glucose were analyzed over a 605-day period. The reactor maintained constant pH and chemical oxygen demand removal during th is period. Thirty-six rm clones from each of seven sampling events were ana lyzed by amplified ribosomal DNA restriction analysis (ARDRA) for the Bacte ria and Archaea domains and by sequence analysis of dominant members of the community. Operational taxonomic units (OTUs), distinguished as unique ARD RA patterns, showed reproducible distribution for three sample replicates. The highest diversity was observed in the Bacteria domain. The 16S ribosoma l DNA Bacteria clone library contained 75 OTUs, with the dominant OTU accou nting for 13% of the total clones, but just 21 Archaea OTUs were found, and the most prominent OTU represented 50% of the clones from the respective l ibrary. Succession in methanogenic populations was observed, and two period s were distinguished: in the first, Methanobacterium formicicum was dominan t, and in the second, Methanosarcina mazei and a Methanobacterium bryantii- related organism were dominant. Higher variability in Bacteria populations was detected, and the temporal OTU distribution suggested a chaotic pattern . Although dominant OTUs were constantly replaced from one sampling point t o the next, phylogenetic analysis indicated that inferred physiologic chang es in the community were not as dramatic as were genetic changes. Seven of eight dominant OTUs during the first period clustered with the spirochete g roup, although a cyclic pattern of substitution occurred among members with in this order. A more flexible community structure characterized the second period, since a sequential replacement of a Eubacterium-related organism b y an unrelated deep-branched organism and finally by a Propionibacterium-li ke species was observed, Metabolic differences among the dominant fermenter s detected suggest that changes in carbon and electron flow occurred during the stable performance and indicate that an extremely dynamic community ca n maintain a stable ecosystem function.