Flexible community structure correlates with stable community function in methanogenic bioreactor communities perturbed by glucose

Methanogenic bioreactor communities mere used as model ecosystems to evalua te the relationship between functional stability and community structure. R eplicated methanogenic bioreactor communities with two different community structures were established. The effect of a substrate loading shock on pop ulation dynamics in each microbial community was examined by using morpholo gical analysis, small-subunit (SSU) rRNA oligonucleotide probes, amplified ribosomal DNA (rDNA) restriction analysis (ARDRA), and partial sequencing o f SSU rDNA clones. One set of replicated communities, designated the high-s pirochete (HS) set, was characterized by good replicability, a high proport ion of spiral and short thin rod morphotypes, a dominance of spirochete-rel ated SSU rDNA genes, and a high percentage of Methanosarcina-related SSU rR NA. The second set of communities, designated the low-spirochete (LS) set, was characterized by incomplete replicability, higher morphotype diversity dominated by cocci, a predominance of Streptococcus-related and deeply bran ching Spirochaetales-related SSU rDNA genes, and a high percentage of Metha nosaeta-related SSU rRNA. In the HS communities, glucose perturbation cause d a dramatic shift in the relative abundance of fermentative bacteria, with temporary displacement of spirochete-related ribotypes by Eubacterium-rela ted ribotypes, followed by a return to the preperturbation community struct ure. The LS communities were less perturbed, with Streptococcus-related org anisms remaining prevalent after the glucose shock, although changes in the relative abundance of minor members were detected by morphotype analysis. A companion paper demonstrates that the more stable LS communities were les s functionally stable than the HS communities (S. A. Hashsham, A. S. Fernan dez, S, L. Dollhopf, F. B. Dazzo, R, F, Hickey, J, M. Tiedje, and C, S. Cri ddle, Appl. Environ. Microbiol. 66:4050-4057, 2000).