Taxon-specific associations between protozoal and methanogen populations in the rumen and a model rumen system

Methanogen populations in the rumen and in model rumen systems (operated over a 240-h period) were studied using the small subunit (SSU) rRNA phylogenetic framework for group-specific enumerations.Representatives of the family Methanobacteriaceae were the most abundant methanogen population in the rumen, accounting for 89.3% ( t 1.02%) of total archaea in the rumen fluid and 99.2% ( k 1.8%) in a protozoa1 fraction of rumen fluid.Their percentage of archaea in the model rumen systems declined from 84% ( k 8.5%) to 54% ( + 7.8%) after 48 h of operation, correlated with loss of protozoa from these systems.The Methanomicrobiales, encompassed by the families Methanomicrobiaceae. Methanocorpusculaceae, and Methanospirillaceae were the second most abundant population and accounted for 12.1% ( & 2.15%) of total SSU rRNA in rumen fluid.Additionally this group was shown to be essentially free living, since only a negligible hybridization signal was detected with the ruminal protozoa1 fraction.This group constituted a more significant proportion of total archaea in whole rumen fluid, 12.1's ( k 2.1'%1) and model rumen fluid containing no protozoa (26.3 f 7.7%)).In contrast, the Methanosarcinales, generally considered the second most abundant population of rumen methanogens, accounted for only 2.8% ( k 0.3%)) of total archaeal SSU rRNA in rumen fluid.0 1998