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Gnotobiotically reared lambs harboring cellulolytic species that differed in their capacity to produce hydrogen were used to determine the influence of the composition of the fibrolytic microbial community on the development and activities of hydrogen-utilizing microorganisms in the rumens of gnotobiotically reared lambs. The addition of fumarate to the rumen contents which stimulated [H.sub.2] utilization only by the ruminal microbiota inoculated with Fibrobacter succinogenes suggested the critical contribution of microbial communities in fumarate reduction in vivo.

The formyltetrahydrofolate synthetase gene (fhs) and acetyl-CoA synthase gene (acsB) libraries were employed to examine the acetogen diversity in the bovine rumen and forestomach of the tammar wallaby (Macropus eugenii), a native Australian marsupial. The observed differences in the community structures of the acsB and fhs libraries from the rumen and the tammar wallaby forestomach might have significance for overall hydrogenotrophy in both ecosystems.