Abstract
Three different ruminal anaerobic fungi, Neocallimastix frontalis PNK2, Sphaeromonas communis B7, and Piromonas communis B19, were grown axenically or in coculture with Methanobrevibacter smithii on xylan. N. frontalis and S. communis in monoculture and coculture accumulated xylobiose, xylose, and arabinose in the growth medium; arabinose was not metabolized, but xylobiose and xylose were subsequently used. The transient accumulation of xylose was much less evident in cocultures. Both the rate and extent of xylan utilization were increased by coculturing, and metabolite profiles became acetogenic as a result of interspecies hydrogen transfer; more acetate and less lactate were formed, while formate and hydrogen did not accumulate. For each of the three fungi, there were marked increases in the specific activities of extracellular xylanase (up to fivefold), alpha-l-arabinofuranosidase (up to fivefold), and beta-d-xylosidase (up to sevenfold) upon coculturing. The stimulating effect on fungal enzymes from coculturing with M. smithii was independent of the growth substrate, and the magnitude of the stimulation varied according to the enzymes and the incubation time. For an N. frontalis-M. smithii coculture, the positive stimulation was maintained during an extended (18-day) incubation period, and this affected not only hemicellulolytic enzymes but also polysaccharidase and glycoside hydrolase enzymes that were not involved in xylan breakdown. The specific activity of cell-bound endopeptidase was not increased under the coculture conditions used in this study. The higher enzyme activities in cocultures are discussed in relation to catabolite repression.