The anaerobic fungi in rumen fibre digestion

Large populations of anaerobic fungi have been shown to colonize plant fragments in the rumen of cattle and sheep on fibrous diets. Plant material entering the rumen is rapidly colonized by flagellated zoospores that attach and produce hyphae that grow and penetrate deeply into the plant tissues. The growing fungus develops an extensive rhizoid for anchorage and supply of nutrients, and (later) a sporangium. Rupture of the sporangium and release of zoospores completes the developmental cycle. The fungi colonize mainly plant vascular tissues and highest numbers are found on stem fragments. The more fibrous the diet the greater the fungal population, and anaerobic fungi are absent from rumen contents of sheep grazing soft leafy plant materials. The major route of fungal invasion is via areas of damaged epidermis. The fungi can be grown in culture on a wide range of solid substrates. Cellulase activity has been demonstrated in anaerobic cultures containing strips of filter paper. Fungi similar to the rumen organisms have been shown to be present in the foregut and hindgut of a wide range of different types of herbivorous animals. The extent of colonization and growth on fibrous plant fragments suggests a significant role in cellulose digestion in the rumen. The ability of fungi to penetrate deeply into tissues not normally accessable to bacteria also suggest a special role for anaerobic fungi in rumen fibre digestion.