The rumen anaerobic fungi

The anaerobic fungi represent a new group of organisms inhabiting the rumen ecosystem and possess a life cycle alternating between a motile flagellated form (zoospore) and a non-motile vegetative reproductive form (thallus). In vivo studies show extensive colonization of plant material suspended in the rumen indicating the fungi have a role in fiber digestion. Pure cultures of anaerobic fungi ferment cellulose to give lactate, acetate, CO2 and H2 as the major products. Ethanol and formate may also be produced. Fermentation of cellulose by the fungi in coculture with H2-utilizing methanogens results in a shift in the fermentation pattern favouring the production of H2 (utilized in the formation of CH4) and acetate at the expense of the electron-sink products, lactate and ethanol. It is postulated that the methanogens in reducing the partial pressure of H2, facilitate an increased passage of reducing equivalents towards the production of H2 via a pyridine-nucleotide (PN)-linked hydrogenase reaction. H2 is believed to be produced in microbodies of the fungi called hydrogenosomes which possess all of the enzymes necessary for this function including PN-linked hydrogenase. Absence of mitochondria and key electron transport components in these organisms indicate a dependence wholly on fermentative processes for growth. Anaerobic fungi also participate in hemicellulose and starch degredation but it is not yet clear whether they have a role in the degradation of lignin. Simple sugars (mono- and disaccharides) are readily utilized and their uptake is subject to similar regulatory constraints such as is found with other micro-organisms. Enzymological studies have revealed that anaerobic fungi release substantial amounts of endo-acting cellulase and protease, possibly giving them a competitive advantage over rumen bacteria in the degradation of plant structural material.