ANAEROBIC FUNGI IN HERBIVOROUS ANIMALS

The conviction that all fungi required oxygen for growth was summarized in 1949 by J. N. Foster when he wrote, 'One of the major metabolic differences between moulds and bacteria is that there are no anaerobic moulds, either obligate or facultative'. For obligate anaerobes, this view held until 1975, when C. G. Orpin demonstrated that certain motile cells in the rumen of sheep, previously believed to be protozoan flagellates, were in fact zoospores of an obligately anaerobic fungus. Prior to this discovery it was assumed that only anaerobic bacteria and protozoa were involved in the hydrolysis of plant biomass in the rumen, but now it is acknowledged that anaerobic fungi participate in this process both in ruminants such as sheep and hindgut-fermenting herbivores such as horses. This review describes the life-cycles and physiology of anaerobic fungi, details their interactions with other rumen micro-organisms and assesses their contribution to the digestion of plant material in herbivores.