Rumen anaerobic fungi
Institute of Animal Physiology and Genetics, Czech Academy of Sciences
Prague, Czech Republic
Rumen or strictly speaking gut fungi inhabit the gastrointestinal tract of herbivores, especially ruminants, and make a significant contribution to the digestion of plant structural polysaccharides. Anaerobic fungi represent, among basal Fungi, the phylogenetically unique separate clade of Neocallimastigomycetes. Six cultivable genera have been described up to now, namely the monocentric Neocallimastix, Caecomyces and Piromyces and the polycentric Anaeromyces, Orpinomyces and Cyllamyces. Recent research however indicates the existence of several new taxa in ruminant and non-ruminant animals. Every trait of these Eukaryots is singular and exceptional. Gut fungi are the only representatives of kingdom Fungi demanding anaerobiosis, and possessing hydrogenosomes instead of mitochondria. Hydrogenosomes under anoxic conditions decarboxylate malate into acetate, CO2, and H2 with concomitant production of energy in the form of ATP. Hydrogen is utilized by methanogens and the syntrophic co-cultures with Archaea not only increase amount of fungal biomass and markedly improve rate and extent of fungal cellulolysis and xylanolysis, but for some Neocallimastigomycetes species is also of vital importance. Genetically, anaerobic fungi represent organisms with the lowest G+C content (17%) ever found in Nature and with duplication of a variety of important hydrolytic genes, which is a trait not common among eukaryotes. Enzymologically, anaerobic fungi produce a broad range of excellent hydrolases, including cellulases, xylanases, mannases, esterases, glucosidases, and glucanases, which can be organized in cellulosomes, the enzyme factories known only in some bacteria, carrying the most active (hemi) cellulases. Enzymes arranged in cellulosomal complexes containing endo- and exo-cellulases, hemicellulases and esterases have been described for Piromyces, Orpinomyces and Neocallimastix species. Due to this special feature anaerobic gut fungi exhibit higher activities then commercial enzyme products derived from Trichoderma reeseii or Aspergillus niger.