3.3 Neocallimastigomycota

The obligately anaerobic chytrids occur in terrestrial and aquatic anaerobic environments and are outstandingly important economically because they are crucially important to the rumen microbiome within the rumen and hindgut of most large mammalian herbivores, including all farmed animals (Trinci et al., 1994). Right up to the 1980s it was believed that obligately anaerobic fungi did not exist but these chytrids are truly obligate anaerobes that lack mitochondria but contain hydrogenosomes instead. These chytrids are potent producers of enzymes needed to degrade cellulose. Their own carbon metabolism relies on fermentation of glucose to acetate, lactate, ethanol and hydrogen. They possess an organelle called a hydrogenosome that generates ATP and appears to be a degenerate mitochondrion lacking a genome (Trinci et al., 1994; van der Giezen, 2002). CLICK HERE for further information about the metabolism of anaerobic chytrids.

Obligately anaerobic chytrids have a crucial role in the primary colonisation and enzymic degradation of lignocellulose in plant materials eaten by herbivores and have therefore been crucial to the evolution of herbivores, and to the prosperity of animal husbandry since humans first domesticated animals (see Section 15.5 Anaerobic fungi and the rise of the ruminants for more information). Although they are morphologically similar to other chytrids, differences are sufficient for them to be placed in their own phylum, called Neocallimastigomycota, which is a focus for extensive research (Edwards et al., 2017; Gruninger et al., 2014; Hibbett et al., 2007; Powell & Letcher, 2014; Wang et al., 2017).

Anaerobic chytrids may be monocentric or polycentric, and zoospores may be multiflagellate or uniflagellate. About 8 genera (comprising about 20 species) have been described in the single Order Neocallimastigales: examples are Neocallimastix frontalis, the original isolate from the domestic cow and notable for multiflagellate zoospores; Orpinomyces, which occurs widely in cattle; and Piromyces, which has been isolated from horses and elephants.

Rumen chytrid zoospores encyst on plant material in the animal’s rumen and intestine, forming a thallus with a well-developed rhizoidal system that penetrates the plant material. These chytrids are potent producers of enzymes needed to degrade cellulose (animals do not produce their own cellulose-degrading enzymes) so they are crucial to the herbivore’s ability to digest its food. The chytrids pass from mother to offspring, probably through licking or faecal contamination of feed. No sexual stage is known.

Updated July, 2019