The rumen anaerobic fungi

Abstract

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.

Effect of anaerobic fungi on in vitro feed digestion by mixed rumen microflora of buffalo

“…The findings of this experiment indicate that a fungal inoculum in the rumen liquor of buffalo accelerates the rate of fermentation, digestibility of lignocellulosic feeds and the synthesis of enzymes involved in fibre degradation and confirm the previous reports on the role of rumen anaerobic fungi in fibre degradation [2][3][4]. The fungal strain FNG5 identified as Piromyces sp.…”

“…The isolates were identified up to the genus level based on the type of thallus development and number of zoospore flagella [3]. The isolates FNG5 (Piromyces sp.…”

“…Manipulation of rumen fermentation by increasing the number or activity of lignocellulolytic micro-organisms in the rumen is one such step in this direction [1]. Among the rumen microbes, anaerobic fungi are considered to be important due to the production of highly active enzymes for lignocellulose degradation [2,3] and have a unique ability to break and penetrate the fibrous feed particles through fungal mycelium, breaking the feed particles and providing more surface area for the action of other microbes [4]. Thus rumen fungi play an important catalytic role in the digestion of poor quality fibrous feeds.…”

The effects of feed blocks containing tomato and cucumber by-products on <i>in vitro</i> ruminal fermentation, microbiota, and methane production

“…Results from different experiments differ depending on factors such as FB composition, level of concentrate replacement and animal species used as experimental animals. A reduction in the abundance or activity of fungi with FB-diets could also contribute to the decreased digestibility of fibre, as fungi are relevant in fibre degradation and lignin solubilization (Mountfort, 1987).…”

Sequencing of a 1,3-1,4-beta-D-glucanase (lichenase) from the anaerobic fungus Orpinomyces strain PC-2: properties of the enzyme expressed in Escherichia coli and evidence that the gene has a bacterial origin

“…Obligately anaerobic fungi are part of the natural microflora of the alimentary tracts of many herbivorous mammals (44). Since the first anaerobic fungus, Neocallimastix frontalis, was isolated in 1975 from the rumen of sheep (48), at least 13 different anaerobic fungi have been isolated from ruminant and nonruminant herbivores (13,49).…”