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which always ferment. Fungi of this type are found
in nutrient-enriched waters, where fermentable sub-
strates are abundant.
4 A few obligately anaerobicChytridiomycota occur
in the rumen and are discussed below.

The microbial consortium of the rumen
and other gut environments

A specialized group of Chytridiomycota, protozoa,
and bacteria grow in the rumen of animals such as
cows, sheep, goats, and camels. These rumen microbes
(Fig. 8.7) are obligate anaerobeswhich are killed by
exposure to oxygen. They live as part of an intimate
and complex microbial community, or consortium, in
the animal’s foregut, which has been modified to form
a series of fermentation chambers. Here the intake of
food is repeatedly regurgitated, rechewed, and reswal-
lowed, so that it is broken up to expose a maximum
surface area for digestion by the community of rumen
organisms. After a residence time of 1–2 days, the
rumen contents are passed along the digestive tract
where many of the microbes are destroyed, providing
amino acids for the animal host. The breakdown prod-
ucts from plant constituents such as cellulose and
hemicelluloses are similarly absorbed. However, the end
products of digestion in the rumen are not simple
sugars but instead are a range of short-chain volatile
fatty acids (VFAs), principally acetic acid, propionic acid,
and butyric acid, together with CO 2 , methane, ammonia,
and occasionally lactic acid.
Bacteria dominate the rumen community in terms
of both their numbers and types. Methanogenic
(methane-producing) archaea also are prevalent and
generate large amounts of methane as a metabolic
end-product. In addition, there are many protozoa

which ingest bacteria and other small particles. The final

components are the rumen fungi, all of which are obli-

gately anaerobic members of the Chytridiomycota and

currently are classified in five genera: Neocallimastix,

Caecomyces, Piromyces, Orpinomyces, and Ruminomyces.

These fungi have a major role in degrading the plant

structural carbohydrates, such as cellulose and hemi-

celluloses. Their zoospores show chemotaxis to plant

sugars in culture, and rapidly accumulate on chewed

herbage in the rumen. Then they encyst and germinate

to produce rhizoids that penetrate the plant tissue and

release enzymes to degrade the plant cell walls.

Mixed acid fermentation

The rumen chytrids are unusual among fungi because

they have a mixed acid fermentation, similar to

that of the lactic acid bacteria (Theodorou et al. 1996).

The main products of this fermentation are formic acid

(HCOO−), acetic acid, lactic acid, ethanol, CO 2 , and

H 2 , shown in the shaded boxes in Fig. 8.9. The initial

fermentation of plant sugars such as glucose is carried

out by the Embden–Myerhof pathway, leading to

the production of ethanol and lactic acid (derived

from pyruvate). This occurs in the cytosol of the

chytrid cells. But some of the pyruvate is converted

to malate (malic acid) which then enters a special type

of organelle, termed the hydrogenosomebecause it

produces molecular hydrogen. Hydrogenosomes are

characteristic of rumen chytrids and anaerobic protozoa.

They are functionally equivalent to the mitochondria

of aerobic organisms, and are involved in the genera-

tion of ATP by electron transfer (Fig. 8.8). The main

end-products released from the hydrogenosome are

acetate, formate, CO 2 , and H 2. The molecular hydro-

gen can then be used as an energy source for the

ENVIRONMENTAL CONDITIONS 149

Fig. 8.7Part of a rumen consortium,
comprising protozoa, chytrids, and
bacteria. (Reproduced by courtesy of
M. Yokayama & M.A. Cobos; Michigan
State University.)