Fumarate-reducing bacteria were sought from the main ruminal bacteria, Fibr
obacter succinogenes, Selenomonas ruminantium subsp, ruminantium, Selenomon
as ruminantium subsp. lactilytica, and Veillonella parvula reduced fumarate
by using H-2 as an electron donor. Ruminococcus albus, Prevotella ruminico
la, and Anaerovibrio lipolytica consumed fumarate, although they did not ox
idize H-2. Of these bacteria, V, parvula, two strains of Selenomonas, and F
. succinogenes had a high capacity to reduce fumarate. In all the fumarate-
reducing bacteria examined, fumarate reductase existed in the membrane frac
tion. Based on the activity per cell mass and the affinity of fumarate redu
ctase to fumarate, these bacteria were divided into two groups, which corre
sponded to the capacity to use H-2: A group of bacteria with higher activit
y and affinity were able to use H-2 as an electron donor for fumarate reduc
tion. The bacteria in this group should gain an advantage over the bacteria
in another group in fumarate reduction in the rumen. Cellulose digestion b
y R. albus was improved by fumarate reduction by S. lactilytica as a result
of an increased growth of R. albus, which may have been caused by the fact
that S. lactilytica immediately consumed H-2 produced by R. albus. Thus fu
marate reduction may play an important role in keeping a low partial pressu
re of H-2 in the rumen.