Fate of metabolic hydrogen in the rumen.

Introduction T h e first axiom of ruminology appears to be: ‘The rumen is a strictly anaerobic system’ (Hungate, 1966) but before we accept this we should examine the evidence for and against this assertion. Probably the strongest evidence for anaerobiosis comes from the studies of the reactions in the rumen, from the stoichiometries observed and from the low oxidation-reduction potential obtaining in the rumen, although some discrepancies exist (Seeley, Armstrong & MacRae, I 969 ; Demeyer, Nevel, Henderickx & Martin, 1970). Many important rumen organisms grown in pure culture are sensitive to oxygen, but the isolated microbes, apart from being separated by countless generations from the original inocula, might behave quite differently in their natural habitat. The evidence that the rumen might be a partially aerobic system is not conclusive, but plentiful. It can be easily demonstrated in vitro that the rumen contents can utilize oxygen with an alteration of the fermentation pattern but with no inhibition of the rate of utilization of substrates (Czerkawski & Breckenridge, 1969). The rumen contents are surrounded by tissues rich in capillaries containing blood with high-oxygen tension compared with the oxygen tension in the rumen which is never more and rarely less than 1% (see review, Czerkawski, 1969). Facultatively anaerobic micro-organisms exist in the rumen and there is evidence of some processes in the rumen that are more common under aerobic conditions, such as oxidation of n-alcohols (Czerkawski & Breckenridge, 1972) or desaturation of long-chain acids (Patton, McCarthy & Griel, 1968 ; Sklan, Volcani & Budowski, 1971). The metabolic hydrogen and possibly oxygen are of vital importance in the energy exchanges in the rumen, and any attempt to manipulate this balanced ecosystem to increase the efficiency of feed conversion, must take the fate of hydrogen and the possible participation of oxygen into consideration.