Oxygen consumption by ruminal microorganisms: protozoal and bacterial contributions.

The relative contributions to O2 consumption made by the protozoal and bacterial populations present within the rumen were determined by using an open-type oxygen electrode system. Measurements indicated that two separate microbial populations contributed approximately equally to ruminal O2 consumption over the O2 concentration range experienced in situ (0.25 to 1.0 microM). The populations were observed to consume O2 under liquid-phase O2 concentrations of up to 7 microM, above which point rapid inactivation of O2 utilization was observed. Km values for the mixed population of bacteria and protozoa were 0.36 +/- 0.17 and 3.2 +/- 0.4 microM at concentrations of less than 1.6 and greater than 1.6 microM, respectively. O2 affinity values obtained for both the protozoal and bacterial populations were similar. O2 affinities of the isolated entodiniomorphid ciliates Polyplastron multivesiculatum and Eudiplodinium maggii showed O2 inhibition thresholds of 10 and 5, respectively, and apparent half-saturation constants (Km values) of 1.7 and 5.2 microM O2, respectively. Corresponding Vmax values were 7.8 microM O2 per min per 10(5) organisms for P. multivesiculatum and 3.6 microM O2 per min per 10(5) organisms for E. maggii. Mass spectroscopic analysis detected average rates of H2 production of 12.0 and 3.7 microM H2 per min per 10(5) organisms for P. multivesiculatum and E. maggii, respectively. Trace levels of dissolved O2 (less than 0.25 microM) stimulated the H2 production rate of E. maggii eightfold but inhibited that of P. multivesiculatum by 18%.