Acetate is the predominant end product of the chemoheterotrophic lifestyles of the mixed population of microorganisms that live in the reticulo-rumen.  Many species produce acetate but one group; the homoacetogens are particularly important because they can live both as chemoautotrophs (like methanogens) generating ATP in association with a chemiosmotic mechanism, and as chemoheterotrophs deriving ATP also by phosphoryl group transfer in a fermentation pathway.  They have only recently been identified in the reticulo-rumen and since they produce mainly acetate as the end product of their metabolism they are undoubtedly partly responsible for the predominance of acetate as an end product of microbial metabolism.