The microbial ecology of the rumen is very complex. Different species of bacteria, protozoa, and fungi are involved in digestion of plant material in ruminants. In spite of complicated interrelationships among the various groups of microorganisms in the rumen ecosystem, Bacteria and Archaea are believed to play a major role because of their numerical predominance and metabolic diversity. In this work we are presenting the results for microbial population dynamics of rumen microbes during two-stage anaerobic digestion of grass. The reactors were inoculated with fresh rumen content. Fluorescent in situ hybridization, confocal laser scanning microscopy and epifluorescence microscopy were employed for microbial investigation. It was observed that Bacteria dominated in the hydrolytic reactor (1st stage) whereas Archaea were predominant in the methanogenic reactor (2nd stage). The stability of the methanogenic reactor was result of the dominance of Methanosaeta species (mainly the filamentous type).

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