Soluble microbial products (SMP) are organic compounds produced by activated sludge microorganisms as they degrade substrates. They include by-products of microbial activity, death and lysis. The available literature does not reveal how SMP influence microbial community composition. In this regard, we microscopically studied changes in composition of microbial communities, especially protozoa and metazoa, under the influence of increased as well as reduced levels of SMP. The presence of SMP at high level significantly caused changes in microbial community composition. Microbial species shifted from attached ciliates (12-175 μm) to free-swimming and crawling ciliates (35-330 μm) and then invertebrates, which included rotifers (0.2-1 mm) and nematodes (1-50 mm). The shift of small-size microorganisms to large ones was observed as one of the most significant influences of SMP. Attached ciliates reappeared when we removed the SMP that had accumulated in the bioreactors - we have called this as the resurrection phenomenon of microorganisms. Such rapid changes in microbial community composition were not observed in the experiment with low concentration of SMP. Overall, the results suggest that accumulation of SMP is one of the intrinsic regulatory mechanisms that control viability and dormancy of microbial communities in activated sludge.
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