Slow sand filters (SSF) are widely used to treat water for potable use. The process is dependent on the activities of complex microbial communities in the biofilm (schmutzdecke) layer. In this study, we generated a comprehensive view of three-domain microbiological complexity within a model SSF. DNA was analysed using a high-density microarray (PhyloChip) and rRNA library analysis. The Eukaryotic community was dominated by Cercozoa (Ebridd-type protists); these are likely to be involved in predation of other organisms in the schmutzdecke layer, thus proving opportunity for successional development. Ciliate protozoa, green microalgae, stramenopiles, amoeboid protozoa and fungi in the Phylum Ascomycota and the deep-branching Chytridiomycota were also detected. The Archaeal community was dominated by Euryarchaeota, and most were Halobacteriales. These organisms may contribute to filter function through removal of dissolved organic carbon, a primary treatment goal of these filters. Given that the Eukaryotic and Archaeal communities were examined using clone libraries, the expected richness of taxa present is expected to be greater than detected here and dependent on sampling effort (library size). The bacterial community was rich in taxa (21 Phyla) but was not dominated by any phylogenetic group. The successful function of SSF's relies on interactions between taxa (e.g. grazing of Cercozoa protists on bacteria), and removal of dissolved organic matter from the influent. Understanding the taxa and functions in these systems will aid in monitoring and managing SSF for optimal water quality.

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