The tendency to aggregate is a common trait among microbial cells in contrived experimental laboratory environments, controlled industrial processes, and in nature.1-3 When aggregation takes place on a surface, it often leads to the formation of biofilms,4,5 while aggregation of cells in aqueous suspensions results in the formation of flocs.6 Flocs formed in the water column of the ocean have been termed "marine

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snow."7 Regardless of what form the aggregation process takes, cells engaged in this behavior become physically associated with each other through extracellular polymeric substances (EPS) secreted by at least a portion of the aggregated microbial community. Some of the cells secrete EPS that contributes to the structure of the floc or biofilm, which promotes the formation of close, stable, physical associations between cells of the same and different populations.8 The physical associations lead to cooperation among cells of the populations present and the formation of consortia.9,10 EPS also form matrices that function as diffusion barriers between the cells and the external environment,11,12 which in some instances, protect the cells from insults of the surrounding environment.13 Extracellular enzymes secreted by floc-encapsulated microorganisms represent another class of EPS that participate in the transformation of toxic chemical species in the environment to nontoxic products,14 as well as the transformation of nutritionally valuable chemical species in the environment for uptake and utilization by members of the floc-associated microbial community.15

In this chapter, we will describe the exoenzymes involved in the degradation of polymeric organic compounds in the activated sludge process of wastewater treatment. Specifically we will describe the distribution of phosphatase (PO4ase) exoenzymes within flocs produced during the activated sludge process, and the floc-associated microbial populations that synthesize and secrete this important class of enzymes.

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