The bio-geochemical cycle of phosphorus is sig-ni®cantly in¯uenced by microbes in the aquatic environment. Organic phosphorus compounds are decomposed and mineralized by enzymatic complexes such as phosphatases produced by microbes. Enzymatic catalysis results in the production of orthophosphate, which can be used readily by primary producers. Even the smallest concentration of phosphate in water has an in¯uence over the production process in aquaculture systems. Extracellular alkaline phosphatase activity was observed in water and sediment media of aquaculture ponds with different management practices. Heterotrophic bacterial populations as well as phosphatase-producing bacterial populations were higher in sediments compared with water. In the freshwater ®sh ponds, Bacillus spp. were the dominant forms of bacteria producing phosphatase. The alkaline phosphatase activity of sediment was always higher than that of water. The partitioning of extracellular alkaline phosphatase in pond water by a 0.22-mm membrane ®lter revealed that a proportion was often free rather than cell associated and might have originated as free enzymes released by enriched sediments or by ®sh or microbes. In the case of water, although the dissolved alkaline phosphatase activity was lower than the total alkaline phosphatase activity, the former was nevertheless unimportant, as it constituted about 20% of the`total' activity. Free alkaline phosphatase activity shared a negative correlation with the orthophosphate concentration of water, whereas gross alkaline phosphatase activity was positively correlated with the total phosphorus and bacterial population of water.

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