Responses of alkaline phosphatase activity to phosphorus stress in Daphnia magna

We examined how alkaline phosphatase (AP) activity within the bodies and in the materials released by the crustacean Daphnia magna responds to variable algal food phosphorus (P)-content. We found that Daphnia eating P-poor food (C:P approximately 700) had significantly higher AP activity in their bodies on a mass-specific basis compared with individuals eating P-rich food (C:P approximately 100). This dietary P effect on AP activity was not altered by Daphnia starvation but was partially related to differences in the P concentration of animal body homogenates. By contrast, poor P-nutrition of Daphnia lowered AP activity in released materials compared with that measured from their P-sufficient conspecifics. Moreover, AP activity in Daphnia release was lowest in animals consuming P-poor food for longer time periods. Our results support the hypothesis that AP activity increases inside P-limited Daphnia as a mechanism to increase P-acquisition and retention from ingested algae in these nutritionally stressed animals. The lower level of AP activity present in the water of P-deprived animals could reflect a change from largely free to membrane-bound AP isotypes in the digestive tracts of P-starved animals or a decrease in the shedding of membrane-anchored AP from their intestinal lining. These results supplement accumulating evidence that P-poor algal food reduces the dietary mineral P available to Daphnia. In addition, animal body AP activity measurements, with some refinement, may prove useful as an in situ indicator of P-stress in aquatic consumers.