1. Results and Discussion
Alkaline phosphatase activity increases in phosphorus-limited Daphnia magna Stephen D. McCarthy1, Steven P. Rafferty2, and Paul C. Frost1 1 Department of Biology 2 Department of Chemistry
Introduction
Although zooplankton are known to release phosphatases (which cleave phosphate from dissolved phosphoesters), the effects of phosphorus nutrition on zooplankton phosphatase activity remains largely unexamined.
External APA
Results and Discussion
To determine how soon Daphnia respond to changes in P nutrition in their diet, we fed Daphnia P-rich algae (C:P ratio of 100) and then switched their food with P-poor algae (C:P ) for 2 or 4 days before the 15 day experiment ended.
After eating P-poor food for only 4 days, Daphnia begin to show an increase in their body C:P ratio and a decrease in external APA in the water.
This suggests that external APA in Daphnia responds to a change in the P content of their algae food source within days.
Internal APA
2 days on low P food
4 days on low P food
15 days on low P food
Daphnia C:P APA
Objectives
Determine whether the alkaline phosphatase activity (APA) in the zooplankton crustacean, Daphnia magna, is sensitive to the P-content of their algae food.
Assess whether D. magna change their APA within their body (internal APA) or their environment (external APA) with respect to their P-nutrition.
Of the three ages of Daphnia examined, high food C:P ratios increased the Daphnia body C:P ratio and their internal alkaline phosphatase activity.
Greater internal APA investment made by young and P-limited Daphnia supports the theory that mineral limitation in the diet hinders vital metabolic pathways (P uptake in this case).
Research Question
What affects APA in Daphnia magna?
Hypothesis
APA is affected by Daphnia P-nutrition, given the central role of alkaline phosphatase in P acquisition.
Conclusions
Alkaline phosphatase activity of D. magna is sensitive to the P content of the algal food.
Internal APA responds differently from external APA due to an unspecified mechanism
Changes in external APA can materialize within 4 days of switching Daphnia to P-poor food.
External APA
Anticipated Significance
Measuring internal alkaline phosphatase activity in zooplankton could be used a a bioindicator of P-deficiency in wild zooplankton populations.
Consideration of zooplankton phosphatases could help ecologists better understand the nutrient dynamics between bacteria, algae an zooplankton, and how they effect food web interactions.
Surprisingly, alkaline phosphatase activity was lowest for Daphnia consuming high C:P ratio food.
D. magna appear to internalize APA when they are fed algae of lower P content.
Methods
Daphnia neonates (<1 day old) were placed into one of seven food C:P ratios ( ) of Scenedesmus acutus algae. Juveniles were grown in a laboratory for 5, 10 or 15 days in P-free media, with 10 individuals per jar. The P-content of algae and Daphnia was determined in animals removed after each experiment.
APA assay: Alkaline phosphatase activity was determined by monitoring with a spectrophotometer the hydrolysis of para-nitrophenyl phosphate (pNPP) into phosphate (Pi) and the yellow para-nitrophenolate (pNP). Once removed from an experiment, Daphnia were either incubated in fresh water for 1 h (to measure external APA) or homogenized (to measure internal APA).
Daphnia C:P APA
Acknowledgments
Support was provided by an NSERC Discovery grant to P. Frost. S. D. McCarthy was partially supported with an NSERC USRA award in We thank Melanie Kingsbury, Nicole Goulding, Michelle Marcus and Ola Zalewski for their assistance with the experiments.
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