Alkaline phosphatase activity in symbiotic dinoflagellates (zooxanthellae) as a biological indicator of environmental phosphate exposure

Alkaline phosphatase activity (APA) is an indicator of phosphorus status in marine plants. We examined APA in the symbiotic dinoflagellate Symbiodinium bermudense, which occurs with the sea anemone Aiptasia pallida. We have developed an assay for APA that serves as a biological indicator of inorganic phosphate (Pi) exposure in coral reef environments and phosphorus deficiency in zooxanthellae. Zooxanthellae maintained in culture responded to a lack of Pi with elevated levels of APA. Laboratory experiments with A. pallida demonstrated that this effect occurs in symbiosis, although at a longer timescale. The addition of Pi to ambient sea water resulted in higher zooxanthellae density in unfed anemones compared to unfed controls maintained without additional Pi, indicating that S. bermudense can utilize exogenous sources of Pi. APA was measured in zooxanthellae from the coral Montastraea faveolata collected from 2 reef sites with different phosphorus exposure. Zooxanthellae from corals at the low Pi site had significantly higher APA levels than those from the high Pi site. These results indicate the potential of this assay as an indicator of phosphorus exposure in a coral reef environment.