Alkaline Phosphatase Activity in an Equatorial Coral Reef Ecosystem

Alkaline phosphatase is a key enzyme in microbial phosphorus (P) cycling, and its activity rate (APA) provides an index of the potential rate of P cycling. APA is often controlled by dissolved inorganic P (DIP) availability, but most research on marine APA has focused on the oligotrophic open ocean, and far less is known about APA in coastal waters, especially in the tropics. However, nutrient cycling is critical to coastal, tropical ecosystems such as coral reefs, and more studies of nutrient cycling in these systems are therefore needed. We measured APA at an equatorial coral reef time-series site in Singapore both in the water column and associated with corals. Coral-associated APA was measured for 3 species by incubating coral nubbins with 4-methylumbelliferyl-P immediately after removal from the reef. The monsoon system drives large seasonal variation in dissolved inorganic nitrogen (DIN, 0.14–5.4 µM) and DIP (<0.03–0.3 µM), at our site. Water column APA ranged from 1.5-38 nM h^-1, averaging 7.4 nM h^-1, but showed no seasonality, and did not correlate with DIP or DIN:DIP ratio. In seawater nutrient addition experiments, APA showed only a small and non-significant decrease after DIP addition, but a strong increase after glucose and glucose + DIP addition, indicating that water column APA at our site is not primarily controlled by DIP. Coral-associated APA showed no seasonality but varied between species, averaging 28–79 µM m^-2 h^-1. Thus, 1 m² of coral surface can account for as much APA as a 5–10 m water column. Although coral mucus harbours a rich microbial community, we found that isolated mucus accounted for only 12–27% of total coral APA, suggesting that most of the APA originates from the coral polyps, possibly as a digestive enzyme. Overall, our results show a persistently high rate of organic P recycling in a coral reef that is not driven by ambient nutrient availability, and that corals themselves can make a large contribution to reef P cycling.