A possible role for urea in mediating coral responses to pulsed doses of ammonium

; (). A possible role for urea in mediating coral responses to pulsed doses of ammonium. Proceedings of the 10th international Coral Reef Symposium, Okinawa 831-837. Peer reviewed.

In the early 1970s it was observed that some reef-building corals contained surprisingly high levels of urea for aquatic invertebrates. A hypothesis was proposed at the time, whereby hydrolysis of urea might neutralise the protons formed by deposition of CaCO3, thereby enhancing the rate of calcification. However, since that early work the role of urea in coral metabolism has received little attention. We propose that the primary role of urea is to allow the coral-zooxanthellae association to take advantage of episodic peaks in nitrogen availability in an otherwise nutrient-poor environment. Urea would accumulate during times of nitrogen excess to be hydrolysed to release ammonium to the zooxanthellae in times of shortage. An increase in calcification rates could be a beneficial sideeffect of a pulsed nitrogen supply. Preliminary testing of this hypothesis has shown 1. that urea concentrations can be measured in a field laboratory using a colorimetric kit designed for use with human blood plasma 2. that urea concentrations increase significantly in corals exposed to ammonium pulses and return to those of control corals in low-ammonium seawater, 3. that calcification rates in corals exposed to a pulse of ammonium tend to increase relative to controls in low-nutrient seawater. We predict that, under conditions of chronically elevated nitrogen, if urea concentrations in the coral exceed a threshold level, then a decline in growth and health of the coral could follow. The levels of urea in coral tissue may provide another tool by which the nitrogen sufficiency of a coral might be monitored.