Nutrient uptake in the reef-building coral Acropora palmata at natural environmental concentrations

Experiments were carried out over 15 mo to assess the net exchange of dissolved inorganic nitrogen (DIN) by the symbiotic coral Acropora palmata (Lamarck) under near-natural environmental conditions. Specimens were enclosed in situ in transparent acrylic chambers and changes in the ambient seawater concentrations of nitrate, nitrite and ammonium determined over 1 h closed incubat ion~ . There was a consistent depletion of nitrate and a more variable but significant mean depletion of ammonium within the experimental chambers, indicating simultaneous net uptake of both ions by the coral. No exchange of nitrite could be detected. No significant changes in concentration of any form of DIN occurred in ambient seawater control chambers. Contrary to previous predictions from studies of uptake kinetics in isolated zooxanthellae and other coral species, mean net rates of nitrate uptake exceeded that of ammonium uptake by a factor of 2. Nitrate uptake was dependent on substrate concentration, with a linear response over the range of ambient seawater concentrations encountered in the study. Rate of ammonium uptake was apparently independent of substrate concentration and the rate of uptake of each ion was independent of the concentration of the others. A diurnal rhythm was noted in nitrate uptake rates, after changes in concentration had been taken into account, which suggests that although uptake continued over the complete 24 h period, maximal rates of uptake were not maintained throughout the night. These results support previous studies which have demonstrated the capability for dual uptake of both nitrate and ammonium by zooxanthellae, but suggest that under natural conditions nitrate rather than ammonium provides the primary external source of dissolved inorganic nitrogen in this species.