Nitrate uptake in the scleractinian coral Stylophora pistillata

We assessed the uptake rates of nitrate by the scleractinian coral Stylophora pistillata by following N-15 from seawater into the coral tissue. Two sets of corals were first prepared, with "nitrate-enriched" corals grown in 5 mumol L-1 NO3- and control corals grown in less than or equal to1 mumol L-1 NO3-. Uptake rates at 0.3 and 3 mumol L-1 [N-15]NO3- were then measured. Most of the WIN enrichment occurred in the zooxanthellae fraction. Uptake rates were not significantly different between nitrate-enriched and control corals, suggesting that they were not dependent on a nitrate acclimation. These rates increased with the in situ nitrate concentration and varied from 1.2 +/- 0.2 ng h(-1) cm(-2) N to 6.1 +/- 1.1 ng h(-1) cm(-2) N in the algal fraction at 0.3 and 3 mumol L-1 [N-15]NO3-, respectively. In a second experiment, two sets of corals were prepared, with "ammonium-enriched" corals grown in 5 mumol L-1 NH4+ and control corals grown in <1 mumol L-1 NH4+. Uptake rates at 3 mumol L-1 [N-15]NO3- were measured. These rates were significantly lower with high NH4+ concentrations in seawater. In the algal fraction, they ranged from 0.1 to 0.6 ng h(-1) cm(-2) N in NH4+-enriched corals and from 2.2 to 4.5 ng h(-1) cm(-2) N in control corals. Nitrate can therefore be considered as an important source of nitrogen for corals, at least when ammonium concentrations are low in seawater.