Nitrate uptake in the scleractinian coral <i>Stylophora pistillata</i>

Abstract

We assessed the uptake rates of nitrate by the scleractinian coral Stylophora pistillata by following 15 N from seawater into the coral tissue. Two sets of corals were first prepared, with “nitrate‐enriched” corals grown in 5 µmol L −1 NO 3 − and control corals grown in ≤µmol L −1 NO 3 − . Uptake rates at 0.3 and 3 µmol L −1 [ 15 N]NO 3 − were then measured. Most of the % 15 N 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 µmol L −1 [ 15 N]NO 3 − , respectively. In a second experiment, two sets of corals were prepared, with “ammonium‐enriched” corals grown in 5 µmol L −1 NH 4 + and control corals grown in &lt;1 µmol L −1 NH 4 + . Uptake rates at 3 µmol L −1 [ 15 N]NO 3 − were measured. These rates were significantly lower with high NH 4 + concentrations in seawater. In the algal fraction, they ranged from 0.1 to 0.6 ng 14 h −1 cm −2 N in NH 4 + ‐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.