Uptake of ammonium by the scleractinian coral Stylophora pistillata: Effect of feeding, light, and ammonium concentrations
Limnol. Oceanogr., 47(3), 2002, 782-790 | DOI: 10.4319/lo.2002.47.3.0782
ABSTRACT: Experiments were designed to assess the uptake rates of 0.2, 1, and 5 µM ammonium by the scleractinian coral Stylophora pistillata maintained under different feeding regimes (highly fed, slightly fed, and starved) for 4-8 weeks. 15NH4 was used to follow the incorporation of nitrogen in the zooxanthellae or in the animal tissue and to calculate uptake rates. After 4 or 8 weeks, fed corals contained significantly higher concentrations of protein, chlorophyll, and zooxanthellae than starved nubbins. They also contained significantly higher amounts of carbon and nitrogen per unit surface skeleton. Results obtained showed that the algal fraction was enriched with 15N at up to 10 times the rate of the host, which suggests that the zooxanthellae are the primary site of assimilation. Uptake rates (measured in the algal fraction) varied according to the nitrogen concentration in seawater. They were ~20 times lower at 0.2 than at 1 or 5 µM 15NH4 enrichment (2-30 vs. 120-510 ng N h-1 cm-2) for both fed and starved nubbins. These rates were also affected by the feeding history of the host, because they were significantly lower for fed than for starved nubbins (analysis of variance, P < 0.005), at both high and low ammonium concentrations. On the basis of the nitrogen content of the zooxanthellae, we suggest that an external concentration of ammonium equal to 0.6 mM can sustain the growth of the zooxanthellae population.