Response of a scleractinian coral, Stylophora pistillata, to iron and nitrate enrichment.

The purpose of this study was to determine whether the addition of iron alone or in combination with nitrate affects growth and photosynthesis of the scleractinian coral, Stylophora pistillata, and its symbiotic dinoflagellates. For this purpose, we used three series of two tanks for a 3-week enrichment with iron (Fe), nitrate (N) and nitrate+iron (NFe). Two other tanks were kept as a control (C). Stock solutions of FeCl(3) and NaNO(3) were diluted to final concentrations of 6 nM Fe and 2 &mgr;M N and continuously pumped from batch tanks into the experimental tanks with a peristaltic pump. Results obtained showed that iron addition induced a significant increase in the areal density of zooxanthellae (ANOVA, p=0.0013; change from 6.3+/-0.7x10(5) in the control to 8.5+/-0.6x10(5) with iron). Maximal gross photosynthetic rates normalized per surface area also significantly increased following iron enrichment (ANOVA, p=0.02; change from 1.23+/-0.08 for the control colonies to 1.81+/-0.24 &mgr;mol O(2) cm(-2) h(-1) for the iron-enriched colonies). There was, however, no significant difference in the photosynthesis normalized on a per cell basis. Nitrate enrichment alone (2 &mgr;M) did not significantly change the zooxanthellae density or the rates of photosynthesis. Nutrient addition (both iron and nitrogen) increased the cell-specific density of the algae (CSD) compared to the control (G-test, p=0.3x10(-9)), with an increase in the number of doublets and triplets. CSD was equal to 1.70+/-0.04 in the Fe-enriched colonies, 1.54+/-0.12 in the N- and NFe-enriched colonies and 1.37+/-0.02 in the control. Growth rates measured after 3 weeks in colonies enriched with Fe, N and NFe were 23%, 34% and 40% lower than those obtained in control colonies (ANOVA, p=0.011).