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 alo ne or in combination with nitrate affects growth and photosynthesis of the scleractinian coral, Stylophora pistillata, and its symbiotic dinoflagellat es. For this purpose, we used three series of two tanks for a 3-week enrich ment with iron (Fe), nitrate (N) and nitrate + iron (NFe). Two other tanks were kept as a control (C). Stock solutions of FeCl3 and NaNO3 were diluted to final concentrations of 6 nM Fe and 2 muM N and continuously pumped fro m batch tanks into the experimental tanks with a peristaltic pump. Results obtained showed that iron addition induced a significant increase in the ar eal density of zooxanthellae (ANOVA, p = 0.0013; change from 6.3 +/- 0.7 x 10(5) in the control to 8.5 +/- 0.6 x 10(5) with iron). Maximal gross photo synthetic rates normalized per surface area also significantly increased fo llowing iron enrichment (ANOVA, p = 0.02; change from 1.23 +/- 0.08 for the control colonies to 1.81 +/- 0.24 mu mol O-2 cm(-2) h(-1) for the iron-enr iched colonies). There was, however, no significant difference in the photo synthesis normalized on a per cell basis. Nitrate enrichment alone (2 muM) did not significantly change the zooxanthellae density or the rates of phot osynthesis. Nutrient addition (both iron and nitrogen) increased the cell-s pecific density of the algae (CSD) compared to the control (G-test, p = 0.3 x 10(-9)), with an increase in the number of doublets and triplets. CSD wa s 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%, 3 4% and 40% lower than those obtained in control colonies (ANOVA. p = 0.011) . (C) 2001 Elsevier Science B.V. All rights reserved.