Effects of light intensity and ammonium enrichment on the hermatypic coral Stylophora pistillata and its zooxanthellae

The response of zooxanthellate corals to ammonium enrichment depends on light intensity. In these corals the growth of both algal symbionts and the host animal is controlled by nitrogen and carbon fluxes and their ratios. The combined effects of light intensity [100% (HL), 50% (ML), 10% (LL), and less than 1% (D) of sunlight] and ammonium concentration (<1 and 20 μM) on the symbiotic coral Stylophora pistillata were examined. Algal density depended on light intensity; being lowest (4.60x10⁵ cells/cm²) for colonies maintained under <1% of sunlight, and highest (1.97x10⁶ cells/cm²) under high light (HL). Under low light intensity (LL), the algal population density also increased from 8.89x10⁵ to 1.95x10⁶ cells/cm² as a response to ammonium enrichment, whereas under other light intensifies there was no such response. Chlorophyll concentration per algal cell increased as light intensity decreased, concomitantly with structural change in the chloroplast. There was an increase in surface density of thylakoid per cell without change in surface density of thylakoids per chloroplast. Contrary to changes in light intensity, ammonium concentration did not cause pigment changes or any ultrastructural changes in the algae. There was no clear effect on the metabolism of the colonies, since both respiration and maximal photosynthesis remained nearly constant under all treatments.