Symbiosis, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 24 (1998) 127-146 Balaban, Philadelphia/Rehovot 127 zyxwvutsrqpo Effects of Light Intensity and Ammonium Enrichment on the Hermatypic Coral zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE Stylophora pistillata and its Zooxanthellae NOGA STAMBLER Department of Life Sciences, Bar-Ilan University, Ramat-Gan 52100, Israel. Fax. +972-3-5351824 Received May 13, 1997; Accepted July 14, 1997 Abstract The response of zooxanthellate corals to ammonium enrichment depends on light intensity. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 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 /cm-) 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 intensities 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. Keywords: Coral, zooxanthellae, light, enrichment 0334-5114/98/$05.50 ©1998 Balaban