Effect of exogenous nitrogen levels on ultrastructure of zooxanthellae from the hermatypic coral Pocillopora damicornis

Branches of the hermatypic coral Pocillopora damicornis (Linnaeus) were exposed for 2, 4, 6, and 8 weeks to ammonium concentrations of < 1 uM (nutrient-stripped), 2 uM (seawater as a control), 20 uM, and 50 uM (enriched), after which their symbiotic zooxanthellae were examined for changes in their ultrastructure. No significant differences among treatments were detected in cell diameter or in relative volume of any of the cellular organelles of zooxanthellae subjected to the various nitrogen levels. The surface density of thylakoids was higher in cells from the elevated-nitrogen treatments. However, there was a significant increase in accumulation of starch grains and lipid droplets in zooxanthellae in corals maintained in unenriched and nutrient-stripped seawater, occupying about 15% of the cell volume. Storage of these N-free compounds showed that under N-limited conditions photosynthate cannot be used as carbon skeletons in synthesis of amino and nucleic acids, both required for cell doubling. We believe that our results further demonstrate the uncoupling of photosynthesis from population growth under C : N ratios deviating from those needed to support balanced growth.