Are free amino acids responsible for the 'host factor' effects on symbiotic zooxanthellae in extracts of host tissue?

Symbiotic dinoflagellates ('zooxanthellae') typically release short-term photosynthetic products and have enhanced photosynthesis when exposed to extracts of host tissue. Published evidence has indicated that free amino acids (FAA) at concentrations exceeding 40 mM are responsible for these 'host factor' effects on zooxanthellae from sea anemones and corals. We have compared the ninhydrin-positive FAA concentrations of extracts of the symbiotic sea anemone Aiptasia pallida with their efficacy in eliciting these responses and found little effect on carbon release by freshly isolated A. pallida symbionts at concentrations up to 0.6 mM, the highest concentration of our samples. Extracts, of the coral Montastraea annularis induced release from these algae at less than 0.1 mM FAA, but there was no correlation between total ninhydrin-positive FAA concentration and 'host factor' activity. However, all of these preparations stimulated photosynthesis. We tested a range of concentrations (less than or equal to50 mM) of glycine, alanine and glutamic acid with the isolated A. pallida symbionts. There was a significant increase in the release of fixed carbon with increasing alanine concentrations, but not with the other two amino acids. There were no effects on photosynthesis. Our observations support other reports indicating that other compounds, or specific amino acids such as taurine and the mycosporine-like amino acids, are responsible for 'host factor' effects.