ACCLIMATION AND ADAPTATION TO IRRADIANCE IN SYMBIOTIC DINOFLAGELLATES .1. RESPONSES OF THE PHOTOSYNTHETIC UNIT TO CHANGES IN PHOTON FLUX-DENSITY

The photosynthetic responses to variations in photon flux density were determined for Symbiodinium microadriaticum, the symbiont of the Caribbean jellyfish Cassiopeia xamachana, S. kawagutii, the symbiont of the Indo-Pacific stony coral Montipora verrucosa, and S. pilosum, the symbiont of the Carribbean zoanthid Zoanthus sociatus. Photosynthetic responses were characterized in terms of cellular pigment content, photosynthesis versus irradiance (P-I) relationships, and number and size of the photosynthetic unit (PSU). Analyses of the responses under 2 light regimes of 40 and 250 mumol quanta m-2 s-1 indicate that: (1) the 3 different species cultured under identical conditions possess different photosynthetic characteristics; (2) the 3 species acclimate to low photon flux density by simultaneously increasing the number and size of their PSU; (3) PSU characteristics were not correlated with the parameters of their respective P-I curves; (4) the 3 species analyzed have different photo-acclimatory capabilities which can be correlated with their respective ecological distribution as endosymbionts. As different species demonstrate characteristic responses, photo-acclimatory adjustment in symbiotic dinoflagellates may be under genetic constraints, and thus may represent photo-adaptation (sensu O. Bjorkman). As a whole, the results suggest that symbiont photo-adaptation may constitute an important axis of niche diversification for the intact associations.