Acclimation and adaptation to irradiance in symbiotic dinoflagellates. I. Responses of the photosynthetic unit to changes in photon flux density

Abstract

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 Caribbean zoanthld 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 p 0 1 quanta m-' S-' 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 respechve P-lcurves; (4) the 3 species analyzed have different photoacclimatory capabilities which can be correlated with their respective ecological distribution as endosymbionts.As different species demonstrate characteristic responses, photo-acclunatory adjustment in symbiotic dinoflagellates may be under genetic constraints, and thus may represent photo-adaptation (sensu 0. Bjorkman).As a whole, the results suggest that symbiont photo-adaptation may constitute an important axis of niche diversification for the intact associations.