Acclimation and adaptation to irradiance in symbiotic dinoflagellates. II. Response of chlorophyll–protein complexes to different photon-flux densities

Abstract

The response of chlorophyll–protein complexes to super- and sub-saturating photon-flux densities, PFD (250 μmol quanta m⁻² s⁻¹ and 40 μmol quanta m⁻² s⁻¹, respectively) were analyzed for Symbiodinium microadriaticum Freudenthal, the symbiont of the Caribbean jellyfish Cassiopeia xamachana; S. kawagutii Trench and Blank, the symbiont of the Indo-Pacific scleractinian Montipora verrucosa; and S. pilosum Trench and Blank, the symbiont of the Carribbean zoanthid Zoanthus sociatus. The results indicate that each species exhibits a quantitatively distinct chlorophyll (chl) a distribution among its chl–protein complexes when cultured under standardized high and low light conditions. In response to sub-saturating PFD, the three species differentially increased the cellular concentrations of most of the chl–protein complexes. Increases in P₇₀₀ (reaction center of Photosystem I) under sub-saturating PFD correlate with an increase in the cellular concentrations of the Photosystem I-enriched complexes. Similarly, increases in photosynthetic unit (PSU) size correlate with an increase in the cellular concentrations of the water-soluble peridinin–chl-a–protein (PCP) complexes and the membrane-bound chl a–chl c ₂–peridinin–protein (acpPC) complexes, which together represent the light-harvesting components of this group. In S. microadriaticum, acclimation to sub-saturating PFD uniquely includes the preferential enrichment of the dimeric form of PCP. Under super-saturating PFD, an enrichment in photo-protective xanthophylls was detected in acpPC from S. microadriaticum and S. pilosum, but not from S. kawagutii. Each species demonstrated a characteristic photo-acclimatory response which correlates with its distribution as endosymbiont in nature, supporting the concept that different species of symbiotic dinoflagellates are adapted (sensu Björkman 1981) to different photic environments. The study was conducted between May 1992 and November 1994.