Photoacclimatization in the zooxanthellae of Pocillopora verrucosa and comparison with a pelagic algal community Photoacclimatation comparée des zooxanthelles de Pocillopora verrucosa et d’une communauté algale pélagique

Photoacclimatization of zooxanthellae extracted from the coral Pocillopora verrucosa was studied through the determination of pigments, light absorption and photosynthetic parameters, for samples collected in summer and winter between 1 and 40 m on a northwestern reef of Tahiti (French Polynesia). The same measurements were also performed on phytoplanktonic samples collected at a stable oceanic site north of the island. For the zooxanthellae, the variations with depth of all the parameters were generally of small amplitude. Seasonal differences were also observed. The photosynthetic to non-photosynthetic pigments ratio was higher at depth in both seasons and was higher in winter. The intracellular concentration of chlorophyll a and photosynthetic pigments was higher in winter, as was the photosynthetic pigments/chlorophyll a ratio, whereas the non-photosynthetic pigments/chlorophyll a ratio was higher in summer. Variations in the light absorption properties were also small. The photosynthetic parameters showed limited changes with depth with the largest variations (a factor of ∼2) observed for Pmax. The trends observed for the phytoplankton assemblage were generally of much higher amplitudes than for the zooxanthellae (e.g. for photosynthetic to non-photosynthetic pigments ratio or the saturation parameter, Ek). These results suggest that, in the very clear Polynesian waters, the amount of energy that reaches the zooxanthellae of P. verrucosa is not variable enough in the 1–40 m depth range to result in a drastic modification of the photosynthetic apparatus of the algae. © 2002 Ifremer/CNRS/IRD/Éditions scientifiques et médicales Elsevier SAS. All rights reserved.