Photoadaptation of zooxanthellae in the sponge Cliona vastifica from the Red Sea, as measured in situ

In the Red Sea, the zooxanthellate sponge Cliona vastifica (Hancock) is mai nly present at >15 m depth or in shaded areas. To test whether its scarcity in unshaded areas of shallower waters is linked to the functional ineffici ency of its photosymbionts at high irradiances, sponges were transferred fr om 30 m to a six times higher light regime at 12 m depth, and then returned to their original location. During this time, photosynthetic responses to irradiance were measured as rapid light curves (RLCs) in situ by pulse ampl itude modulated (PAM) fluorometry using a portable underwater device, and s amples were taken for microscopic determinations of zooxanthellar abundance . The zooxanthellae harboured by this sponge adapted to the higher irradian ce at 12 m by increasing both their light saturation points and relative ph otosynthetic electron transport rates (ETRs). The ETRs at light saturation increased almost fourfold within 15-20 days of transfer to the shallower wa ter, and decreased back to almost their original values after the sponges w ere returned to 30 m depth. This, as well as the fact that the photosynthet ic light responses within an individual sponge were in accordance with the irradiance incident to specific surfaces, shows that these photosymbionts a re highly adaptable to various irradiances. There was no significant change in the number of zooxanthellae per sponge area throughout these experiment s, and the different photosynthetic responses were likely due to adaptation s of the photosynthetic apparatus within each zooxanthella. In conclusion, it seems that parameters other than the hypothesised inability of the photo symbionts to adapt adequately to high light conditions are the cause of C. vastifica's rareness in unshaded shallow areas of the Red Sea.