Photosynthesis of the Mediterranean green alga Caulerpa prolifera measured in the field under solar irradiation

Photoinhibition of photosynthesis was investigated in the Mediterranean green alga Caulerpa prolifera by using purse amplitude modulation (PAM) chlorophyll fluorescence and oxygen evolution. After exposure to solar radiation at the water surface, oxygen production decreased drastically within 20 min; this inhibition of photosynthesis was only partially restored during the subsequent hours in the shade. Oxygen measurements at different depths showed optimal photosynthesis at a depth of 5 m. Exposure to solar radiation at the surface also caused a drastic decline in the effective photosynthetic quantum yield. Also these effects only partially recovered indicating that they are mainly caused by reversible photoinhibition and to a smaller extent to nonreversible photodamage. Photoinhibition occurs in this alga even in its natural habitat when the sun is at large angles. Despite the fact that W-B accounts for a very small fraction of solar radiation, it has a considerable effect on photosynthesis while UV-A seems to contribute only a small amount to photoinhibition in C. prolifera.