Photoinhibition in the Mediterranean Green Alga Halimeda tuna Ellis et Sol Measured in situ

— Photoinhibition of photosynthesis was investigated in the Mediterranean green alga Halimeda tuna measuring pulse amplitude modulation (PAM) chlorophyll fluorescence and oxygen evolution in situ under solar radiation. Exposure to solar radiation at the surface caused a drastic decline in the photosynthetic quantum yield accompanied by a decline in the photochemical quenching, while the nonphotochemical quenching dramatically increased. The algae recovered from these effects within a few hours indicating that these are mainly due to reversible photoinhibition and only to a smaller extent to nonreversible photodamage. Individuals harvested from deeper waters were more affected than those from shallower waters. Photoinhibition occurs in this alga even in its natural habitat when the sun is at high angles as shown by measuring the fluorescence parameters at hourly intervals during the course of the day. Photoinhibition was less pronounced when the short wavelength band was increasingly removed from solar radiation using cut-off filters. After exposure of thalli to solar radiation at the water surface, oxygen production decreased drastically within 30 min; this inhibition was even more pronounced in algae harvested from deeper waters. Oxygen measurements at different depths showed optimal photosynthesis at a depth of 1 m. Also for photosynthetic oxygen production inhibited by high solar irradiance at least partial recovery could be observed within several hours. Despite the fact that UVB accounts for a very small fraction of solar radiation, it has a considerable effect on photosynthesis, whereas UVA seems to contribute only little to photoinhibition in H. tuna.