PHOTOINHIBITION IN THE MEDITERRANEAN GREEN-ALGA HALIMEDA-TUNA ELLIS ET SOL MEASURED IN-SITU

Photoinhibition of photosynthesis was investigated in the Mediterranea n green alga Halimeda tuna measuring pulse amplitude modulation (PAM) chlorophyll fluorescence and oxygen evolution ill situ under solar rad iation, Exposure to solar radiation at the surface caused a drastic de cline in the photosynthetic quantum yield accompanied by a decline in the photochemical quenching, while the nonphotochemical quenching dram atically increased, The algae recovered from these effects within a fe w hours indicating that these are mainly due to reversible photoinhibi tion and only to a smaller extent to nonreversible photodamage. Indivi duals harvested from deeper waters were more affected than those from shallower waters. Photoinhibition occurs in this alga even in its natu ral habitat,when the sun is at high angles as shown by measuring the f luorescence parameters at hourly intervals during the course of the da y. Photoinhibition was less pronounced when the short wavelength band was increasingly removed from solar radiation using cut-off filters, A fter exposure of thalli to solar radiation at the water surface, oxyge n production decreased drastically within 30 min; this inhibition was even more pronounced in algae harvested from deeper Waters, Oxygen mea surements at different depths showed optimal photosynthesis at a depth of 1 m, Also for photosynthetic oxygen production inhibited by high s olar irradiance at least partial recovery could be observed within sev eral hours, Despite the fact that UVB accounts for a very small fracti on of solar radiation, it has a considerable effect on photosynthesis, whereas UVA seems to contribute only little to photoinhibition in H. tuna.