Photoinhibition by solar radiation in the Mediterranean alga Peyssonnelia squamata measured on site

Photoinhibition of photosynthesis, defined as reversible decrease in the ef fective photosynthetic quantum yield, was measured in the Mediterranean red alga, Peyssonnelia squamata, using pulse amplitude modulation (PAM) chloro phyl fluorescence and oxygen production on site. This alga is adapted to ve ry low fluence rates of solar radiation and is easily inhibited by exposure to excessive radiation. At high solar angles its photosynthetic capacity i s impaired even in its natural habitat, in the protective shade of overhang ing rocks. oxygen production was maximal at 5 m depth and decreased to almo st zero at the surface. When exposed at the surface oxygen production cease d within 16 min. The optimal photosynthetic quantum yield, defined as F-v/F -m, was about 0.45 in dark-adapted specimens. After 30 min of exposure to u nattenuated solar radiation the (effective, F-v'/F'(m)) quantum yield decre ased to below 0.1. Removing solar UV (especially UV-B) significantly reduce d photoinhibition: the quantum yield of a sample exposed under a UV-B cut-o ff filter was double that of a sample exposed to full solar radiation after 30 min exposure. Recovery from photoinhibition took several hours and was not complete after prolonged exposure (1.5 h) to direct solar radiation. Th e degree of photoinhibition depended on the depth at which the thalli were exposed. Recovery from photoinhibition was complete within 2 h except when the algae were exposed at the surface. When measured over the whole day, th e effective photosynthetic quantum yield significantly decreased by about 2 5% from initially high values toward early afternoon and rose again towards evening. The data indicate that this alga is adapted to very low irradianc es and is easily inhibited by excessive solar radiation; solar UV contribut es substantially to the observed photoinhibition.