PHOTOSYNTHETIC OXYGEN AND FLUORESCENCE MEASUREMENTS IN ULVA LAETEVIRENS AFFECTED BY SOLAR IRRADIATION

Photoinhibition of photosynthesis was investigated in the marine macroalga Ulva laetevirens by measuring chlorophyll fluorescence (PAM) and oxygen evolution. After exposure to unfiltered solar radiation the ratio of the variable fluorescence to the maximal fluorescence F-v/F-m (F-v = F-m-F-O) as well as the oxygen production declined rapidly with increasing duration of the exposure. Fluence rate-response curves for the oxygen production showed a decrease in the slope with exposure time and a shift of the compensation point to higher fluence rates as well as a decrease in the photosynthetic oxygen production. Solar radiation deprived of most of the W-B (long pass above 320 nm) induced a similar degree of photoinhibition of the ratio F-v/F-m as unfiltered solar radiation, whereas the oxygen production was less affected. The photoinhibitory effect of solar radiation was diminished when the solar radiation was filtered by a filter which passes wavelengths above 400 nm. Even though UV-B accounts for a very small fraction of solar radiation, it has a considerable effect on photosynthesis. In addition, UV-A and visible radiation contribute to photoinhibition in Ulva.