Abstract
Photoinhibition of photosynthesis was investigate in the marine macroalga Ulva laetevirens by measuring chlorophyll flourescence (PAM) and oxygen evolution. After exposure to unfiltered solar radiation the ratio of the variable flourescence to the maxiaml flourescence 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 photosunthetic oxygen production. Solar radiation deprived of most of the UV-B (long pass above 320nm) 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 400nm. 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 photoinhibitionin Ulva .
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