Photoinhibition of photosynthesis in the marine brown alga Fucus serratus as studied in field experiments

In field experiments photoinhibition of photosynthesis'in the intertidal brown alga Fucus serratus was investigated by means of both fluorescence and oxygen measurements. Samples were taken from thalli floating just below the water surface. Dunng falling tides the ratio variable fluorescence/maxlrnum fluorescence (FJF,) decreased with increasing photon fluence rates. As during rising tides only samples could be measured that were previously desiccated, F,/F, increased. Plants living in a rock pool and, hence, not emersing during falling tides showed a considerable decrease of photosynthetic capacity due to photoinhibition. Measured algae samples were collected after experiments, kept for about 24 h in dim light for recovery and then measured again. These values were used as references in order to calculate the extent of photoinhibition. Oxygen measurements support results obtained with the fluorescence technique. In the morning photosynthetic capacity decreased with increasing photon fluence rates. Photosynthetic capacity recovered in the late afternoon and early evening when photon fluence rates decreased. However, if on a sunny day an algal sample oriented perpendicularly to the sun's rays was continuously irradiated, a strong decrease of photosynthetic capacity was observed which was not reversible under the dim light conditions of the afternoon and early evening. This indicates photodamage.