Effects of high light stress on photosynthesis of polar macroalgae in relation to depth distribution

The capability of several polar macroalgal species to protect photosynthesis against excessive irradiation by dynamic photoinhibition was investigated and related to the specific depth distribution of the species. Photoinhibition of photosynthesis was induced by exposure of the algae to a photon fluence rate of 500 mu mol m(-2) s(-1) for 2 h. Changes in the oxygen production rate and in vivo chlorophyll fluorescence were recorded. For oxygen measurements gross P-max and the slope (alpha) of the fluence rate-response curve were determined before and after photoinhibitory treatment and after recovery of photosynthesis in dim light. In fluorescence measurements the kinetics of change of the variable fluorescence during the inhibitory time phase and the recovery phase were determined. Significant differences in the reactions during exposure and recovery were found in the different algal classes. Within each class a correlation between species depth distribution and the ability to cope with high light stress was found. Algae growing in nature close to the water surface or in the intertidal were generally not severely stressed. Algae growing in the middle and upper subtidal zone showed a decrease in photosynthetic activity during high light stress with full recovery of photosynthesis in subsequent dim light. Fluorescence measurements showed that the reaction kinetics of photoinhibition and recovery were slower than in algae growing close to the water surface or in the intertidal. Algae growing in the lower subtidal also showed a decrease in the photosynthetic parameters due to high light stress; however, photosynthesis recovered only slightly and very slowly during subsequent dim light conditions. In these algae the decrease in the photosynthetic activity was caused by photodamage rather than by dynamic photoinhibition. In conclusion these experiments indicate that algae already cultured for a long time in the laboratory, retain a certain genetic adaptation to the natural light environment. This is true not only for the lower light Limit, but also for the upper light limit.