Abstract:Macroalgae of the upper sublittoral zone of Arctic coastal ecosystems are subjected to darkness or low light for several monthsduring winter and have to withstand large changes in irradiance after the breakup of sea ice in the Arctic spring. Changes inphotosynthetic response to high PAR (photosynthetically active radiation) and UV-B radiation (UV-B) in the cold temperatebrown alga Alaria esculenta were monitored with a PAM fluorometer to study photoinhibition, recovery and acclimation ofmaximal quantum yield of photochemistry. Plants collected in the field, as well as specimens raised in the laboratory, wereexposed to various radiation conditions including different levels of PAR and UV radiation (UV-A + UV-B). Measurements ofvariable chlorophyll fluorescence of photosystem II revealed that the photosynthetic apparatus in A. esculenta was able toacclimate to the respective high light and UV treatments within several days. However, two different mechanisms of acclimationseem to be involved. Initially, the rate of recovery of maximal quantum yield increased after only a few exposures to high lightor UV. Second, after several exposure cycles, the degree of inhibition was reduced. Data on fluorescence induction kinetics andquenching analysis showed that exposure to the respective UV radiation resulted in an increase of non-photochemical quenching,while effective quantum yield of photochemistry was hardly affected.Keywords:Acclimation, Alaria esculenta, chlorophyll fluorescence, light stress, photosynthesis, UV radiation.