To estimate the potential eVects of climate change on polar marine macroalgae, studies on interactive stress eVects of multiple climate-related parameters are essential. Interactions of temperature, radiation and salinity on two diVerent life history stages of Alaria esculenta (L.) Greville from the Kongsfjord (Spitsbergen) were investigated for the Wrst time within this study. Adult macroscopic sporophytes of A. esculenta were exposed to diVerent temperatures between 4 and 21°C combined with artiWcial irradiation conditions [photosynthetically active radiation, ultraviolet (UV) radiation: UV-A/UV-B, Wrst experiment] and with diVerent salinities [34, 28, 20 practical salinity units (p.s.u.)ß second experiment]. EVects of photosynthetic activity were determined by measuring variable chlorophyll Xuorescence of photosystem II. Germination success of young microscopic zoospores of A. esculenta was studied under multifactorial stress. Zoospore suspensions were exposed to the three diVerent salinities and irradiation conditions at four temperatures between 2 and 16°C. Overall, A. esculenta exhibited a highly stagespeciWc susceptibility towards the experimental treatments.In both experiments using sporophytes, photosynthetic activity showed signiWcant temperature eVects and only very few signiWcant radiation and salinity eVects. Microscopic stages of A. esculenta were shown to be more sensitive than the adult macroscopic stages, since germination capacity of zoospores was signiWcantly aVected by temperature and salinity changes, and interactions of both. These results suggest that multiple stress factors interact synergistically. Temperature seems to be a predominant environmental parameter for the kelp A. esculenta. Overall, A. esculenta proved to be relatively tolerant and adaptable to increasing temperature and UV radiation, as well as to diluted salinities, but only up to a speciWc limit.

Stratospheric ozone depletion and the concomitant increase in ultraviolet (UV) B radiation at the earth's surface represent major threats to polar marine ecosystems. Whereas in coastal rocky shore environments macroalgae constitute an assemblage of particular significance to ecosystem function, benthic diatoms dominate microphytobenthic assemblages, which typically grow on shallow-water sediments as highly productive and stabilising phototrophic biofilms. This review summarises present knowledge on how UV radiation affects the physiology of polar benthic algae with an emphasis on cell biological and structural changes, molecular targets and repair mechanisms, induction of reactive oxygen species and antioxidative strategies, photosynthesis and growth, photoprotective mechanisms, interactive effects between UV radiation and other abiotic factors, and finally ecological consequences. Although available data indicate that there are specific characteristics and adaptations in polar benthic micro- and macroalgae that explain their ecological success and limits under environmentally extreme conditions, much more research is needed to understand the underlying mechanisms. In particular, more ecosystem approaches and studies on interactive effects, as well as modern genomic, proteomic and metabolomic approaches could help address all open questions and depict a more holistic picture.

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