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Global warming is driving shifts in the geographical distribution of seaweeds, such as the contraction of the Southern limit of cold-temperate Atlantic macroalgae, including the foundation intertidal fucoid Fucus serratus. However, the ecophysiological mechanisms explaining the decline of seaweeds is not fully understood. In vivo chlorophyll a fluorescence is used as an estimator of photosynthetic activity and physiological state of seaweeds. Collected at its southern distribution limit, Fucus serratus was cultured for two weeks in different temperatures (from 8 to 30 degrees C with increase of 2 degrees C). Growth rate, reproductive stage and fluorescence parameters were measured as maximal quantum yield (F-v/F-m), Electron transport rate (ETR), non-photochemical quenching (NPQ) and the yield losses (YNO and YNPQ). At high temperature, the photosynthetic responses mimic the pattern of shade adapted plants, i.e., increase of photosynthetic efficiency (alpha(ETR)) and decrease of irradiance for saturation of photosynthetic rate (Ek(ETR)). At low irradiances (< 66 mu mol m(-2) s(-1)) and mainly at extreme temperatures (high and low), yield loss is dominated by YNO, suggesting heat dissipation due to closure of PSII reaction centers. However, at high irradiances (1150-1500 mu molm(-2) s(-1)) and in a wide range of temperatures (8-28 degrees C), yield loss is mainly due to Y(NPQ), related to the energy dissipation as heat throughout regulated photoprotective NPQ mechanisms such as the xanthophyll cycle. The optimal temperature for growth was 14-20 degrees C as ETRmax, whereas F-v/F-m maintains high values in a wider range of temperatures compared to ETRmax and Yield losses, indicating that F-v/F-m is not a good parameter to discriminate the effect of temperature on algal production. We discuss the use of ETR and yield losses to relate photosynthesis with growth, survival or reproduction under stress conditions. Our study shows that warming can increase photoprotection through yield loss and decrease the photosynthetic activity in macroalgae.