1. The distributional limits of species in response to environmental change are usually studied at large temporal and/or geographical scales. However, organismal scale habitat variation can be overlooked when investigating large-scale averages of key factors such as temperature. We examine how microhabitat thermal conditions relate to physiological limits, which may contribute to recent range shifts in an intertidal alga. 2. We defined the onset and maximum temperatures of the heat-shock response (HSR) for a southern edge population of Fucus vesiculosus, which has subsequently become extinct. The physiological threshold for resilience (assayed using chlorophyll fluorescence) coincided with declining HSR, determined from the temperature-dependent induction of seven heat-shock protein transcripts. 3. In intertidal habitats, temperature affects physiology directly by controlling body temperature and indirectly through evaporative water loss. We investigated the relationship between the thermal environment and in situ molecular HSR at microhabitat scales. Over cm to m scales, four distinct microhabitats were defined in algal patches (canopy surface, patch edge, subcanopy, submerged channels), revealing distinct thermal and water stress environments during low-tide emersion. 4. The in situ HSR agreed with estimated tissue temperatures in all but one microhabitat. Remarkably, in the most thermally extreme microhabitat (canopy surface), the HSR was essentially absent in desiccated tissue, providing a potential escape from the cellular metabolic costs of thermal stress. 5. Meteorological records, microenvironmental thermal profiles and HSR data indicate that the maximum HSR is approached or exceeded in hydrated tissue during daytime low tides for much of the year. Furthermore, present-day summer seawater temperatures are sufficient to induce HSR during high-tide immersion, preventing recovery and resulting in continuous HSR during daytime low-tide cycles over the entire summer. 6. HSR in the field matched microhabitat temperatures more closely than local seawater or atmospheric data, suggesting that the impacts of climatic change are best understood at the microhabitat scale, particularly in intertidal areas.

FE-2014-00335_Gene Expression DataGene expression data for the field and laboratory experiments. Cycle threshold (Ct) values and normalized relative expression values of seven Hsp transcripts.FE-2014-00335_FvFm DataFv/Fm of Fucus vesiculosus following 24 h recovery at 15 ºC from 3 h heat-shock at 24, 28, 32 and 36 ºC and controls.FE-2014-00335_temperature dataTemperature data used in Figures 2, 5, S2 and S3. Microhabitat temperatures for Ria Formosa on sampling days. Air temperatures in Faro for 2008 and 2012. Water temperatures in the Ria Formosa in 2012. Daily temperatures and monthly averages for 1973 – 2011.