The potential to acclimate photosynthesis to sub- and supra-optimal temperatures was investigated in seven isolates of Valonia utricularis (Roth) C. Agardh, a green macrophyte with a tropical to warm-temperate distribution. Photosynthesis-light response curves were obtained by measuring chlorophyll a fluorescence characteristics of algae grown at optimal (25 degrees C), sub- and supra-optimal temperatures. Suboptimal temperatures were chosen to support 30% of the maximum relative growth rate in each isolate. Thermal acclimation was investigated by comparing short-term and long-term temperature effects on the initial rate of increase of the relative electron transport rate (rETR) and the maximum rETR under light-saturating conditions. Isolates from the northeast Atlantic and the Mediterranean all showed a strong potential to acclimate maximum rETR to suboptimal growth temperatures, i.e. short-term temperature effects were diminished after acclimation. However, photoinhibition, measured as a decrease of the maximal quantum yield (F(v)/F(m)), was found when plants were grown at 30 degrees C. The isolates reduced light harvesting at 30 degrees C by decreasing total chlorophyll content and by increasing the chlorophyll a/b ratio. Up-regulation of photoprotective processes by the xanthophyll cycle pigments was not observed. In contrast, isolates from the Indo-west Pacific were unable to acclimate photosynthesis to suboptimal growth temperatures and these temperatures were strongly photoinhibiting, even though adjustments on the pigment level were observed. All Indo-west Pacific isolates reached comparable maximum rETR values at 30 and 25 degrees C. Thus, the Atlantic/Mediterranean isolates had a stronger potential to acclimate photosynthetic rates at suboptimal growth temperatures compared to the Indo-west Pacific isolates, which was accompanied by losses at 30 degrees C. The results are discussed in a biogeographical context.