Differences in thermal acclimation of chloroplast functioning in two ecotypes of Valonia utricularis (Chlorophyta)

Chloroplast functioning in two temperature ecotypes of the tropical to warm-temperate green macrophyte Valonia ultricularis was monitored by measuring chlorophyll a fluorescence parameters. One ecotype from the Mediterranean Sea is, with respect to growth and survival, more cold-adapted and eurythermal than the cold-sensitive and stenothermal ecotype from the Indian Ocean. Short-term temperature responses of the effective quantum yield (PhiPSII) between 5 and 35degreesC and the potential for acclimation of photosystem II to suboptimal growth temperatures (15, 18, 20degreesC) were investigated. When grown at optimal temperature (25degreesC), the two ecotypes did not differ to a great extent in the short-term temperature response of PhiPSII, even though the temperature optimum was lower in the Mediterranean compared with the Indian Ocean isolate (26 vs 33degreesC). In both ecotypes, decreases in PhiPSII were primarily caused by decreases in the efficiency of excitation capture (Fv'/Fm') and by the decreased fraction of open reaction centres (qP). PhiPSII declined in both isolates to 20-30% of the maximum when measured at 5degreesC. However, the ecotypes showed distinct differences in their potential to acclimate to suboptimal growth temperatures. The Indian Ocean ecotype was not capable of cold acclimation and suffered from photoinhibition when grown at 15-20degreesC, even though it was capable of adjusting its pigment composition, i.e. by decreasing light absorption combined with increasing the capacity for zeaxanthin-induced energy dissipation. The Mediterranean ecotype was capable of acclimating its photosynthetic efficiency to temperatures of 15-20degreesC, achieving optimal photosynthetic activity at 18-20degreesC. Changes in Fv'/Fm' were mainly responsible for the acclimation response of PhiPSII. It is suggested that acquisition of cold adaptation and the potential for temperature acclimation in populations of the northeast Atlantic/Mediterranean Sea have enabled V. utricularis, a species with a tropical origin, to extend its biogeographic range into warm-temperate regions.