Photosynthetic efficiency of endosymbiotic algae of Paramecium bursaria originating from locations with cold and warm climates

Paramecium bursaria (Ciliophora) is a cosmopolitan unicellular organism that plays a significant role in aquatic ecosystems. P. bursaria contains symbiotic algae and this association is a mutual symbiosis. The aim of the present study was to determine the activity of photosystem II (PSII) in Chlorella sp. inside P. bursaria cells. Ciliates were incubated for 7 days at different temperatures from 6 to 18 degrees C, under the circadian cycle: 12 h light/12 h dark, at light intensity of 200 mu mol m(2) s(1) and under constant darkness conditions. The control group was kept at a temperature of 18 degrees C under constant light conditions. Changes in PSII were monitored using different fluorescence parameters. Differences in responses between endosymbiotic algae of two P. bursaria strains - Ard7 from a warm climate and KD64 from a cold climate - were determined. The highest photosynthetic activity of P. bursaria green endosymbionts was observed at a temperature of 18 degrees C, regardless of the light conditions. Algae from warm climate were more sensitive to cold temperature stress than algae from P. bursaria collected in cold climate.