The terrestrial cyanobacterium Nostoc commune Vaucher ex Bornet et Flahault occurs worldwide, including in Japan and on the Antarctic continent. The terrestrial green alga Prasiola crispa (Lightf.) Kütz. is also distributed in Antarctica. These two species need to acclimate to the severe Antarctic climate including low ambient temperature and desiccation under strong light conditions. To clarify this acclimation process, the physiological characteristics of the photosynthetic systems of these two Antarctic terrestrial organisms were assessed. The relative rate of photosynthetic electron flow in N. commune collected in Japan and in Antarctica reached maxima at 900 and 1,100 μmol photons · m-2 · s-1, respectively. The difference seemed to reflect the presence of high amounts of UV-absorbing substances within the Antarctic cyanobacterium. On the other hand, the optimal temperatures for photosynthesis at the two locations were 30°C-35°C and 20°C-25°C, respectively. This finding suggested a decreased photosynthetic thermotolerance in the Antarctic strain. P. crispa exhibited desiccation tolerance and dehydration-induced quenching of PSII fluorescence. Re-reduction of the photooxidized PSI reaction center, P700, was also inhibited at fully dry states. Photosynthetic electron flow in P. crispa reached a maximum at 20°C-25°C and at a light intensity of 700 μmol photons b m-2 b s-1. Interestingly, the osmolarity of P. crispa cells suggested that photosynthesis is performed using water absorbed in a liquid form rather than water absorbed from the air. Overall, these data suggest that these two species have acclimated to optimally photosynthesize under conditions of the highest light intensity and the highest temperature for their habitat in Antarctica.
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