ACCLIMATION TO LOW TEMPERATURE OF TWO ARTHROSPIRA PLATENSIS (CYANOBACTERIA) STRAINS INVOLVES DOWN-REGULATION OF PSII AND IMPROVED RESISTANCE TO PHOTOINHIBITION(1).

This study aimed to compare the ability of two Arthrospira platensis (Nordst.) Gomont strains, M2 and Kenya, isolated from two different habitats, to acclimate to low temperature (15°C). Both strains had similar growth rates at 30°C, but once acclimated to low temperature, M2 showed a greater decline in growth (59% vs. 41% in the Kenya strain). We suggest that the Kenya strain acclimated better to low temperature by down-regulating its photosynthetic activity through (i) decreasing antenna size and thus reducing energy flux into the photosystems; (ii) decreasing reaction center density (RC/CSX ) and the performance index, thus decreasing the trapping probability and electron transport rate while maintaining electron transport probability for electron transport beyond QA (-) unchanged; (iii) increasing the energy dissipation flux. In contrast, the M2 strain showed no difference in antenna size and exhibited a much lower decrease in RC/CSX and a lower dissipation rate. Hence, the Kenya strain minimized potential damage on the acceptor side of PSII compared to the M2 cells. Furthermore, acclimation to low temperature was accompanied by an improved mechanism for handling excess energy resulting in an enhanced ability of the Kenya strain to rapidly repair damaged PSII RCs and withstand a high photon flux density (HPFD) stress; this finding might be defined as a cross-adaptation phenomenon. This study may provide a tool to identify strains suitable for outdoor mass-production in different regions characterized by different climate conditions.