ONLINE MONITORING OF CHLOROPHYLL FLUORESCENCE TO ASSESS THE EXTENT OFPHOTOINHIBITION OF PHOTOSYNTHESIS INDUCED BY HIGH OXYGEN CONCENTRATION AND LOW-TEMPERATURE AND ITS EFFECT ON THE PRODUCTIVITY OF OUTDOOR CULTURES OF SPIRULINA-PLATENSIS (CYANOBACTERIA)

The saturating pulse fluorescence technique was applied to study photo inhibition of photosynthesis in outdoor cultures of the cyanobacterium Spirulina platensis (Nordstedt) Geitler strain M2 grown under high ox ygen and low temperature stress. Diurnal changes in maximum photochemi cal yield (F-v/F-m), photon yield of PSII (Delta F/F'(m)), and nonphot ochemical quenching (qN) were measured using a portable, pulse-amplitu de-modulated fluorometer. When solar irradiance reached the maximum va lue, the F-v/F-m and Delta F/F'(m) ratios of the Spirulina cultures gr own under high oxygen stress decreased by 35% and 60%, respectively, a s compared with morning values. The depression of the F-v/F-m and Delt a F/F'(m) ratios reached 55 % and 84 %, respectively, when high oxygen stress was combined with low temperature (i.e. 10 degrees C below the optimal value for growth). Photoinhibition reduced the daily producti vity of the culture grown under high oxygen stress by 33% and that of the culture grown under high oxygen-low temperature stress by 60%. Cha nges in the biomass yield of the cultures correlated well with changes in the daily integrated value of the estimated electron transport rat e through the PSII (Delta F/F'(m) x photon flux density). The results indicate that on-line chlorophyll fluorescence measurement is a powerf ul tool for assessing the photosynthetic performance of outdoor Spirul ina cultures.