• cyanobacteria;
  • fluorescence;
  • O2 concentration;
  • photobioreactor;
  • photoinhibition;
  • Spirulina platensis;
  • temperature

The saturating pulse fluorescence technique was applied to study photoinhibition of photosynthesis in outdoor cultures of the cyanobacterium Spirulina platensis (Nordstedt) Geitler strain M2 grown under high oxygen and low temperature stress. Diurnal changes in maximum photochemical yield (Fv/Fm), photon yield of PSII (ΔF/F′m), and nonphotochemical quenching (qN) were measured using a portable, pulse-amplitude–modulated fluorometer. When solar irradiance reached the maximum value, the Fv/Fm and ΔF/F′m ratios of theSpirulina cultures grown under high oxygen stress decreased by 35% and 60%, respectively, as compared with morning values. The depression of the Fv/Fm and ΔF/F′m ratios reached 55% and 84%, respectively, when high oxygen stress was combined with low temperature (i.e. 10° C below the optimal value for growth). Photoinhibition reduced the daily productivity of the culture grown under high oxygen stress by 33% and that of the culture grown under high oxygen–low temperature stress by 60%. Changes in the biomass yield of the cultures correlated well with changes in the daily integrated value of the estimated electron transport rate through the PSII (ΔF/F′m × photon flux density). The results indicate that on-line chlorophyll fluorescence measurement is a powerful tool for assessing the photosynthetic performance of outdoorSpirulina cultures.