Effect of light and temperature on the photosynthetic activity of the cyanobacterium Spirulina platensis

The response of the photosynthetic activity of Spirulina platensis M2 to temperature and light stress was studied. The optimal temperature for photosynthesis was 35°C, while dark respiration was highest at 45 C. At temperature extremes outside those optimal for growth, both respiratory and photosynthetic activity declined. However, the sensitivity of respiration to such extremes was significantly greater than the sensitivity of photosynthesis under the same conditions. Under conditions where respiration was completely inhibited, photosynthetic oxygen evolution was maintained at about 30% of the optimum value. Exposing Spirulina cells to high photon flux densities results in a significant reduction in all the photosynthetic parameters (i.e. initial slope, light-saturated rate and the convexity of the photosynthesis-light response curve). The photoinhibitory stress resulted in much larger decreases in the quantum yield than in the light-saturated rate of photosynthetic oxygen evolution. The extent of photoinhibition was much higher when applied at temperatures over or below the optimum for photosynthesis. The implications of these findings are discussed with respect to the outdoor cultivation of Spirulina.