Since the 1950s, microalgae have been grown commercially in man-made cultivation units and used for biomass production as a source of food and feed supplements, pharmaceuticals, cosmetics and lately biofuels, as well as a means for wastewater treatment and mitigation of atmospheric CO₂ build-up. In this work, photosynthesis and growth affecting variables—light intensity, pH, CO₂/O₂ exchange, nutrient supply, culture turbulence, light/dark cell cycling, biomass density and culture depth (light path)—are reviewed as concerns in microalgae mass cultures. Various photosynthesis monitoring techniques were employed to study photosynthetic performance to optimize the growth of microalgae strains in outdoor cultivation units. The most operative and reliable techniques appeared to be fast-response ones based on chlorophyll fluorescence and oxygen production monitoring, which provide analogous results.

Light and salt stress affect the growth of plants and microorganisms, causing photo-oxidative stress. The cyanobacterium Synechocystis is notable for its adaptability to and sustainability in seawater. In this study, the synergistic effects of different light intensities and salt concentrations on the growth and biomass composition of Synechocystis were examined. Cultures were grown in BG11 medium (control) and with 20 and 40 g L⁻¹ marine salts (obtained from a commercial sea water preparation) at 100, 200, and 400 μmoles photons m⁻² s⁻¹ (LL, ML, and HL, respectively) to assess the interactive effects of salinity stress and increasing light intensity. The effect of salinity stress was most pronounced under LL and ML, where the highest accumulation of all major carotenoids was observed; under HL, the contents of most carotenoids significantly increased mainly at the highest salt concentration but to a lesser extent). Under LL and ML echinenone reached the highest values (2.71-fold and 3.75-fold higher than in the control, respectively), whereas β-carotene showed the highest increase at LL, reaching concentrations three times those of the control. At HL myxoxanthophyll exhibited the highest increase with marine salt (1.9-fold higher than in the control). The results show that Synechocystis could grow at all light intensities and marine salt concentrations via increased synthesis of carotenoids in response to physiological stress.