Synthesis of carotenoids in Spirulina platensis biomass is not only a light-dependent process, but also a thermo-dependent one. A series of experiments of inducing heat stress during cultivation of spirulina was performed in the laboratory of Ficobiotechnology. Thus, starting with the third day of spirulina cultivation, the temperature has been reduced from 400C to 250C during 4 days, after this period the temperature was reset to the initial value. Figure: Changes in beta-carotene content of spirulina biomass during cultivation under optimal thermal regime and under the induced thermal stress Reducing the temperature by 15 degrees modifies the synthesis of carotenoids. On second day of heat stress, carotene content increased by 15% compared with carotene in spirulina biomass cultivated under control thermal conditions. On the third day of stress, carotenoid content already increased by 28%. During the restoration of the thermal regime, the carotene content in the control experimental variant increased by 46%. The maximum carotene content was determined on day 8 of the cultivation cycle. Therefore, heat stress modifies carotenoid synthesis in spirulina biomass. The carotene synthesis in spirulina biomass during the induced thermal stress is determined by the recovery of biosynthetic activity in the cell.