Spirulina is, like most cyanobacteria, an obligate photoautotroph, i.e. it cannot grow in the dark on media containing organic carbon compounds. It reduces carbon dioxide in the light and assimilates mainly nitrates. The main assimilation product of spirulina photosynthesis is glycogen. Spirulina shows an optimum growth between 35 and 37 °C under laboratory conditions. Outdoors, it seems that an increase in temperature up to 39 °C for a few hours does not harm the blue-green alga, or its photosynthetic ability. Thermophilic or thermotolerant strains of spirulina can be cultivated at temperatures between 35 and 40 °C. Such a property has the advantage of eliminating microbial mesophilic contaminants. The minimum temperature at which growth of spirulina takes place is around 15 °C during the day. At night, spirulina can tolerate relatively low