Temperature is known to be a parameter of great importance for the growth of all the microorganisms, since it affects all metabolic activities as well as nutrient availabil­ity and uptake [109].

Vonshak [109] reported for S. platensis cultivation an optimum temperature ranging from 24 to 38°C, depending on the strain. However, satisfactory growth was shown starting at 20°C [ 81] . The usual optimal temperature for cultivation of Spirulina spp. is in the range 35-38°C [109]. However, it must be pointed out that this range of temperature is arbitrary. Many Spirulina strains will differ in their optimal growth temperature, as well as in their sensitivity to extreme ranges [109].

It was previously proposed [28, 98] that cell growth in different bioprocesses can be kinetically described assuming its direct dependence on the activity of one enzyme limiting the overall metabolism. Based on this supposition, the thermody­namics of the system was described resorting to the so-called thermodynamic approach, according to which the specific growth rate increases with temperature would be contrasted by a progressive activity decrease owing to “thermal inactiva­tion” [87]. These authors, who worked with urea as a nitrogen source by a fed-batch process, demonstrated that the best growth temperature was 30°C, which is in accordance with previous results [48, 83]. Besides, Danesi et al. [33] performed S. platensis cultivation at 36.8°C, where there was decline in cell growth, thus sug­gesting possible thermal inactivation of this microorganism, as observed for other microorganisms [37, 90, 98]. Contrary to the use of nitrate as a nitrogen source [109],this cyanobacterium grew reasonably well even at lower temperature (23.3°C) when urea was used as a nitrogen source [87]. It could have happened due to the fact that urea hydrolysis to ammonia has less energy requirements than the reduction of nitrate to ammonia; therefore, the thermal situation at this temperature may be sufficient to sustain nitrogen metabolism. Moreover, when using nitrogen sources that lead to the presence of ammonia in the cultivation medium, the availability of the nitrogen source in the A. platensis cultivation depends on temperature. In fact, when urea is used as a nitrogen source, it can be hydrolyzed to ammonia by urease [93] and/or spontaneous hydrolysis in alkaline medium [33], releasing ammonia, which may be lost by off-gassing [86]. The temperature would then have a dual influence on the culture: besides the fact that higher temperatures can promote microorganism growth, it can also increase the off-gassing of ammonia to the envi­ronment and may lead to a nitrogen limitation in the culture medium. Such fact is particularly important when working with open ponds.