# _Arthrospira platensis_ as a Feasible Feedstock for Bioethanol Production
## 2 Growth Requirements for Biomass Production  
The attempt in this first section is just to point out some key parameters influencing biomass production, while the objective of the review is to further highlight specific conditions promoting glycogen accumulation. _Spirulina_ cultivation has been studied for a long time as it is probably the most cultivated photosynthetic microorganism worldwide. Under this generic name, several cyanobacteria may be encountered, especially species belonging to the genus _Arthospira_. In a recent publication from [PERSON] et al. [33], phylogenetic analyses based on the 16S rRNA gene have allowed for a new classification of most of the commercially and bank deposited strains, including creation of a new genus under the name _Limitospira_ to be established. However, this paper will not discriminate between these genus as it is not possible to clearly compare previous works with one or the other. Since the historical work of [PERSON] [34] on _Spirulina maxima_, numerous studies have been conducted focusing on abiotic factors such as light intensity, temperature, medium composition, pH, and salinity. Both indoor and outdoor conditions were studied in photobioreactors or open ponds. Subsequently, the individual or the combined effects of these environmental factors on biomass productivity are now well documented. It is well known that numerous parameters influence the growth or protein content of microalgae: light, temperature, salinity, CO\({}_{2}\) addition, nutrient addition, inoculation size, stirring, pH, etc. [34; 35; 36]  
Most _Arthospira_ species currently grown in mass culture were isolated from alkaline and saline or brackish waters characterized by high levels of carbonate-bicarbonate and high pH levels. [PERSON] [34] determined the precise composition of the water in Lake Tchad Lake, a natural habitat for the _Arthospira maxima_ strain used in his study, allowing him to propose a synthetic medium that is still the basis of all cultivation media classically used for _Arthospira_ species. However, study of [PERSON] et al. [37] has shown that all compounds in Zarrouk medium were not necessary to support _Arthospira_ growth, thus proposing a simplification of the medium. The pH of these media varies between 8.3 and 10, as [PERSON] [34] showed that the growth rate was similar across this range. [PERSON] and [PERSON] [38] confirmed that the maximal growth rate for _Arthospira_ was obtained in the 9.5-9.8 range. When incubated at pH 7.0, the growth rate of _Arthrospira_ was severely inhibited and was only 20% of that under the optimal conditions. Moreover, an external pH of 9.5 will lead to an internal pH of 7.5, which is the optimal pH for RuBisCo [39]. This high pH (\(>\)8) requirement clearly defines _Arthrospira_ as an obligatory alkalighile [40]. Generally, the waters populated by _Arthrospira_ have a mean salinity of 37 g L\({}^{-1}\). However, _Arthrospira_ has been found at salinity levels ranging from 8.5 to 200 g L\({}^{-1}\) and in at least one case, up to 270 g L\({}^{-1}\)[41]. Salinity values used for laboratory or mass cultivations generally range from 22 to 60 g L\({}^{-1}\). In nature, _Arthrospira_ is found in permanent or temporary water bodies at relatively high temperatures. The optimal temperature for the cultivation of this organism is about 36 \({}^{\circ}\)C [34; 42], but many studies at 30 \({}^{\circ}\)C can also be found. A detailed study on the response of a _Spirulina_ strain marked M-2 was performed by [PERSON] and [PERSON] [43] and the optimal temperature for photosynthesis was 35 \({}^{\circ}\)C. However, many _Arthrospira_ strains differ in their optimal growth temperature as well as in their sensitivity to extreme values. [PERSON] [8] tested three different strains, one with a relatively low temperature optimum of 30-32 \({}^{\circ}\)C, while another grew well at a temperature of up to 40-42 \({}^{\circ}\)C.  
Without a doubt, light is the most important factor affecting photosynthetic organisms.