Standard culture medium for Arthrospira spp. cultivation is rich in bicarbonate and carbonate. This medium is alkaline due to the presence of these ions. At pH values below 6.4, carbon dioxide is the predominant form of carbon source. At pH values between 6.4 and 10.3, the predominant form is bicarbonate. Above pH 10.3, the predominant form is carbonate. Miller and Colman [65] reported that bicarbonate is the carbon form preferentially assimilated by cyanobacteria, which explains the decreasing of Spirulina biomass concentration in the pond at pH values above 10.2­

10.4 [49].

Considering ammonia from ammonium salts or urea, in the chemical equilib­rium established at pH 9.3, both ammonium ions and ammonia are present. At pH values below 7.0, the predominant form is ammonium ion and above pH 12.0, only the ammonia is present in culture medium.

Ammonia uptake by S. platensis is pH-dependent. In alkaline conditions, ammonia enters the cell by simple diffusion, driven by a pH gradient, and is intracellularly assimilated by the action of the enzyme glutamine synthetase [10].

It should also be mentioned that ammonia, depending on its concentration and pH of the medium, is toxic to most microorganisms [1], including S. platensis [6]. These authors found that at pH 7.0, S. platensis LB1475/a and Anabaena sp. were not affected by ammonia. However, at pH 10.0 with ammonia concentrations of 10 mM, only 50% of photosynthetic activity remained for the S. platensis LB1475/a, while for Anabaena sp. photosynthetic activity has ceased. Thus, since the use of ammo­nium salts or urea as nitrogen sources lead to a release of ammonia in the culture medium, it would be limited by process by which the cultivation is carried out.

Therefore, pH is a very important variable to be studied, since it may affect the form of the carbon reservoir, urea decomposition and how ammonia is presented in the culture medium (protonated or unprotonated). Sanchez-Luna et al. [87] studied the effect of pH on A. platensis cultivation using urea as nitrogen source and found that A. platensis grows well at pH 9.0-10.0, with an optimum pH of 9.5. This pH range is in agreement with Belay [ 5] , who suggests maintaining the pH of the medium above 9.5 in outdoor cultivations aiming to avoid contamination by other microalgae.