Figure 1 shows the growth of the cyanobacterium S. platensis for the two conditions assessed (1 and 2 g.L^-1 of NaHCO₃). In the experiment, the concentration of NaHCO₃ of 2.0 g.L^-1 resulted in the greatest cell concentrations (1.0 g.L^-1). In cultures carried out with 1.0 g.L^-1 of NaHCO₃, the production rates (via photosynthesis) and consumption of biomasses (via respiration) were equal after the sixth day. This phenomenon is known as the compensation point and occurs when the photosynthetic activity carried out during the day equals the heterotrophic activity that occurs during the night. In this assay, the compensation point is associated with the limitation of growth due to lack of nutrients (in this case, the carbon).

When the NaHCO₃ concentration was 2.0 g.L^-1, the rate grew continuously throughout the cultivation period, and there was no compensation point in this condition. The maximum cell concentration obtained in these experiments was twice as high as that obtained when 1.0 g.L^-1 of NaHCO₃ was added, showing that there is a direct relationship between the concentration of dissolved inorganic carbon with the cellular concentration maintained in the cultures (Table 1).

The bicarbonate concentration in the medium determines the rate of formation of carbon dioxide ( ), where k is the kinetic constant of the reaction. The growth rate and cell concentration determine the culture’s carbon dioxide demand; thus, the bicarbonate concentration maintained in the  medium  depends  on  these  parameters.  Table 1 presents the kinetic parameters obtained during the experiments. The same maximum specific growth was obtained for both experiments (0.76 day^-1), demonstrating that growth is not impaired under any of the conditions. However, the parameter that must be followed in this case is the cell concentration to be maintained, as this metric will determine whether the carbon is excessive (which would cause the loss of carbon into the atmosphere) or in short supply (thus limiting the growth).

Figure 1 show that, during the light phase (day time), growth occur at higher rates than those observed when evaluated over several days of cultivation. This is due to the consumption of the biomass resulting from the activation of the heterotrophic metabolism during the dark phase (night). Under environmental conditions, the cultures are not illuminated during the night. Vonshak and Richmond (1988) reported that the loss of biomass in Spirulina cultures caused by respiration during the night can represent 35% of the biomass produced during the day. Figure 2 shows the growth curve determined over the light phase on the fourth day with the medium containing 2.0 g.L^-1 of NaHCO₃. One alternative to significantly increase growth rates would be to keep the cultures illuminated at night, thereby avoiding the consumption of biomass. Another option to avoid the nocturnal loss of carbon is the use of organic carbon sources, such as glucose or acetate, during the night (Ogbonna and Tanaka, 1996). Such measures would ensure  biomass  growth,  even  when  the   heterotrophic (Ogbonna and Tanaka, 1996). Such measures would ensure biomass growth, even when the heterotrophic metabolism is activated, although the consumption of carbon dioxide would be zero during the entire period in which the microalga stopped photosynthesizing (Andrade and Costa, 2007).

The productivity of the culture  (Figure 3)  increased  for the test containing 2.0 g.L^-1 of NaHCO₃ during the study period. However, when 1.0 g.L^-1 of NaHCO₃ was added, the productivity fell after the sixth day of cultivation. On the first day of cultivation, when the cellular concentration was low, the difference in the concentrations of inorganic carbon did not influence the productivity of the cultures. However,   when   the  cell  concentration  increased,  the growth was limited by the carbon source.

In the study by Pelizer et al. (2003), the best results with related to specific growth rate were obtained when the cell concentration used in the inoculum was 50 mg.L^-1. However, according to Vonshak et al. (1982), the density of the Spirulina population must be 400-500 mg.L^-1. These authors showed that the maximum daily production rates of Spirulina occurred in this range, with a significant reduction in concentrations above these values.

Radmann et al. (2007) evaluated the blend concentration that must be maintained and the Zarrouk medium dilution in semi-continuous cultivations of S. platensis. According to the results obtained in this study, the highest specific growth rates were seen when the blend concentration was 400 mg.L^-1 in Zarrouk medium containing 20% v/v of the original formulation, which produced a NaHCO₃ concentration of 3.36 g.L^-1.

Figure 4a shows the behavior of the pH of the medium as a function of time of the cultivation for the experiments with 1.0 g.L^-1 of NaHCO_3. During the light phase, the pH increases, whereas during the dark phase, the pH falls, returning to its initial value. This cycle is due to two phenomena that occur during the daily cycle. During the light phase, the autotrophic metabolism consumes CO₂ and increases the medium’s pH; during the dark phase, the heterotrophic metabolism releases CO₂ in the medium, thereby reducing the pH.

Shiraiwa et al. (1993) reported an increase in the pH from 6-9 in the medium of Chlorella cultures after just a few minutes in the presence of inorganic carbon (in the form of HCO₃^-) and light. This pH increase has been linked with the carbon consumption and the production of O₂ during photosynthesis. The conversion of HCO₃^- into CO₂ and OH^- was the main cause of the change in the medium’s pH. The medium pH is a function of biological activity; thus, during the light phase when carbon dioxide is being consumed, the pH increases. During the night, when the respiration rate causes the release of carbon dioxide, the pH drops.

In the assay containing 2.0 g.L^-1 of NaHCO₃ (Figure 4b), the pH did not follow the same patterns as in the assay with 1.0 g.L^-1 of NaHCO₃; instead, the pH increased during the first days of cultivation although normally it would drop overnight. In this case, the high concentration of inorganic carbon present under these conditions led to a transfer of CO₂ to the external medium, raising the pH. In the experiment during which 2.0 g.L^-1 of NaHCO₃ was initially added, the CO₂ concentration in the medium during the first days of cultivation exceeded the demand for the cultivation. This made the pH to rise, even at night when the photosynthetic activity ceased and when the consumption of biomass would usually decrease the medium’s pH.