## 3 Results and Discussion  
In the different photoperiods studied, _Spirulina_ sp. LEB 18 cultures that grew in a medium supplemented with glycerol attained higher biomass concentrations than culturesthat did not contain this source of organic carbon; the glycerol-added cultures showed 2.3, 3.9, and 4.6 times higher concentrations than the control cultures (24:0, 12:12, and 0:24, respectively). In all assays supplemented with glycerol, from 0 to 24 h, a lag phase occurred, where there was an adaptation of the cells to the culture medium. After 24 h, an exponential growth phase was observed. The control experiments did not show exponential growth phases (Figure 1a). According to [PERSON] et al. [27], high cell densities can be obtained during cultivation using combinations of organic and inorganic carbon sources due to the availability of large quantities of energy, which promotes a combination of aerobic respiration and catabolism of the glycerol present in the medium during photosynthesis. Among the experiments supplemented with glycerol, the one with the 12:12 photoperiod showed the highest biomass concentration (1.24 g L\({}^{-1}\), \(p\) > 0.05) and maximum productivity (28.36 mg L\({}^{-1}\) h\({}^{-1}\), \(p\) >0.05), with a generation time of 7 h (Table 1). The specific growth rate under this condition was 0.10 h\({}^{-1}\), with no significant differences from the other photoperiods using glycerol supplementation studied. This condition also resulted in 100% glycerol consumption at 36 h. The other photoperiods resulted in consumption  
Figure 1: Growth curves for (**a**) cell productivity, (**b**) glycerol consumption, and (**c**) _Spirulina_ sp. LEB 18 cultured using glycerol supplementation (G) and control (C) under different photoperiods: continuous light (24:0), 12 h light/12 h dark (12:12), and no light (0:24). levels of 50.2% and 54.5% (24:0 and 0:24, respectively, Figure 1c). Experiments 24:0 and 0:24 did not show significant differences (\(p>0.05\)) in biomass concentration (0.91 g L\({}^{-1}\) and 1.01 g L\({}^{-1}\), respectively) or productivity (20.16 mg L\({}^{-1}\) h\({}^{-1}\) and 22.02 mg L\({}^{-1}\) h\({}^{-1}\), respectively). For all cultures grown using the organic carbon source, the acceleration in productivity coincided with the beginning of the exponential phase of cell growth at 24 h (Figure 1b). Heterotrophic growth and photosynthesis can occur simultaneously and independently in mixotrophic cultures, and the presence of an organic carbon source alters both photosynthetic and heterotrophic metabolism, enhancing cell growth and biomass productivity. This behavior was reported by [PERSON] and [PERSON] for the microalgae _Spiulina_ sp. grown in a medium supplemented with glucose in a mixotrophic culture [28]. This may explain the higher biomass concentration and productivity of _Spiulina_ sp LEB 18 cultures grown in the 12:12 photoperiod in a medium supplemented with glycerol compared with the other investigated photoperiods (Table 1). [PERSON] et al. [9] cultured _Spiulina_ sp. LEB 18 using media supplemented with glycerol (2.5 g L\({}^{-1}\)) and reported a similar maximum cell concentration of 1.08 g L\({}^{-1}\) at 36 h for a culture grown in a photoperiod of 12:12. According to [PERSON] et al. [29], the presence of organic carbon can reduce the production of proteins involved in photosystem II, and ribulose-1,5-bisphosphate carboxylase/oxygenase in the Calvin cycle can repress photoautotrophic growth in algae. A synergistic effect between light and organic substrates on microalgal growth was previously reported by [PERSON] et al. [30]. The authors found that under continuous illumination (24:0) in a medium supplemented with glucose, _Nanochloropis gagitana_ reached the stationary phase earlier with lower productivity (29 mg L\({}^{-1}\) day\({}^{-1}\)) than mixotrophic cultures. [PERSON] et al. [31] evaluated the growth of _Chorella sorkiniana_ under different light intervals and showed that the presence of dark stages results in better productivity compared with continuous lighting.