## 3 Results and Discussion
According to [PERSON] et al. [3], the absence of light is necessary during the synthesis of essential metabolic molecules for microalgal growth from ATP and NADPH. In the 0:24 culture, only the organic carbon source was used, as the inorganic source could not be metabolized during the dark stage. This was confirmed by the fact that the dark control showed no change in biomass concentration and was probably maintained by the energy produced before the start of cultivation. According to [PERSON] et al. [32], during mixotrophic growth, organic carbon is used to produce biomass and energy via a \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & \(\mathbf{X_{max}}\) & \(\mathbf{P_{max}}\) & \(\mathbf{\mu_{max}}\) & \(\mathbf{t_{g}}\) & \(\mathbf{Proteins}\) & **Carbohydrates** & **Lipids** \\  & (g L\({}^{-1}\)) & (mg L\({}^{-1}\) h\({}^{-1}\)) & (h\({}^{-1}\)) & (h) & (\% w w\({}^{-1}\)) & (\% w w\({}^{-1}\)) & (\% w w\({}^{-1}\)) \\ \hline  & & & & & & & \\ \hline Glycerol & 0.91 b \(\pm\) 0.05 & 20.16 b \(\pm\) 2.91 & 0.08 a \(\pm\) 0.01 & 8.88 a \(\pm\) 0.45 & 45.3 b \(\pm\) 2.73 & 23.8 a \(\pm\) 1.17 & 7.3 a \(\pm\) 0.54 \\ Control & 0.39 c \(\pm\) 0.01 & 4.76 c \(\pm\) 0.42 & - & - & 47.6 b \(\pm\) 0.36 & 14.5 b \(\pm\) 2.86 & 8.4 a \(\pm\) 0.58 \\ \hline  & & & & & & & \\ \hline Glycerol & 1.24 a \(\pm\) 0.01 & 28.36 a \(\pm\) 1.38 & 0.10 a \(\pm\) 0.01 & 7.01 b \(\pm\) 0.44 & 52.2 b \(\pm\) 3.92 & 15.9 b \(\pm\) 0.35 & 8.0 a \(\pm\) 0.94 \\ Control & 0.32 d \(\pm\) 0.02 & 3.01 d \(\pm\) 0.47 & - & - & 50.5 b \(\pm\) 2.13 & 14.7 b \(\pm\) 0.47 & 8.9 a \(\pm\) 0.48 \\ \hline  & & & & & & \\ \hline Glycerol & 1.01 b \(\pm\) 0.01 & 22.02 b \(\pm\) 1.60 & 0.10 a \(\pm\) 0.01 & 7.10 b \(\pm\) 0.35 & 58.1 a \(\pm\) 1.69 & 11.4 c \(\pm\) 3.69 & 7.3 a \(\pm\) 0.98 \\ Control & 0.22 c \(\pm\) 0.01 & 1.35 e \(\pm\) 0.89 & - & - & 43.1 b \(\pm\) 1.76 & 11.4 c \(\pm\) 1.00 & 8.9 a \(\pm\) 0.54 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Maximum biomass concentration (\(\mathbf{X_{max}}\)), maximum productivity (\(\mathbf{P_{max}}\)), maximum specific growth rate (\(\mathbf{\mu_{max}}\)), generation time (\(\mathbf{t_{g}}\)), protein, carbohydrate, and lipid contents for _Spiulina_ sp. LEB 18 cultured using glycerol supplementation under different photoperiods: continuous light (24:0), 12 h light/12 h dark (12:12), and no light (0:24). Average \(\pm\) standard deviation. The same superscript in the same column indicates that the averages do not differ significantly at the 95% confidence level (\(p>0.05\)). heterotrophic metabolism, which generates CO2 that can be assimilated during the light-driven photoautotrophic stage; this does not occur during cultivation in the absence of light and results in lower productivity. According to [PERSON] et al. [4], photoperiod modification causes changes in chemical composition, photosynthetic activity, and pigment concentration in microalgal cultures. In the present study, proteins were the predominant macromolecules in _Spirulina_ biomass grown under different photoperiods with and without glycerol supplementation, followed by carbohydrates and lipids, respectively. With respect to the influence of photoperiods on biomass composition, the 24:0 culture showed an increase in the concentration of carbohydrates (23.8% w w\({}^{-1}\)). On the other hand, the 0:24 culture showed positive influences on the concentration of proteins (58.1% w w\({}^{-1}\), \(p\) > 0.05). According to [PERSON] et al. [31], under constant lighting, an increase in productivity and biomass production may occur during cultivation with organic carbon due to CO2 conversion into carbohydrates upon consumption of the substrate. The 12:12 culture showed no variation in biomass composition compared with its control; however, due to high biomass productivity, the lipid and protein yields (99.45 mg L\({}^{-1}\) and 647.28 mg L\({}^{-1}\), respectively) were higher for this photoperiod relative to the others. For the concentration of carbohydrates, the 24:0 culture showed the highest yield (216.19 mg L\({}^{-1}\)).