## 3 Results and Discussion
### Effects of Light Intensity and Photoperiod  
Light intensity is an environmental factor that plays a vital role in the growth and CO\({}_{2}\) fixation of microalgae. In photosynthesis production, microalgae require a light/dark system. They require a light system to produce organic cofactors (ATP and NADPH) and a dark system to synthesize biomolecules for algae growth (e.g., carbohydrates, protein, and lipids) [32]. The light intensity levels for the inhibition and saturation of algal growth depend upon the propriety of other environmental factors such as CO\({}_{2}\) concentration, temperature, and nutrient supplementation [33]. In this study, the microalgae, _C. variabilis_ RSM09, was cultivated under different light intensities. Figure 6 illustrates the cell growth curve of _C. variabilis_ RSM09 at light intensities of 3000, 5000, and 7000 Lux, respectively, in the 24:0 h (light/dark) photoperiod. In the first two days, all the cultures showed slow growth, indicating that the cells were in the adaptation period (lag phase). From day 2 to 6, all the cultures entered the exponential growth phase during which the cells multiplied fast. From day 6 onwards, the cultures entered the early stationary phase. The highest optical density (4.20 \(\pm\) 0.14) was achieved at the light intensity of 5000 Lux whereas the lowest cell density (3.87 \(\pm\) 0.10) was obtained at the light intensity of 7000 Lux. Figure 5: Dry weight and lipid content of _C. variabilis_ RSM09 under different CO\({}_{2}\) concentrations. Different letters represent statistically significant difference (\(p<0.05\)). Figure 6: Growth curve of _C. variabilis_ RSM09 under different light intensities in 24:0 h photoperiod. Error bars show \(\pm\) S.D. of three replicates. Figure 7 shows the biomass and lipid concentrations of _C. variabilis_ RSM09 under different light intensities during a 14-day culture period. From Figure 7, the dry weights of _C. variabilis_ RSM09 were found between 1.55 \(\pm\) 0.17 g/L and 1.79 \(\pm\) 0.25 g/L under different light intensities. The dry weight is highest under 5000 Lux, accounting for 1.79 \(\pm\) 0.25 g/L. The result is similar to that reported by [PERSON] et al. [34]. They found that the biomass concentration reached the maximum at 5000 Lux with 1884 \(\pm\) 12.55 mg/L. For photoautotrophic microalgae, the light supply significantly impacts cell development and CO\({}_{2}\) fixation in the photosynthetic system. It may also alter the biochemical makeup of the microalgae, such as its content of lipids or carbohydrates [35]. In this study, the lower total lipid content of _C. variabilis_ RSM09 was detected under 3000 lux, while it is higher under 5000 Lux (Figure 7). [PERSON] et al. [36] reported that a light intensity of 5000-6000 Lux could promote the lipid accumulation of _Scenedesmus abundans_ with a maximum lipid content of 32.77 \(\pm\) 0.6% wt. The effect of three different lengths of photoperiod conditions at a light intensity of 5000 Lux on the growth of _C. variabilis_ RSM09 for 14 days of the cultivation period is illustrated in Figure 8. The results indicate that microalgae growth increased as the photoperiod extended from 12:12 to 24:0 h (L/D cycle). Figure 9 shows the biomass concentration and lipid content of _C. variabilis_ RSM09 under different photoperiods. It found that the photoperiod had a high effect on biomass concentration. The increase in dry cell weight was associated with increased light duration. The lowest dry weight (1.35 \(\pm\) 0.43 g/L) was observed at 12:12 h L/D, and the highest dry weight (1.86 \(\pm\) 0.21 g/L) was found at 24:0 h L/D at the end of the cultivation period. More extended light periods encourage photosynthesis, and biomass concentration may be raised by altering the intensity and wavelength. [PERSON] et al. [37] reported that when compared with the control (12:12 photoperiod), the biomass of _Oscillatoria_ sp. OSCI_UFPS001 was increased significantly by up to 1.6 g/L at a photoperiod of 24:0 (L/D cycle).