| Summary: | The biosequestration of CO<sub>2</sub> using microalgae has emerged as a promising means of recycling CO<sub>2</sub> into biomass via photosynthesis, which could be used to produce biofuels as an attractive approach to CO<sub>2</sub> mitigation. We investigated the CO<sub>2</sub> fixation capability of the native nonaxenic microalgal culture using a 2 L photobioreactor operated in batch mode. The cultivation was carried out at varying concentrations of total dissolved CO<sub>2</sub> (Tco<sub>2</sub>) in the bulk media ranging from 200 to 1000 mg L<sup>−1,</sup> and the temperature and light intensities were kept constant. A maximum CO<sub>2</sub> fixation rate was observed at 400 mg L<sup>−1</sup> of Tco<sub>2</sub>. Characteristic growth parameters such as biomass productivity, specific growth rate, maximum biomass yield, and biochemical parameters such as carbohydrate, protein, and lipids were determined and discussed. We observed that the effect of CO<sub>2</sub> concentration on growth and biochemical composition was quite significant. The maximum biomass productivity was 22.10 ± 0.70 mg L<sup>−1</sup> day<sup>−1</sup>, and the rate of CO<sub>2</sub> fixation was 28.85 ± 3.00 mg L<sup>−1</sup> day<sup>−1</sup> at 400 mg L<sup>−1</sup> of Tco<sub>2</sub>. The maximum carbohydrate (8.17 ± 0.49% dry cell weight) and protein (30.41 ± 0.65%) contents were observed at 400 mg L<sup>−1</sup>, whereas the lipid content (56.00 ± 0.82% dry cell weight) was the maximum at 800 mg L<sup>−1</sup> of Tco<sub>2</sub> in the bulk medium.
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