Modulation of biomass composition by CO2 in two highly productive green microalgae as a sustainable system for byproduct generation

Microalgae have the potential to mitigate atmospheric CO₂ and generate sustainable byproducts. However, how CO₂ impacts C-flux in different species is not clear. This study evaluated the effect of CO₂ concentration from industry exhaust gas on growth and biochemical composition of Desmodesmus abundans and Chlorella vulgaris. Gas supply (25 % v/v CO₂/air) stimulated growth of both species (1.38 ± 0.02 and 1.56 ± 0.02 g L⁻¹ for D. abundans and C. vulgaris, respectively). Proteins were the major metabolite at high CO₂, especially for C. vulgaris, while D. abundans showed no variation (51 ± 0 and 49 ± 3 % d.w., respectively). Both species exhibited 1.2–2.2-fold higher carbohydrate and starch content under air. Also, under air, C. vulgaris showed an increase of 2 to 3-fold in lipids, contrary to D. abundans that increased under high CO₂ during exponential growth. However, as biomass production was optimal at high CO₂, all metabolite concentrations were highest at 25 % CO₂. Interestingly, D. abundans exhibited 2 to 3-fold higher pigment content under high CO₂, suggesting a differential adaptation to high CO₂. Lastly, a rapid and cost-effective protein extraction protocol was optimized (95 °C, 15 min, NaOH 1N) with comparable yields to the standard Micro-Kjeldahl method during exponential growth and some variation at stationary phase (80–89 % yield). The effect of high CO₂ was species-specific, where D. abundans appears as a robust species. Further studies should evaluate the nature of the protein extract for potential applications and consider technical, economic, and environmental feasibility for system scale-up.