Synergistic effects between autotrophy and heterotrophy in optimization of mixotrophic cultivation of Chlorella sorokiniana in bubble-column photobioreactors

Abstract

Mixotrophic cultivation of microalgae enables inorganic carbon capture through photosynthesis along with simultaneous organic carbon uptake. Extremophilic microalga Chlorella sorokiniana, capable of tolerating high CO2 levels, was cultivated mixotrophically in 1 L bubble-column photobioreactors. This study shows how the biomass yield increases as the algal CO2 uptake process shifts from being driven by Carbon Concentrating Mechanism (CCM) to being driven by direct diffusion, as the CO2 concentration increases from very low (0%) and low (atmospheric: 0.04%) to high (1%, 2%, 3%, 5%). The biomass yield and productivity increase from 1.05 g/L and 0.33 g/L/day, respectively, at atmospheric CO2 to 1.85 g/L and 0.48 g/L/day, respectively, at 3% CO2, with a corresponding doubling of the lipid yield. Using acetic acid as the organic carbon source allows it to directly enter the metabolic pathways at the acetyl-CoA junction and enhance fatty acid synthesis, while enhanced reactor illumination (below photo-inhibition) synergizes the phototrophic and photo-heterotrophic pathways. These three reactor scale parameters, namely, organic (acetic acid) and inorganic (CO2) carbon loading and illumination, are co-optimized to activate the synergies between the metabolic pathways for autotrophy and heterotrophy in mixotrophic algal growth, which, in turn, significantly enhances the biomass and the macromolecular yields at the cellular scale. This multiscale synergy leads to a four-fold increase in the biomass yield (from 0.7 to 2.79 g/L), a five-fold increase in biomass productivity (from 0.124 to 0.593 g/L/day), and a six-fold increase in the total lipid yield (from 0.12 to 0.75 g/L), while eliminating night-time biomass losses. Such an approach of co-optimising reactor-scale parameters to synergize the autotrophic and the heterotrophic metabolic pathways can be implemented in mass cultivation of microalgae, resulting in faster carbon sequestration and higher yields of biomass, lipids and other value-added co-products.