CO2 capture and inorganic carbon assimilation of gaseous fermentation effluents using Parachlorella kessleri microalgae

Abstract Microalgal cultivation represents a promising technique for the photosynthetic fixation of CO2 present in gas streams while offering the possibility to produce valuable microalgae-based bioproducts and biofuels. CO2 can be captured using an alkaline solution and subsequently used as an inorganic carbon (IC) source for microalgal cultivation. First, the IC removal and growth properties of P. kessleri were investigated using three synthetic NaHCO3 solutions (1, 5 and 10 g/L) with or without pH control. Results indicated that regularly adjusting the pH to 8.5 had a positive effect on the IC removal as well as on the overall microalgal proliferation. However, even if significant IC removal was achieved (> 86.4 %), only a small fraction of the carbon was directly used by P. kessleri. This phenomenon could be due to the partial loss of CO2 during carbonic anhydrase activity under the effect of constant agitation. These findings were used to design a two-step process to first capture CO2 produced during anaerobic fermentation and then convert it into fresh microalgal biomass. Fermentation of sugar-beet molasses generated around 32 g/L of ethanol while most of the CO2 was absorbed in a NaOH-BBM solution using 3.5 L photobioreactors. After P. kessleri inoculation and under low agitation, microalgae accumulated up to 44.9 % of carbohydrates in 34 days achieving 97 % of IC removal and 95 % of biofixation efficency. This study shows the potential of an integrated microalgae-based CO2 treatment process with the concomitant production of microalgal metabolites including but not limited to carbohydrates.