Effect of photoperiods and CO2 concentrations on the cultivation of carbohydrate-rich P. kessleri microalgae for the sustainable production of bioethanol

Abstract

Biofuels production from microalgal biomass has been investigated as an alternative approach to replace conventional fossil fuels, contributing to the reduction of greenhouse gas emissions. However, several constraints such as high cultivation and harvesting costs are still limiting the techno-economic viability of microalgae-based biofuels. The integration of wastewater and CO2 streams shows strong potential for lowering the process costs as well as the environmental impacts of conventional microalgal culture systems.In this study, 25 microalgae strains were individually screened for their ability to survive and grow under various CO2 concentrations ranging from 0.04% to 100%. After selecting the most resistant strain, cultivation assays were performed in 3.5 L bubble columns reactors to investigate the effect of different light/dark cycles (4 h/20 h, 12 h/12 h and 20 h/4 h) and CO2 concentrations (0.04%, 2.5%, 5.0% and 10%) on the algal growth properties, the CO2 fixation efficiency as well as the microalgal biochemical composition.The cultivation of Parachlorella kessleri microalgae in the presence of 2.5% (v/v) of pig manure combined with 5% CO2 and light/dark cycle of 20 h/4 h generated the highest specific growth rate (0.58 d−1), biomass productivity (104 mg/L/d) and CO2 fixation rate (211 mgCO2/L/d). These culture conditions triggered the accumulation of carbohydrates in P. kessleri microalgae, resulting in the highest carbohydrate content and productivity of 44.2% and 46.1 mg/L/d, respectively. Subsequently, the carbohydrate-rich microalgal biomass was successfully used as feedstock to produce bioethanol using Saccharomyces cerevisiae yeasts.