Step-wise SO2-feeding strategies for microalgae-based CO2 fixation from flue gas and bioenergy production

Abstract

Aiming at eliminating the negative effect of SO2 on microalgae CO2 fixation caused by inappropriate SO2-feeding strategies, physiology-based step-wise SO2-feeding strategies were proposed to enhance the bioenergy production of microalgae. The step-wise SO2-feeding strategy (four-step gradient increase from 100 to 400 ppm followed by four-step gradient decrease from 400 to 100 ppm) successfully realized the gratifying proliferation of microalgae by maintaining a better activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), total adenosine triphosphatase (Total ATPase), superoxide dismutase (SOD), and catalase (CAT) enzyme. Moreover, the step-wise SO2-feeding strategy, which considered the microalgal tolerance to sulfur at different growth stages, reduced the accumulation rate of toxic S-compounds in the culture medium by 47.2% and ensured a favorable solution pH of around 7 without the addition of alkaline chemical reagents. Sulfur replenishment and utilization were well-matched (SO2 → HSO3-+SO32-→SO42-→intracellular ammonia acid) under a step-wise SO2-feeding strategy, contributing to a maximum microalgal biomass concentration of 3.26 g L-1, a photosynthetic efficiency of 10.89 %, and biomass energy of 18.97 kJ, which was 85.2%, 185%, and 390% higher than the microalgae cells cultivated under 300 ppm SO2, respectively. Moreover, an increment of 81.4% in CO2 utilization efficiency was also achieved. Overall, this study provided an effective method for achieving microalgal carbon capture from industrial flue gas with high SO2 concentrations.