Enhancing CO₂ bio-mitigation by genetic engineering of cyanobacteria

The increasing of atmospheric CO₂, which is considered as a major greenhouse gas, plays a crucial role in global warming and climate change. In addition to reducing CO₂ emissions from anthropogenic activities, it is more urgent to actively remove CO₂ from the air. Carbon capture and storage (CCS) is a feasible but high-cost technology to remove CO₂ from the flue gases of coal-fired power plants. On the other hand, CO₂ sequestration by biological approaches shows potential and has the benefit that the biomass generated from the fixed CO₂ can then be utilized for other purposes. However, CO₂ bio-mitigation technology is still under development because the efficiency of CO₂ capture and fixation is too low to be applicable in industry. In this study, we enhanced a photobioreactor-based microalgal CO₂ mitigation system by combining the chemical capture/transformation of CO₂ by carbonic anhydrase (CA) with the biological fixation of captured CO₂ by cyanobacteria. We genetically engineered the cyanobacteria to produce and secrete CAs in the medium. The secreted CAs efficiently transformed dissolved CO₂ to HCO₃^-. And HCO₃^- was taken up by the cyanobacteria and further fixed into biomass through photosynthesis. To our knowledge, we have demonstrated for the first time that CO₂ can be sequestrated in a sustainable way through combining the chemical transformation of CO₂ with the biological CO₂ fixation in a microalgal photobioreactor system.