Enhanced CO2 capture for photosynthetic lycopene production in engineered Rhodopseudomonas palustris, a purple nonsulfur bacterium

Abstract

Lycopene has been widely applied in the fields of food additives, drugs and cosmetics due to its anti-oxidative, anti-cancer, and anti-inflammatory activities. The traditional plant extraction of lycopene suffers from the unstable supply of raw materials, and for the chemical synthesis of lycopene, residues of toxic chemical reagents have limited the quality of lycopene. Lycopene in heterotrophic microorganisms suffers from the requirement of carbohydrates as feedstocks and inevitable carbon loss during cell metabolism. We here report direct lycopene production from CO₂ in photoautotrophic bacteria, Rhodopseudomonas palustris, using sustainable and carbon-neutral methods. Metabolic engineering strategies were carried out to improve the metabolic flux directed toward lycopene in R. palustris. Then, the light-driven CO₂ fixation efficiency of R. palustris was improved greatly by adaptive laboratory evolution and glycerol addition. The remarkable synergetic effect of glycerol assimilation and CO₂ fixation was demonstrated for the first time. The NADH produced in the glycerol degradation pathway provided the electron required for CO₂ fixation, and dihydroxyacetone phosphate (DHAP) produced from glycerol is beneficial for ribulose-1,5-bisphosphate (Ru1,5P) regeneration, an essential step in the Calvin–Benson–Bassham (CBB) cycle. The final engineered strain, RPLYC45, could accumulate a lycopene yield of 283.2 mg g⁻¹ DCW from CO₂ and glycerol, which was 87.4 times greater than that of the original strain and represented the highest isoprenoid production using purple non-sulfur bacteria (PNSB). Our results demonstrated the large potential of R. palustris to be engineered as a microbial cell factory using CO₂ and waste glycerol as feedstocks, a process more sustainable than the lycopene production in heterotrophic microorganisms. The enhanced CO₂ fixation rate is beneficial for solving environmental and unsustainability problems caused by the extensive utilization of fossil fuels. The utilization of glycerol as a feedstock, an excessively available by-product of the biodiesel industry, is an attractive option to green up the biodiesel industry.