Comparison analysis on the energy efficiencies and biomass yields in microbial CO 2 fixation

Abstract

Abstract CO 2 fixation by a hydrogen-oxidizing bacterium, Cupriavidus necator, was evaluated in a packed bed bioreactor under a constant flow rate of gas mixtures (H 2 , O 2 , CO 2 ). The overall energy efficiency depends on the efficiencies of CO 2 fixation into carbohydrate and the reduced carbon into biomass and bioproducts, respectively. The efficiencies varied with the limiting gas substrate. Under O 2 limitation, the efficiency (20–30%) of CO 2 fixation increased with time and was higher than the overall efficiency (12–18%). Under H 2 limitation, the efficiency of CO 2 fixation declined with time while the biomass yield was quite similar to that under O 2 limitation. A cellular metabolic model was suggested for the lithoautotrophic growth of C. necator , including CO 2 fixation into carbohydrate followed by the main metabolic pathway of reduced carbon. Under CO 2 limitation, most H 2 energy was wasted, resulting in a very low biomass yield. Under a dual limitation of O 2 and nitrogen, biosynthesis of poly(3-hydroxybutyrate) was triggered, and the energy efficiency or yield of biopolyester was lower than those of microbial cell mass. Compared with a green microalga Neochloris oleoabundans that produces lipid under nutrient limitation, C. necator exhibited a much higher (3–6 times) energy efficiency in producing biomass and bioproducts from CO 2 .