Rapidly growing demands for energy and environmental concerns about carbon dioxide emissions make development of renewable biofuels more and more attractive. Tremendous academic and industrial efforts have been paid to produce bioethanol, which is one major type of biofuel. The current commercialization production of bioethanol is limited due to food competition (from food-based biomass) or cost effectiveness (from lignocellulose-based biomass). As a candidate for biofuel-producing microbial systems, cyanobacteria are attractive because they incorporate the favorable characteristics of prokaryotes and plants.

Metabolic engineering group, led by Prof. LU Xuefeng, at Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, have been working on photosynthetic bioethanol production in genetically engineered cyanobacteria. The research group applied a consolidated bioprocessing (CBP) strategy to integrate photosynthetic biomass production and microbial conversion producing ethanol together into the Synechocystis sp. PCC6803, a photosynthetic bacterium, which can directly convert CO₂ to ethanol in one single biological system.

Prof. LU’s group constructed a Synechocystis sp. PCC6803 mutant strain, overexpressing heterologous pyruvate decarboxylase from Zymomonas mobilis and endogenous alcohol dehydrogenase through homologous recombination at two different sites of the chromosome and disrupting the biosynthetic pathway of poly-b-hydroxybutyrate, that displayed significantly higher ethanol-producing efficiency compared to previous research. The ethanol production of this genetically engineered cyanobacteria strain reached to 5.5 g/L (212 mg/L/day), which is the highest yield ever reported.

The group have cloned nine alcohol dehydrogenases from different cyanobacterial strains and then tested their ethanol-producing efficiency via expressing in E. coli. They also evaluated the effects of different culturing conditions including tap water, metal ions, and anoxic aeration on ethanol production. The research findings not only demonstrate that biosynthesis of biofuel molecules in the single photosynthetic cyanobacterial cell with direct conversion of solar energy and carbon dioxide is technically feasible, but also prove that the potential for improving biosynthetic efficiency of biofuels in cyanobacteria by metabolic and genetic engineering is huge.

The research work has been published online in the journal of Energy and Environmental Science (DOI: 10.1039/C2EE22675H) on September 26, 2012; and supported by the grant from National High-Tech Research and Development Program of China (863).