S137: Photosynthetic conversion of CO2 and xylose to ethylene and organic acids using cyanobacteria

Algae are very efficient in capture of solar energy and in fixing CO₂, some strains including our model cyanobacterium Synechocystis 6803 can use glucose. Three challenges are identified in the development of algal strains for the production of fuels and chemicals. 1), lipids harvesting is very energy intensive; a product that naturally separates from the cells is preferred. 2), most algae do not naturally grow on xylose and some other sugars, preventing the use of potentially abundant supply of lignocellulosic sugars to supplement photosynthesis for higher productivity in natural day/night cycles. 3), cell growth competes for carbon flux, reducing productivity while increasing cost. Therefore an ideal alga would be able to use both biomass sugars and CO₂, secrete the product out of the cells, with minimal cell growth. Progress has been made at National Renewable Energy Laboratory towards creating such an ideal photosynthetic whole cell catalyst. To address the first challenge, ethylene was produced photosynthetically from CO₂ and it naturally separates from cells and accumulates in culture headspace. To address the second challenge, our cyanobacterium was engineered to utilize xylose, and it enhanced ethylene production. To address the third challenge, we used nitrogen limitation to arrest cell growth, and redirected carbon flux by blocking the synthesis of storage compound glycogen. The cells, in a unique physiological state termed photocatalytic conversion, continued to fix CO₂ and excreted into the medium significant amounts of α-ketoglutarate and pyruvate, with little or no biomass growth.