A metabolic pathway in cyanobacteria could yield better bioproducts

Scientists from the Energy Department's National Renewable Energy Laboratory (NREL) have discovered that a metabolic pathway previously only suggested to be functional in photosynthetic organisms is actually a major pathway and can enable efficient conversion of carbon dioxide to organic compounds.

The discovery shines new light on the complex metabolic network for carbon utilization in cyanobacteria, while opening the door to better ways of producing chemicals from carbon dioxide or plant biomass, rather than deriving them from petroleum.

NREL scientists engineered a cyanobacterium, Synechocystis, that is unable to store carbon as glycogen into a strain that could metabolize xylose (a main sugar component of cellulosic biomass), thus turning xylose and carbon dioxide into pyruvate and 2-oxoglutarate, organic chemicals that can be used to produce a variety of bio-based chemicals and biofuels. While testing this mutant strain under multiple growth conditions, the scientists discovered, unexpectedly, that it excreted large amounts of acetic acid.

Acetic acid is a chemical produced in high volumes for a wide variety of purposes. The chemical industry produces more than 12 million tons per year of acetic acid, primarily from methanol, which in turn is mainly produced from natural gas. The potential to produce acetic acid from photosynthesis could reduce the nation's reliance on natural gas.

The researchers found that the phosphoketolase pathway was solely responsible for the production of acetic acid in the dark and also contributed significantly to carbon metabolism in the light when xylose was supplied.