Metabolic pathway in cyanobacteria could yield better biofuels

Scientists from the US 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 utilisation in cyanobacteria, while opening the door to better ways of producing chemicals from carbon dioxide or plant biomass, rather than deriving them from petroleum.

The discovery was led by NREL senior scientist Jianping Yu and Wei Xiong, an NREL Director's Postdoc Fellow.

The findings were published in the online edition of Nature Plants.

The latest NREL discovery followed on the heels of recent work involving cyanobacteria, commonly known as blue-green algae.

NREL scientists engineered a cyanobacterium, Synechocystis, that is unable to store carbon as glycogen into a strain that could metabolise 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 biofuels and bio-based chemicals.

While testing this mutant strain under multiple growth conditions, the scientists discovered, unexpectedly, that it excreted large amounts of acetic acid, which is a chemical produced in high volumes for a wide variety of purposes.

The chemical industry produces more than 12 million tonnes 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 US reliance on natural gas.

While the potential applications are promising, the researchers were mainly intrigued that they could not explain the production of acetic acid from known pathways.

Traditional pathways involving pyruvate dehydrogenase did not fit the facts and they knew that an enzyme called phosphoketolase could be involved, as it had previously been suggested to be active in cyanobacteria.

The researchers were able to identify the gene slr0453 as the likely source of the phosphoketolase in Synechocystis, and disabling it in both the wild and mutant strains of Synechocystis slowed the growth in sunlight.

This demonstrated that the gene played a role in photosynthetic carbon metabolism and the strains with the disabled gene did not excrete acetic acid in the light in the presence of xylose, but did so in the dark.

Yu says there are two aspects that are important in this discovery.

‘One is that it is an important native metabolic pathway in the cyanobacterium whose role was not studied previously. Second is that this pathway is more efficient than the traditional pathways, so it can be exploited to increase photosynthetic productivity,’ he says.

The work was supported by the US Department of Energy's Office of Science and in part by the Office of Energy Efficiency and Renewable Energy's (EERE) Bioenergy Technologies Office.

Previous foundational research and development supported by EERE's Fuel Cell Technologies Office was instrumental in enabling these achievements.

NREL is the US Department of Energy's primary national laboratory for renewable energy and energy efficiency research and development.