Better Biofuels and Bioproducts from Photosynthesis?

National Renewable Energy Laboratory (NREL) scientists have discovered that a metabolic pathway previously thought to be functional only in photosynthetic organisms is actually a pathway that can enable efficient conversion of carbon dioxide to organic compounds. The discovery shines light on the metabolic network for carbon utilization in cyanobacteria. It also may open the door to ways of producing chemicals from carbon dioxide or plant biomass other than deriving them from petroleum.

The discovery, led by NREL senior scientist Jianping Yu and Wei Xiong, an NREL Director's Postdoc Fellow, follows recent work involving cyanobacteria, or blue-green algae. 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). This turned 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 strain under multiple growth conditions, the scientists discovered that it excreted large amounts of acetic acid.

Acetic acid is a chemical produced in high volumes for a variety of purposes. The chemical industry produces more than 12 million tons per year, 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. While potential applications are promising, the researchers were mainly intrigued that they couldn't explain the production of acetic acid from known pathways. They knew that an enzyme called phosphoketolase could be involved, as it had previously been suggested to be active in cyanobacteria.

Starting from a previously studied phosphoketolase, the researchers were able to identify the gene slr0453 as the likely source of the phosphoketolase in Synechocystis. The next step was to disable the gene. Disabling it in both the wild and mutant strains of Synechocystis slowed the growth in sunlight—that is, conditions dependent only on CO2 assimilation by photosynthesis—demonstrating that the gene played a role in photosynthetic carbon metabolism. The strains with the disabled gene did not excrete acetic acid in the light in the presence of xylose.

The clincher was that Synechocystis was able to produce acetic acid in the dark when fed with sugars. Strains with the disabled gene could not. The researchers found that the phosphoketolase pathway was solely responsible for producing acetic acid in the dark and also contributed to carbon metabolism in the light when xylose was supplied.

"From a basic science point of view, this is a major pathway that has a potentially important function in regulating photosynthetic energy conversion," says Yu. Xiong then quantified the contribution of the newly discovered pathway by using carbon isotopes to track how xylose and carbon dioxide were converted into other organic chemicals. The results showed that the phosphoketolase pathway carried a significant proportion of central carbon metabolism. "It turns out that the phosphoketolase pathway is a major pathway under our experimental conditions," says Yu. "And because it avoids the carbon loss associated with traditional pathways, a wide variety of bioproducts and biofuels can be made more efficiently using this pathway."

According to Yu, two aspects of this discovery are important. One is that phosphoketolase is an important native metabolic pathway in the cyanobacterium whose role was not studied previously. Second is that this pathway is more efficient than traditional pathways, meaning it can be exploited to increase photosynthetic productivity.