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NREL team identifies major metabolic pathway in cyanobacteria for efficient conversion of CO2; better biofuels and bioproducts

Scientists from the 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 provides new insight into 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.

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 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.

It was a big surprise.

—Jianping Yu

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.

While the potential applications are promising, the researchers were mainly intrigued that they couldn’t explain the production of acetic acid from known pathways. Traditional pathways involving pyruvate dehydrogenase didn’t quite fit the facts. 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.

Disabling the gene in both the wild and mutant strains of Synechocystis slowed the growth in sunlight—i.e., 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.

Synechocystis was able to produce acetic acid in the dark when fed with sugars, but the strains with the disabled gene could not. 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.

From a basic science point of view, this is a major pathway that has a potentially important function in regulating photosynthetic energy conversion. We didn’t start with the hypothesis that there was an additional pathway actively involved in carbon metabolism; we just followed our own findings and made this discovery.

—Jianping 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 actually carried a significant proportion of central carbon metabolism.

It turns out that the phosphoketolase pathway is a major pathway under our experimental conditions. 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.

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.

—Jianping Yu

This work was supported by the US Department of Energy’s (DOE) Office of Science. It was also supported in part by DOE’s 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.


  • Wei Xiong, Tai-Chi Lee, Sarah Rommelfanger, Erica Gjersing, Melissa Cano, Pin-Ching Maness, Maria Ghirardi & Jianping Yu (2015) “Phosphoketolase pathway contributes to carbon metabolism in cyanobacteria” Nature Plants Article number: 15187 doi: 10.1038/nplants.2015.187



Store the CO2 in empty natural gas wells then use that for fuels using solar hydrogen and power plant waste heat.


Recovering CO2 back out of the atmosphere would reduce GHG; ideally, to be viable, there should be a saleable product to offset costs.


This could be very interesting - another way to make solar fuels, or at least solar vinegar in large quantities.


"Acetic acid is the second simplest carboxylic acid (after formic acid) and is an important chemical reagent and industrial chemical, mainly used in the production of cellulose acetate for photographic film and polyvinyl acetate for wood glue, as well as synthetic fibres and fabrics. In households, diluted acetic acid is often used in descaling agents. In the food industry, acetic acid is used under the food additive code E260 as an acidity regulator and as a condiment. The largest single use of acetic acid is in the production of vinyl acetate monomer [In 2008, this application was estimated to consume one third of the world's production of acetic acid.], closely followed by acetic anhydride and ester production. The volume of acetic acid used in vinegar is comparatively small."

Thomas Lankester

It is nice with chips and a battered sausage too.


On the topic of biofuels, Neste has HPR diesel made from plant oils, hydrolyzed and refined to make a drop in renewable replacement. This is NOT biodiesel, it is renewable diesel.


Acetic acid used to be used in "stop bath" when developing monochrome photographs - another use for it gone.


... sobs quietly into the fixer.

(for Kodak)

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