Microbes engineered to produce carbon-neutral fuel

Biologists and engineers worked together to modify a microbe, Rhodopseudomonas palustris TIE-1, to produce biofuel using carbon dioxide, solar panel-generated electricity and light. R. palustris is a bacterium found in all sorts of environments and notable for its ability to change between four different metabolic modes, giving it potentially useful biotechnology applications. In particular, R. palustris TIE-1 has been shown to obtain energy through extracellular electron transfer.

Previous work from Professor Arpita Bose’s laboratory at Washington University revealed how they use electrons to ‘fix’ CO2 and could be used to create sustainable bioplastics. In the latest project from the Bose lab, researchers have used R. palustris TIE-1 to create a biofuel, n-butanol. This is a carbon-neutral fuel alternative that can be blended with diesel or petrol.

“Microorganisms have evolved a bewildering array of techniques to obtain nutrients from their surrounding environments,” said Bose. “Perhaps one of the most fascinating of these feeding techniques uses microbial electrosynthesis (MES). Here, we have harvested the power of microbes to convert carbon dioxide into value-added multi-carbon compounds in a usable biofuel.”

According to first author Dr Wei Bai, who worked in the Bose lab until last year: “The fuel we made, n-butanol, has a high energy content and low tendency to vaporise or dissolve in water without combustion. This is especially true when compared with ethanol, which is a commonly used biofuel.”

Much research has focused on using microbes to produce sustainable biofuels, in particular CO2 to biofuel conversion. However, microbes like cyanobacteria produce oxygen during photosynthesis, limiting their efficiency for synthesising biofuels (as many of the enzymes involved in the biosynthetic pathways are oxygen sensitive).

To investigate how R. palustris TIE-1 could be exploited to produce biofuel, Bai and Bose engineered a form of the microbe that cannot fix nitrogen. They then introduced an artificial n-butanol biosynthesis pathway into this mutant. The resulting microbe is unable to grow when nitrogen gas is its only nitrogen source. So instead, this microbe channels its efforts into producing n-butanol, increasing its yield of biofuel without increasing electricity consumption significantly.

The researchers concluded in their Communications Biology paper that this showcases R. paulstris TIE-1 as an “attractive microbial chassis” for carbon-neutral biofuel production.

“To the best of our knowledge, this study represents the first attempt for biofuel production using a solar panel-powered [MES] platform, where carbon dioxide is directly converted to liquid fuel,” said Bai. “We hope that it can be a stepping stone for future sustainable solar fuel production.”

A fully sustainable biofuel production cycle can be achieved using [MES] with electricity from solar panels. According to Bose, the US and EU have recognised the potential to use MES in future climate change solutions.

She said: “Ultimately, by exploiting a microbial metabolism that evolved in the distant past, we hope that new methods will emerge to help address some of the most pressing problems of our time.”