Bacteria brew energy-dense biofuels

Long-haul trucking, aviation, and shipping all need sustainable, energy-dense fuel. A group led by Jay Keasling at Lawrence Berkeley National Laboratory has now taken the first steps toward making such fuels with engineered bacteria. The microbes brew chains of three-membered rings that are more energy dense than some rocket fuels (Joule 2022, DOI: 10.1016/j.joule.2022.05.011). Previously, scientists have chemically stitched together chains of cyclopropanes to create fuel, but the breakthrough in the new work is the ability to make the compounds using biology, says study lead author Pablo Cruz-Morales, now at the Technical University of Denmark.

The researchers found inspiration from an antifungal compound called jawsamycin, which is produced by Streptomyces bacteria and gets its name from five spiky, toothlike cyclopropane rings in its chain. The team found enzymes that produce polycyclopropanes in another Streptomyces strain and engineered that microbe’s pathway to produce chains without jawsamycin’s bulky ending group, which makes the molecule difficult to functionalize into a fuel.

The researchers call the resulting fatty acids fuelimycins, and they can esterify the molecules to create burnable fuels. Simulations suggest that the fuelimycins have energy densities of around 40 MJ/L. By comparison, jet and rocket fuels pack about 35 MJ/L. But no one will be filling up with these bacterial biofuels just yet, Cruz-Morales says. “It’s just proof of concept.” Scaling up the synthesis will be challenging, he adds. “But if there was nothing to scale up, then we wouldn’t be talking about it.”