An illustration of two fuel cans with images of bacteria on the front, with a rocket launching in the background into a night sky

Bacteria for Blastoff: Using Microbes to Make Supercharged New Rocket Fuel

Scientists turned to an eccentric molecule made by bacteria to create a new type of fuel that can be used for all types of vehicles, including rockets. (Credit: Jenny Nuss/Berkeley Lab)

Cconverting petroleum into fuels involves crude chemistry first invented by humans in the 1800s. Meanwhile, bacteria have been producing carbon-based energy molecules for billions of years. What do you think is better at work?

A group of biofuel experts led by the Lawrence Berkeley National Laboratory (Berkeley Lab), well aware of the benefits offered by biology, took inspiration from an extraordinary antifungal molecule made by Streptomyces bacteria to create a totally new type of fuel. to be developed with a higher energy density. than the most advanced heavy fuels in use today, including the rocket fuels used by NASA.

“This biosynthetic pathway provides a clean route to highly energy-rich fuels that, prior to this work, could only be produced from petroleum using a highly toxic synthesis process,” said project leader Jay Keasling, a synthetic biology pioneer and CEO of the department of energy Joint BioEnergy Institute (JBEI)† “Since these fuels would be produced from bacteria fed on plant material — which is made from carbon dioxide taken from the atmosphere — burning them in engines will significantly reduce the amount of added greenhouse gas relative to any fuel generated from petroleum. .”

The incredible energy potential of these candidate fuel molecules, called POP-FAMEs (for polycylcopropane fatty acid methyl esters), arises from the fundamental chemistry of their structures. Polycylcopropane-containing molecules contain multiple triangular three-carbon rings that force each carbon-carbon bond into an acute 60-degree angle. The potential energy in this strained bond translates into more energy for combustion than can be achieved with the larger ring structures or carbon-carbon chains typically found in fuels. In addition, these structures allow fuel molecules to be packed closely together into a small volume, increasing the mass — and thus the total energy — of fuel that can fit in a given tank.

“With petrochemical fuels you get a kind of soup of different molecules and you have little control over those chemical structures. But that’s what we’ve used for a long time, and we’ve designed all our engines to run on petroleum derivatives,” said Eric Sundstrom, an author on the paper describing POP fuel candidates published in the journal Jouleand a research scientist at Berkeley Lab’s Advanced Process Development Unit for Biofuels and Bioproducts (ABPDU).

“The larger consortium behind this work, Co-Optima, was funded to think not only about remaking the same fuels from biobased feedstocks, but also how we can make new fuels with better properties,” said Sundstrom. “The question that led to this is, ‘What kind of interesting structures can biology make that petrochemistry can’t make?'”

A search for the ring(s)

Keasling, also a professor at UC Berkeley, has long had an eye for cyclopropane molecules. He had searched the scientific literature for organic compounds with three carbon rings and found only two known examples, both made by Streptomyces bacteria that are almost impossible to grow in a laboratory setting. Fortunately, one of the molecules had been studied and genetically analyzed due to interest in its antifungal properties. Discovered in 1990, this natural product was given the name jawsamycin, due to the tooth-like appearance of its unprecedented five cyclopropane rings.

A petri dish with white bacterial growth on a brown medium

A culture of the Streptomyces bacterium that makes the jawsamycin. (Credit: Pablo Morales-Cruz)

Keasling’s team, consisting of JBEI and ABPDU scientists, studied the genes of the original strain (S. roseoverticillatus) that code for the jawsamycin-building enzymes and delved deep into the genomes of related Streptomyces, looking for a combination of enzymes that form a molecule with the tooth rings of jawsamycin while skipping the other parts of the structure. Like a baker who rewrites recipes to invent the perfect dessert, the team hoped to remix existing bacterial machines to create a new molecule with ready-to-use fuel properties.

First author Pablo Cruz-Morales was able to gather all the necessary ingredients to make POP-FAMEs after discovering new cyclopropane-making enzymes in a strain called S. albireticuli. “We searched thousands of genomes for paths that naturally create what we need. That way, we avoided the technique that might or might not work and used nature’s best solution,” said Cruz-Morales, senior researcher at the Novo Nordisk Foundation Center for Biosustainability, Technical University of Denmark and the co-principal investigator of the natural yeast. products lab with Keasling.

Unfortunately, the bacteria were not as cooperative when it came to productivity. Streptomyces are ubiquitous in the soil on every continent and are known for their ability to make unusual chemicals. “Many of the drugs in use today, such as immunosuppressants, antibiotics and cancer drugs, are made by engineered Streptomyces,” Cruz-Morales said. “But they are very fickle and not pleasant to work with in the lab. They’re talented, but they’re divas.” When two different engineered Streptomyces failed to make POP-FAMEs in sufficient quantities, he and his colleagues had to copy their newly arranged gene cluster into a more “tame” relative.

The resulting fatty acids contain up to seven cyclopropane rings chained to a carbon backbone, giving them the name fuelimycins. In a process similar to the production of biodiesel, these molecules need only one additional chemical processing step before they can serve as fuel.

Now we cook with cyclopropane

While they still haven’t produced enough candidate fuel molecules for field testing — “you need 10 kilograms of fuel to do a test in a real rocket engine, and we’re not there yet,” Cruz-Morales explained with a laugh — they were able to evaluate Keasling’s predictions about energy density.

Colleagues at the Pacific Northwest National Laboratory analyzed the POP-FAMEs with nuclear magnetic resonance spectroscopy to prove the presence of the elusive cyclopropane rings. And employees at Sandia National Laboratories used computer simulations to estimate how the compounds would perform compared to conventional fuels.

The simulation data suggest that POP fuel candidates are safe and stable at room temperature and will have an energy density greater than 50 megajoules per liter after chemical processing. Regular gasoline has a value of 32 megajoules per liter, JetA, the most common jet fuel, and RP-1, a popular kerosene-based rocket fuel, has about 35.

A small bottle of brown liquid held up to the light in a lab

An extract of jawsamycin extracted from the capricious bacteria. (Credit: Pablo Cruz-Morales)

During their research, the team found that their POP-FAMEs are very close in structure to an experimental petroleum-based rocket fuel called Syntin, which was developed in the 1960s by the Soviet Union’s Space Agency and used for several successful Soyuz rocket launches. in the 1970s and 80s. Despite its powerful performance, Syntin was discontinued due to its high cost and unpleasant process: a series of synthetic reactions with toxic by-products and an unstable, explosive intermediate.

“While POP-FAMEs have similar structures to Syntin, many have superior energy densities. Higher energy densities result in lower fuel volumes, which can increase payload in a rocket and lower total emissions,” said author Alexander Landera, a staff scientist at Sandia. One of the team’s next goals is to create a process to remove the two oxygen atoms from each molecule, which adds weight but has no combustion benefit.” When mixed into an aviation fuel, well-deoxygenated versions of POPs can FAMEs offer a similar benefit,” Landera added.

Since the publication of their proof-of-concept paper, the scientists have begun to improve the production efficiency of the bacteria to generate enough for combustion testing. They are also investigating how to modify the multi-enzyme production pathway to create polycyclopropane molecules of different lengths. “We are working on tailoring the chain length to specific applications,” says Sundstrom. “Longer chain fuels would be solids, well suited for certain rocket fuel applications. Shorter chains could be better for jet fuel, and in the middle could be a diesel alternative molecule.”

Author Corinne Scown, JBEI’s Director of Technoeconomic Analysis, added: “Energy density is everything when it comes to aerospace and rocketry and this is where biology can really shine. The team can create fuel molecules tailored to the applications we need in those rapidly evolving industries.”

Ultimately, the scientists hope to turn the process into a workhorse strain of bacteria that can produce large amounts of POP molecules from plant food waste (such as inedible agricultural waste and brushes cleared for wildfire prevention), helping the ultimate carbon neutral fuel

Who’s up for an eco-friendly space trip?

This work was supported by the US Department of Energy Office of Science and Office of Energy Efficiency and Renewable Energy. JBEI is an Office of Science Bioenergy Research Center.

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