Petroleum geologists normally look for oil underground. Gary Strobel made his strike by pruning a tree. In the current issue of Microbiology, Strobel, a plant pathologist at Montana State University, Bozeman, and colleagues report that Gliocladium roseum--a novel fungus they discovered hidden within a stem from a scraggly tree in northern Patagonia--produces dozens of the same midlength hydrocarbons found in gasoline, diesel fuel, and jet fuel. The fungus may help companies convert the chemical energy stored in plants into liquid fuels capable of replacing fossil fuels.
The discovery is "a really great contribution," says Stephen Del Cardayre, a synthetic biologist and vice president for research and development at LS9, a South San Francisco-based start-up working to use microbes to produce renewable fuels. Even though the new fungus pumps out only small quantities of fuel hydrocarbons, researchers might use its genes to engineer other industrial microbes to do the job more efficiently. "The beauty is that even if the chemical reaction isn't perfect, you can always improve it," he says.
The search for fuel-producing microbes is one of the hottest areas in synthetic biology (Science , 24 October, p. 522). Strobel, an expert on endophytes--organisms that live within the tissues of other creatures--joined it by accident in 1997, when he discovered a fungus in Honduras that naturally produces volatile antibiotics, now being evaluated as a way to preserve fruit during shipping. Strobel has since identified related fungi around the globe that produce different volatile hydrocarbons.
After discovering the new fungus wedged between cells in a stem from an Ulmo tree (Eucryphia cordifolia), Strobel and colleagues cultured the organism, collected the gaseous compounds it produced, and ran the compounds through a mass spectrometer to identify them. When he saw the printout, Strobel says, "every hair on my body stood up." The list included octane, 1-octene, heptane, 2-methyl, and hexadecane--all common components of diesel fuels.
Although other microbes are known to make individual volatile hydrocarbons common in fuels, Strobel says none can match the synthetic repertoire of G. roseum, which makes a staggering 55 volatile hydrocarbons: "No one has ever observed anything like this with any microbe before." He suspects that the fungus produces the hydrocarbon stew to inhibit other organisms from growing nearby.
Strobel and his colleagues also cultured G. roseum by feeding it cellulosic biomass like that from agricultural wastes, although the yield of volatile hydrocarbons declined. Even if the bug turns out not to produce fuels economically, Del Cardayre says, renewable-fuel companies are likely to try to adopt its synthetic prowess to boost the biofuel output of their own organisms, if they use similar metabolic pathways to convert energy-rich starting materials into hydrocarbons. Strobel is teaming up with his son, Scott, an enzymologist at Yale University, and members of his lab to sequence G. roseum's complete genome and identify its component enzymes.
Other energy-making bugs could be on the way, too. The younger Strobel says a recent sample-collection trip he made to South America with a group of Yale undergraduates found other novel endophytic fungi that turn out a wide variety of hydrocarbons. "There is just huge swaths of biodiversity to be discovered out there," Strobel says. That may persuade the next generation of oil explorers to trade in their seismographs for pruning shears.