The microbes of greatest interest to biofuels production are thermophiles -- organisms that thrive at high temperatures. In fact, scientists “bioprospect” for them in hot springs and other extreme environments around the globe.
"The interesting thing is that extremophiles don't simply tolerate extreme conditions. They require them," says Michael W. Adams, Ph.D., professor of biochemistry and molecular biology at the University of Georgia. Organisms that require heat are particularly useful for many applications in the bioeconomy, including detergents, biotechnology, paper manufacturing, and breaking down cellulose in biomass for fuel and chemical synthesis.
Raising the temperature in biofuels production can speed the process and improve efficiency. That’s why finding organisms and enzymes that operate at super-high temperatures could be especially useful, says Adams. "The conditions that hyperthermophiles require -- temperatures as high as the boiling point of water -- essentially eliminate all other organisms, so there's almost no need to worry about contamination in the production process," he explains.
Rajesh Sani, Ph.D. is an associate professor at the South Dakota School of Mines and Technology. He is also among the bioprospectors combing the world’s “hot spots” for microbes that break down cellulose. Rather than prospecting in hot springs – a favored haven for thermophiles -- Sani and colleagues have gone underground, collecting microbes 4,000 feet down a South Dakota mine shaft. “Even in January, when it may be snowing outside, the temperature down there is about 40 degrees C,” he says.
Meanwhile, scientists like Christopher Rao, Ph.D. and his team are engineering strains of thermophiles capable of producing fuel molecules. Rao, an associate professor of chemical and biomolecular engineering at the University of Illinois at Urbana-Champaign and a principal investigator at EBI, has introduced genes into the thermophile Geobacillus glucosidans so that it makes ethanol molecules. “What we do is prevent the microbe from making what it wants to make (organic acids) and get them to make what we want (ethanol molecules). To do so, we first have to delete, or knock out, the microbes’ native genes, then add genes to the organisms.”
All this is pretty challenging in an organism that is 30 to 120 times smaller than the diameter of a human hair, especially because there are no standard techniques to draw on. “The technology for inserting genes into yeast was developed about 30 years ago, but those techniques don’t really exist for these [microbes],” says Rao. “It’s also complicated because bacteria exchange genes with one another and they’re not species-specific – in biology, we’d say they have sex with one another.” His lab uses a process called conjugation for manipulating the Geobacillus strain and has successfully “tricked” the thermophiles into producing ethanol and other fuel-like molecules.
Meanwhile, bioprospectors continue to search the planet for organisms that might help produce biofuels cheaply and efficiently. They’ve found complex organisms thriving in places no one would have expected. Here are some of the most promising:
WHERE FOUND: A freshwater volcanic spring in the Valley of Geysers on Russia's Kamchatka Peninsula.
WHAT IT LIKES: Temperatures around 78 °C.
CLAIM TO FAME: One of the most heat-loving bacteriums capable of breaking down cellulose, C. bescii can break down raw, unprocessed biomass. An enzyme in C. bescii has been shown to digest cellulose twice as fast as other known microbial enzymes. It also breaks down xylose, a component of plant cell walls that many commonly used biofuel microbes, like yeast, cannot use. Recently, the bacterium was tweaked at a University of Georgia laboratory to perform one-step conversion of lignocellulose into fermentable sugars for ethanol, cutting out the costly pretreatment processing.
FACTOID: First discovered in 1990, the bacterium was named after the BioEnergy Science Center (“BESC”) at Oak Ridge, Tenn.
WHERE FOUND: Since the 1980s, various species have been discovered in hot springs and/or geysers in China, Iceland, Russia, Yellowstone Park, and other “hot spots”
WHAT IT LIKES: Temperatures that would kill many other bacteria
CLAIM TO FAME: Certain species are coveted workhorses in cellulosic ethanol production. Although the bacteria can’t break down cellulose on their own, they can be cultured with other organisms that do.
FACTOID: Although the output is not yet feasible at a commercial scale, some species are able to churn out high ethanol yields by simultaneously fermenting hexose and pentose sugars, particularly xylose.
WHERE FOUND: Salty sludge at the bottom of Soap Lake, a mineral lake in Washington State.
WHAT IT LIKES: Extremely saline environments, at least 10 times saltier than sea water.
CLAIM TO FAME: H. hydrogeniformans is one of very few halophiles, or salt-loving microbes, that break down biomass efficiently enough to be useful for some applications of biofuels production. In addition to producing hydrogen from sugars in biomass, “it can also produce small amounts of electricity,” said Melanie R. Mormile, PhD, a research professor at Missouri University of Science and Technology and a leading expert on the organism.
FACTOID: The water at the bottom of Soap Lake, where H. hydrogeniformans was found, is so salty that it has the consistency of syrup. At the turn of the last century, Soap Lake was one of the most popular mineral spas in the country, prized for its purported healing powers.
WHERE FOUND: A shallow thermal vent off Volcano Island, Italy.
WHAT IT LIKES: Temperatures at 100 ºC, the boiling point of water.
CLAIM TO FAME: P. furiosus is one of the most heat-loving thermophiles. Its enzymes are already used in biotechnology to copy DNA and produce diols (chemical compounds that contain two alcohols). It has cellulase that tolerates 100º C and is being studied as a possible way to turn atmospheric carbon dioxide into chemicals. Researchers at North Carolina State University and the University of Georgia are tinkering with this microbe in an effort to create liquid fuels directly from carbon dioxide.
FACTOID: The name Pyrococcus furiosus literally means "rushing fireball." It is one of the few organisms with enzymes containing tungsten, an element rarely found in biological molecules.