Thermophiles: Hot Microbes on the Biofuel Frontier

If you’ve ever been lucky enough to visit Yellowstone National Park (or looked at pictures), then you have probably seen the colorful, steaming mats that surround many of the park’s famous geysers and geothermal pools. While they may not look it, these mats are actually living creatures. They form a special class of organisms called “thermophiles” (heat-loving), and they thrive in super-hot temperatures ranging from 41 to 122°C (106-252°F). However, thermophiles are more than just a family vacation novelty: they are the future of fuel production.

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Grand Prismatic Spring in Yellowstone National Park, taken by author

Biofuels Compete with Your Groceries

When you fill up your tank at the gas station, what you’re getting isn’t 100% gasoline. About 10% of the gas we pump into our tanks is actually ethanol made from fermenting and distilling grains such as corn. While it may sound like a great idea to make fuel out of something we can grow, there are some issues. Critics of corn ethanol say that ethanol production drives up food costs and does not significantly reduce emissions. Corn ethanol production increases food prices because it reduces the supply available for animal feedstock. These issues have led to a big push for a non-food alternative: cellulosic biofuel.

Cellulosic ethanol is made from the parts of plants we don’t eat, which can either be agricultural by-products (eg. corn husks) or non-food sources (eg. grass). Fuel production from these sources has a lower carbon footprint than corn ethanol because grasses and other feedstocks can be grown on “marginal lands”.  Marginal lands are rarely used for conventional farming because they are either too dry or are contaminated by pollutants. It also does not compete with food production. However, current efforts to produce cellulosic ethanol have been controversial due to high production costs, leading some to regard the idea of cellulosic biofuels in their current form as a failure. So what makes cellulosic sources so expensive? Hint: it’s right in the name.

The edible parts of corn (eg. the kernel) are made of simple chains of sugar molecules called starch. This starch can easily be converted to ethanol by yeast. However, cellulosic biomass is much more complex. Cellulosic biomass is made up of three parts: cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are also chains of sugars, but they can be organized in lots of different ways that make them confusing to work with. Lignin makes things even more complicated because it acts like a protective shield that keeps enzymes (molecules that break down the cellulose) away from the sugars. Currently, biofuel producers use expensive enzymes to pretreat the cellulosic material before it can be broken down into fuel.

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Bacterial mats at Yellowstone National Park, taken by author

Using Heat to Simplify

So how do our thermophile friends fit into all of this? Well, some researchers are developing “single-step” production of cellulosic biofuels, where pretreatment with enzymes is unnecessary. One way to achieve single-step production is to use thermophiles to ferment the cellulose. At high temperatures, these organisms can convert the biomass directly into ethanol, eliminating a significant cost barrier. It’s a simple and elegant solution that lets the thermophile use its own enzymes to do all the heavy lifting.

While this strategy works in the lab, a lot of work needs to be done before this can be seen as a viable option. In its current form, this route of producing ethanol is “wasteful”. The target product of the fermentation is ethanol, but other products such as acetate and lactate are also produced. We don’t really care about these products, and they waste metabolic potential that could be used to make more ethanol. Imagine that you’re making cookies. You have enough ingredients to make 40 cookies, but someone keeps taking giant globs of cookie dough out of the bowl so you only end up with 20. Frustrating, right? Researchers are getting around this by altering fermentation conditions and using bioengineering to change the organisms themselves. By changing certain genes, you can make a thermophile that is custom-made for biofuel production. Some of this research is even being done right here in Athens at the University of Georgia!

Cellulosic biofuels are cleaner than traditional fossil fuels as well as being renewable. With reports on climate change looking grimmer than ever, it is vital that we find sustainable alternatives to fossil fuels. This won’t be easy, and will require changes in how we work, drive, and live on a global level. However, these big changes will start with small steps, maybe even on the scale of a tiny heat-loving microbe.

Trevor3 Trevor Adams is a Ph.D. Student in the Integrated Life Sciences program at the University of Georgia. He is interested in how the molecular bits of life shape our world. His hobbies include hiking, reading, and hanging out with his cat Bustelo. More from Trevor Adams.