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Carbon-Eating Bacteria Could Be One Answer to Climate Change

Clostridium thermocellum can metabolize CO2.

by Grennan Milliken
Nov 3 2016, 9:00am

Image: Tony Webster/Flickr

The bacterium Clostridium thermocellum has garnered a lot of interest recently because of its ability to break down cellulose (a notoriously tough organic compound found in plants) into usable biofuels without added enzymes. While studying these useful properties, however, scientists at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) made another eye opening discovery: the bacterium can take up and metabolize carbon dioxide (CO2) as well.

This is really exciting: although it doesn't mean that the bacterium could "scrub" the air of CO2, it could be engineered to create more efficient biofuels that are "carbon neutral," meaning they absorb as much CO2 as they emit. The findings are published in Proceedings of the National Academy of Sciences.

"If we can understand the mechanism of why and how [the bacteria takes up CO2]," NREL staff scientist and co-author Katherine J. Chou told Motherboard in a phone call, we can "further engineer this bacteria to take up more." That could help reduce the amount of CO2 released into the environment, overall.

The fact that is at all possible to scientists is still something of a biological mystery. C. thermocellum is a type of bacteria known as a heterotroph. Heterotrophs require organic carbon molecules—like those from cellulose—to build cells and carry out their day to day duties. These are the types of bacteria that ultimately turn carbon that has been cycling around in the living environment back into its inorganic form, CO2. As the bacteria munch on organic carbons like glucose, they churn out CO2 as a waste byproduct. This ultimately cuts into the maximum yield of biofuel these types of bacteria can create, and is known as carbon loss.

"This represents the loss of organic carbon that could have been used to make hydrogen and hydrocarbons," said Chou. She explained that It's also cast a shadow over the legitimacy of using biofuels as a renewable energy source, even if it diversifies our energy base. "If we constantly have to take renewable biomass and waste a third of that to CO2, is this even the right direction?"

Partly why C. thermocellum has gotten so much attention is because it is highly efficient at directly breaking down cellulose into biofuels. Scientists figured this probably has something to do with a low carbon loss, but they weren't exactly sure how it made up for this deficit until now.

Researchers at NREL investigated further into C. thermocellum's powers using a wide range of techniques including mass spectrometry, which essentially allows scientists to look at the different masses of a particular chemical so they can identify unknown compounds. It is particularly useful for studying chemical processes.

They found that, astonishingly, the microorganism is capable of metabolizing CO2, meaning it can scoop up the carbon it releases when it breaks down cellulose—re-absorbing its own waste produce, and preventing it from adding to the total gaseous CO2 in the environment. "How the bacteria is able to be so efficient at consuming cellulose at the same time it's taking up some of that CO2, is intriguing," Said Chou.

This strange characteristic turns biology on its head, because bacteria usually break down either organic carbon, or inorganic carbon—not both. Chou remarked that "we have to redefine this bacteria completely." And perhaps others. How it manages to use both types of carbon without breaking the rules of thermodynamics in energy conservation is unknown (It didn't read the rulebook?).

Regardless, the finding paves the way to potentially using a genetically modified version of this tiny microorganism to efficiently produce biofuels that are carbon neutral. In so doing, it might also bring biofuels back into the conversation as a legitimate way to reduce human-produced carbon emissions. But it still remains to be seen if the bacterium's efficiency can be improved to the point that it would actually make a dent in global CO2 levels, and done so in way that's scientifically and ethically sound.

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