As humans warm the planet by releasing carbon dioxide into the atmosphere, some researchers believe that capturing CO2 and trapping it in buried rocks could lower the risk of catastrophic climate change. Now a team of researchers has shown that bacteria can help the process along. They can even be genetically modified to trap CO2 faster, keeping it underground for millions of years.
When CO2 is pumped into underground porous rocks, it combines with metal ions in the salty water that fills the rock pores and mineralizes into mineral carbonates, such as calcium carbonate (CaCO3). The process can take thousands of years. To see if they could speed things up, biochemist Jenny Cappuccio and colleagues at the Lawrence Berkeley National Laboratory's Center for Nanoscale Control of Geologic CO2 put a diverse mix of common bacterial species in a calcium chloride solution in the lab and then pumped in CO2. They found that calcium carbonate formed faster in areas where the bacteria were living than it did in sterile solutions. The CaCO3 also had a different mineral structure when the bacteria were around. It tended to grow into crystals of white calcite instead of amorphous black lumps (see picture). The bacteria enhanced the formation of calcite even when they were just lying around, not growing or multiplying.
Intrigued, the team guessed that the surfaces of the bacteria were somehow helping the CO2 hook up with calcium ions. To test that idea, they decided to modify one of the bacterial species, Caulobacter vibrioides, shaping its surface to attract calcium ions, and see what happened.
Cappuccio and colleagues inserted a short DNA sequence that coded for a loop of six glutamic acids—a type of amino acid—into C. vibrioides. The loop sticks out of the bacteria's surface protein and is repeated over the entire surface of the bacteria in a hexagonal pattern. Each six-acid loop contains six negative charges. The team reasoned that this "negative loop" could fit neatly around positively charged calcium ions in water, attracting them to the surface of the bacteria and coaxing them to form CaCO3.
It worked. When the researchers pumped CO2 into the tanks where the modified bacteria were living, even more CaCO3 solidified than in tanks with unmodified bacteria. Better yet, more of it was in the crystalline calcite form, which is more stable—and likely to sequester CO2 over geological time—than amorphous CaCO3. Cappuccio reports the team's results today at a meeting of the Biophysical Society in San Diego, California.
Robin Gerlach, a biological engineer at Montana State University in Bozeman, who was not involved with the study, calls the work "very fundamental." He anticipates broad applications, including stabilizing soil in flood zones, isolating radioactive isotopes, and identifying early life in the fossil record by tracking changes in carbonate mineralization.
But Cappuccio is the first to admit that the results need to be demonstrated in conditions closer to real life. She wants to test her modified bacteria at higher pressures, higher temperatures, and lower pH, conditions closer to those that might be found underground. Eventually, her team wants to modify hard-to-grow extremophiles—bacteria that grow in very hot, high-pressure environments—that are more like the microorganisms that colonize the underground rock formations where CO2 would be sequestered.