Bacteria could help turn CO2 to rock under extreme conditions

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Some Bacillus bacteria can convert CO2 to rock in extreme environments

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Bacteria may be able to accelerate the mineralisation of carbon dioxide under extreme conditions. Injecting such microbes underground along with captured CO2 could enable more durable storage of the greenhouse gas.

Gokce Ustunisik at the South Dakota School of Mines and Technology and her colleagues isolated Geobacillus bacteria species from a compost pile in Washington state that were known to tolerate high temperatures and pressures.

In laboratory tests, the researchers compared the rate at which CO2 mineralised when dissolved in water with and without these microbes. They tested the process at different temperatures, pressures and salinities, comparable to the extreme conditions found deep underground where CO2 might be stored. They also tested the process with different types of basalt rock.

Without the microbes, the researchers didn’t observe any CO2 mineralisation. Ustunisik says the process normally takes years, even under ideal geological conditions. “It’s going to take forever,” she says.

When the microbes were present, however, Ustunisik says it took just 10 days for the CO2 to form mineral crystals at 80°C (176°F) and pressures about 500 times that of sea level. That speedy rate under extreme conditions could enable more CO2 to be locked away in storage sites deep underground, such as depleted oil and gas reservoirs.

The key to this rapid mineralisation rate is an enzyme made by the bacteria called carbonic anhydrase, says Ustunisik. Once the CO2 solution has dissolved the rock, she says the enzyme quickly reduces the acidity of the solution so magnesium and calcium released from the rock can form carbonate minerals.

Bret Lingwall, also at the South Dakota School of Mines and Technology, says surface-dwelling microorganisms commonly make this enzyme – humans also make it – but they normally don’t survive for very long in extreme conditions. “It’s a hard life at 5000 feet below ground,” he says.

The researchers presented some details of the work at the American Geophysical Union’s annual conference in San Francisco in December. However, they are waiting to release more information – such as the exact species of bacteria they examined – until they can obtain a patent.

The researchers also plan to test Bacillus bacteria isolated from deep in a former mine shaft in South Dakota, as well as genetically modified strains, to identify which microbes perform best. Ustunisik says the next step then is to test the microbes in an actual storage well. But outside researchers say many factors might not translate.

“There are open questions around the resilience of these organisms, the food source of these organisms, their turnover rates and their ability to work in different alkaline environments,” says Greeshma Gadikota at Cornell University in New York. Nutrients would need to be injected along with the microbes to keep them alive. She says it may also be difficult to control how microbes introduced to the subsurface spread, which could be a particular concern if they are genetically modified.

Andrew Mitchell at Aberystwyth University in the UK says it is hard to know how much the microbes would accelerate CO2 mineralisation at larger scales, but cutting the process down to 10 days would be “quick”. He says finding a microbe that could accelerate mineralisation and survive deep conditions would be useful. “A lot of bugs don’t like high-temperature, high-pressure environments.”

Article amended on 14 February 2024

We clarified which bacteria were tested for mineralisation activity


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