Capturing carbon in rocks could release another greenhouse gas

Trapping carbon dioxide in rocks has been touted as a low-cost strategy for combating climate change. But a recent study shows that trace metals released by certain rock types proposed for CO₂ capture can support the growth of microbes that produce methane, a much more potent greenhouse gas (Environ. Sci. Technol. 2024, DOI: 10.1021/acs.est.4c04751).

“Past studies on rock weathering have measured how the weathering process affects carbon capture, but they’ve overlooked the production of other greenhouse gases,” says Qiang Zeng, an author of the new research.

Methane, for instance, is a greenhouse gas that is naturally produced by soil archaea called methanogens, which Zeng studies at the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences. Typically, the lack of metals such as cobalt and nickel in soil limits the growth of these archaea. But as rocks like basalt and peridotite wear away in the process of sequestering CO₂, they could release those metals, which they contain in trace amounts.

Zeng, Hailiang Dong, and their colleagues wanted to find out if dust from basalt or peridotite could spur the growth of methanogens. Such growth could potentially boost methane production and counteract the CO₂ capture that people aim to achieve through carbon mineralization.

To simulate methanogens’ exposure to metals during rock weathering, the researchers cultured Methanosarcina acetivorans C2A, a common model methanogen, and provided it with either crushed peridotite, basalt, or granite. The microbes that were provided with peridotite or basalt grew at rates similar to those of microbes that were supplied with a solution of cobalt, nickel, and other relevant metal ions, and these groups produced comparable amounts of methane. Granite, which contains little nickel or cobalt, did not stimulate the archaea nor did control conditions.

While the study is a good start and confirms that the broken down silicate rocks such as peridotite and basalt stimulate archaea under controlled conditions and increases the production of methane, it is still unknown whether the same overall effects would occur in a more complex natural environment, says Maija Raudsepp, a biogeochemist at the University of Alberta who was not involved in the study.

“It really depends on the right geochemical environment,” she says. In the methanogens’ natural habitats, other microorganisms in the soil could compete for the trace metals; the trace metals could stimulate other microorganisms that consume methane; or even the availability of oxygen could check the growth of the methanogens, which require anaerobic conditions.

So whether this effect on methane production outweighs the benefits of CO₂ sequestration remains unclear. “I don’t think this study can answer that question,” Raudsepp says. But it makes more urgent the need to fill the knowledge gaps regarding how microorganisms produce and consume methane, she says.

Zeng agrees that the study’s design is simple and says that the team is currently planning more realistic experiments and measurements of other greenhouse gases. For example, the production of oxides of nitrogen might also increase when the right soil microorganisms are stimulated by rock weathering, he says.