This mechanism involves the gradual transformation of carbon dioxide into carbonate through a process called silicate weathering. Through this process, carbon dioxide is converted into rocks such as limestone, or stays dissolved in the oceans in a very stable form called bicarbonate. Thus, silicate weathering represents a natural and very effective way to capture and permanently store carbon.
While promising, this natural process is normally too slow to be effective in the fight against climate change. However, Dr Van Den Berghe’s team shows that microbes can be used to speed up this weathering process by over an order of magnitude, boosting its carbon-capture potential to be effective on human time scales.
A major factor that slows the natural weathering process is the build-up of iron oxide on the surfaces of silicate rocks. Since iron oxides are highly stable, they act as a protective barrier against further weathering. However, certain species of bacteria have the ability to harvest the iron in this protective layer, using it as a critical nutrient for fundamental processes such as respiration and photosynthesis. They do this by releasing biomolecules named ‘siderophores’, which break down iron oxides and allow the bacteria to harvest the iron atoms. By breaking down iron oxide, these microbes ultimately expose the silicate rock underneath, allowing it to undergo further weathering.
In one study, Dr Van Den Berghe and his colleagues measured the impact of siderophore-producing bacteria on the weathering of olivine: a common silicate mineral named for its green colour. Olivine is one of the most common minerals of the Earth’s crust, and when it weathers, it generates alkalinity and thus captures and stores carbon dioxide, making it a prime candidate for carbon capture.
In their study, Dr Van Den Berghe’s team exposed one sample of olivine to a wild, strain of bacteria that produces siderophores. They exposed another sample to a mutant strain of bacteria that they had genetically modified to block the production of siderophores.
The team found that the olivine exposed to the wild bacteria underwent weathering 10 to 15 times more quickly than the sample exposed to the mutant bacteria. In fact, the olivine exposed to the mutant bacteria dissolved no quicker than when exposed to a sterile solution.
Even when the researchers introduced additional siderophores to olivine exposed to the mutant strain, the wild bacteria were still more effective at weathering olivine. This result hints at the importance of other biological mechanisms that can act to further magnify the effects of siderophores, which the team is eager to explore.
These results clearly demonstrate the vital role of siderophore-producing bacteria in speeding up the weathering process. Through a deeper understanding of this relationship, the team at Cytochrome is studying how this process can be exploited for large-scale, cost-effective carbon capture and storage solutions. If achieved, their research could be a key step forward in the fight against global climate change.
