Researchers have genetically engineered a widespread marine bacterium to accelerate rock weathering, a natural process that removes carbon dioxide from the atmosphere and helps regulate Earth’s climate.
The team from Harvard University’s Wyss Institute, Harvard Medical School, and the Stanford Doerr School of Sustainability modified Alteromonas macleodii to increase the weathering of olivine in seawater.
Tested in custom bioreactors, the bacterium accelerated the process by 2.6-fold and increased atmospheric CO2 removal.
Bacteria remove rust from mineral surfaces
Rock weathering begins when silicate minerals such as olivine dissolve after exposure to water, air, and biological activity. The process releases magnesium, iron, and silicate while converting atmospheric CO2 dissolved in water into bicarbonate.
However, the released iron oxidises when exposed to the atmosphere and forms rust over the mineral surface. This coating slows further dissolution and limits the rate at which the rock can capture carbon.
Some bacteria naturally produce siderophores, molecules that bind to oxidised iron and make it soluble. By removing this iron, the microbes effectively strip rust from the mineral surface and allow weathering to continue faster.
The researchers found that natural bacteria stopped producing siderophores once they obtained enough iron for growth.
They therefore engineered A. macleodii to continue producing the molecules regardless of surrounding iron levels.
“To enable enhanced weathering at scale, we engineered A. macleodii to always produce siderophores. We essentially decoupled siderophore production from environmental iron levels,” says first author Neil Dalvie.
Pilot reactors capture carbon from air
After initial experiments showed promise, the researchers built pilot-scale bioreactors containing several kilograms of green olivine sand submerged in raw seawater collected from Boston harbour.
Seawater and bacteria continuously flowed over the mineral, allowing the team to measure weathering under steady-state conditions. The system eventually absorbed 0.5 grams of atmospheric CO2 per day.
The engineered microbes increased olivine weathering by 2.6 times compared with the control conditions.
According to the researchers, the experiment provided proof that synthetic biology could accelerate a geological process that normally unfolds over extremely long periods.
“Our study embraces the concept of biologically inspired engineering and how synthetic biology can be applied to enhance normal climate-regulating processes, which ultimately could have a positive impact on our planet,” says Pamela Silver, a founding core faculty member at the Wyss Institute.
Larger basins could process seawater
The team also completed a life-cycle analysis covering carbon captured and emitted across the biological, geological, and chemical components of the system.
The assessment helped identify which operating parameters would be important for achieving net carbon removal at an industrial scale.
Carbon removal facility. Image: Wikimedia Commons.
Further research is needed to locate economically viable sources of silicate minerals and other feedstocks. Scientists are also studying whether valuable metals could be recovered from the minerals while carbon is sequestered.
One proposed deployment would involve growing the engineered bacteria in large basins similar to those used at sewage treatment facilities.
Untreated seawater would be pumped through the basins before alkaline water containing the captured carbon is released into the ocean.
“We believe this easily applicable, risk-free environmental engineering strategy could be implemented at many places with real-world decarbonisation outcomes,” says Silver.