Engineered bacteria offer a new way to accelerate rock weathering for carbon removal — Phys.org

RESOLVING THE LEDGER

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Add as preferred source Rock weathering, the breakdown and dissolving of rocks and minerals caused by their exposure to water, air and biological life, is a major regulator of Earth's atmospheric CO 2 levels and climate. Throughout Earth's history, rock weathering has been faster during warm periods with increased atmospheric CO 2 levels.

Dissolved minerals ultimately wash into the ocean, where they draw CO 2 from the atmosphere and cool the planet again. While this thermostat is responsible for the temperate climate we enjoy on Earth, the weathering cycle occurs over hundreds of thousands of years.

In search of climate solutions, scientists have asked whether the rock weathering cycle could be sped up, resulting in a number of new companies pursuing enhanced rock weathering (ERW).

Now, a collaborative research team at the Wyss Institute at Harvard University, Harvard Medical School (HMS)'s Department of Systems Biology and the Stanford Doerr School of Sustainability, led by Wyss Institute Founding Core Faculty member Pamela Silver, Ph.D., and Wyss Institute Associate Faculty member Michael Springer, Ph.D., has engineered a potential solution to this problem.

The research team, spearheaded by first author and chemical engineer Neil Dalvie, Ph.D., genetically engineered Alteromonas macleodii, a widespread marine bacterium, to produce much higher amounts of so-called siderophores, molecules that extract iron from silicate minerals.

In customized bioreactors with a continuous flow of seawater, the engineered bacterium sped up the weathering of the silicate mineral olivine by 2.6-fold, boosting the amount of CO 2 removed from the air. Their findings are published in Nature Biotechnology .

"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," said Silver, who also is the Elliot T. and Onie H. Adams Professor of Biochemistry and Systems Biology at HMS and, together with Springer, founded the Synthetic Biology Hive at HMS.

"We believe this easily applicable, risk-free environmental engineering strategy could be implemented in many places with real-world decarbonization outcomes."

During natural rock weathering, silicate minerals like olivine dissolve to release primarily magnesium (Mg), iron (Fe) and silicate (SiO 4 ), trapping atmospheric CO 2 in the water as bicarbonate (HCO 3 - ).

MgFeSiO 4 + 4CO 2 + 4H 2 O → Mg 2+ + Fe 2+ + H 4 SiO 4 + 4HCO 3 −

Specifically, the released iron is not soluble when exposed to the atmosphere. Instead, it covers the mineral surface as rust, slowing down the entire process.

Researchers used custom bioreactors to tease apart when natural bacteria produce siderophores. They found that even a small amount of iron-containing mineral completely inhibited siderophore production, posing a major problem for siderophore production at an industrial scale.

"Once wild bacteria have enough iron to grow, they stop making siderophores completely," said the study's first and co-corresponding author Dalvie, Ph.D., who spearheaded the project as a postdoctoral fellow in Silver's lab. "To enable enhanced weathering at scale, we engineered A. macleodii to always produce siderophores. We essentially decoupled siderophore production from environmental iron levels."

While it took the team roughly one month to engineer the microbes, the real challenge was showing that they sped up rock weathering and removed more CO 2 from the atmosphere. To get a handle on this validation, Dalvie teamed up with co-author Amogh Jalihal, Ph.D., a postdoctoral fellow in Springer's group at the Wyss Institute and HMS.

"We put our heads together and decided that the measurement would be best at steady state. We needed seawater and bacteria to flow continuously over the minerals," said Dalvie.

Conveniently, Springer's group had recently acquired an entire room full of eVOLVERs, small-scale bioreactors originally designed by Ahmad (Mo) Khalil, Ph.D., another associate faculty member at the Wyss Institute and the Hok Lam and Kathleen Kam Wong Professor of Bioengineering and professor of molecular and cellular biology at Harvard University.

After small-scale studies showed promise, the team constructed pilot-scale bioreactors, loaded with several kilograms of green olivine sand and submerged in gallons of raw seawater from Boston Harbor.

"Operating at pilot scale allowed us to start solving scale-up problems: how often do we need to add cells? How do we feed them? Eventually we were able to measure actual uptake of 0.5 g of atmospheric CO 2 into our reactors each day, which was a compelling end result."

The team also carried out a life cycle analysis (LCA), which accounts for all carbon captured or emitted by the entire system over time, including all living, geological and chemical parts. Dalvie and Jalihal collaborated with Abigail Fitzgibbon, a Ph.D. student working with Steven Davis, Ph.D., professor of Earth system science at the Stanford Doerr School of Sustainability at Stanford University.

Davis' group has developed models to quantify the carbon emissions of industrial or agricultural processes and the effects of air quality on human well-being.

"Our collaboration with the Stanford group enabled us to precisely calculate the net carbon balance in our system. We could see which process parameters were key to making it an efficient environmental technology when used at an industrial scale."

Dalvie recently received a fellowship from the Burroughs Wellcome Career Awards at the Scientific Interface (CASI) program, which will fund further work on microbial siderophore production and mineral pro...

Engineered bacteria offer a new way to accelerate rock weathering for carbon removal — Phys.org | AIPROPX