How Soil Bacteria Rapidly Adapt to Toxic Metals in EV Battery Waste

Electric vehicles (EVs) use high-power rechargeable batteries to store electrical energy for their motors. Many EV batteries use a metal mixture called nanoscale lithiated nickel-manganese-cobalt oxide, also known as NMC material.

NMC materials can store large amounts of energy, making them useful for batteries. However, the metals they contain can be toxic to soil and water if used batteries are not recycled properly. Studying how bacteria respond to NMC waste may help scientists predict the environmental effects of EV batteries and other emerging technologies.

Shewanella bacteria develop rapid resistance to NMC metals

NMC batteries leave behind a specific combination of toxic metals. This allows researchers to study how bacteria respond when battery waste enters the environment.

In 2019, researchers at the University of Minnesota found that the common soil bacterium Shewanella oneidensis could develop rapid resistance to NMC materials. Scientists did not know how the bacterium achieved this resistance. Beneficial genetic evolution typically develops over thousands of generations, but S. oneidensis adapted much faster.

In a follow-up study, the research team investigated the genetic mechanisms behind the bacterium’s unusual resistance to toxic metals.

DNA duplication—not mutations—explained the resistance

The researchers used whole-genome sequencing to analyze the complete DNA sequences of 33 NMC-resistant or metal-resistant bacterial colonies. They created colonies resistant to either NMC waste or individual metals, including nickel and cobalt.

The team expected to find mutations in genes that control how bacteria absorb and remove toxic metals. Instead, they found that specific DNA regions had been duplicated 10 to 15 times in all 33 resistant colonies.

These results suggested that temporary adaptation, rather than conventional genetic evolution, was responsible for the bacteria’s resistance. The duplicated regions appeared to provide an immediate response to metal toxicity without requiring mutations in metal-resistance genes.

Nickel and cobalt together trigger gene duplication

To learn how the DNA duplication occurred, the researchers focused on two possible mechanisms. One mechanism creates extra gene copies by directly replicating sections of DNA through a process called gene amplification. The other involves mobile genetic elements, which move DNA segments to different locations and can create additional gene copies.

The team grew S. oneidensis in liquid media containing only cobalt, only nickel, or both metals together. Using quantitative polymerase chain reaction (qPCR), the researchers measured the number of copies of the duplicated DNA region in each bacterial genome.

Neither cobalt nor nickel alone caused the DNA region to replicate. However, bacteria grown in a medium containing both cobalt and nickel developed eight additional copies of the region. This finding suggested that the combination of nickel and cobalt, rather than either metal individually, triggered the gene duplication.

A checkerboard assay tested other metal combinations

The researchers then tested whether other metal combinations could produce the same response. They substituted copper and zinc for nickel and exposed S. oneidensis to different combinations of metals in a 96-well plate using a grid-based checkerboard assay.

The team repeated the qPCR measurements to analyze the duplicated DNA regions. Other metal combinations did not cause the same gene duplication, suggesting that nickel and cobalt are particularly toxic together and that the duplication response is specific to their combined effects.

Duplicated genes produce metal-removing efflux pumps

Finally, the researchers determined the function of the duplicated DNA region. They found that one of its genes activates production of an efflux pump protein. These proteins push metal ions and other toxins out of bacterial cells.

The researchers hypothesized that the efflux pump removes cobalt and nickel from S. oneidensis cells. To test this idea, they created modified bacteria without the relevant efflux pump genes and exposed them to media containing cobalt, nickel, or both metals.

The modified bacteria grew well in nickel alone but failed to grow when exposed to cobalt alone or to the combination of cobalt and nickel. This result indicated that efflux pump proteins are required for S. oneidensis to withstand cobalt toxicity.

What this means for EV battery waste

Based on their experiments, the researchers concluded that nickel and cobalt together trigger DNA duplication in Shewanella oneidensis. The duplicated region increases the number of efflux pump proteins, allowing the bacterium to remove cobalt and helping explain its unusual resistance to NMC materials.

The findings suggest that gene duplication and other temporary adaptations could help scientists track how bacteria respond to electronic waste, EV battery materials, and other new technologies. Understanding these rapid bacterial responses may also improve predictions about the environmental effects of improperly recycled batteries.

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