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Metal-tolerant fungus cleans up mercury

A prevalent soil fungus has the enzymatic tools to break down the toxic pollutant methylmercury

by Alla Katsnelson, special to C&EN
November 25, 2022 | A version of this story appeared in Volume 100, Issue 42

Two maize plants in pots next to a ruler. The plant raised in soil polluted with methylmercury is short, and the other, grown with an engineered fungus, is taller.
Credit: Congcong Wu
A maize plant treated with a genetically engineered fungus (right) grows taller in soil polluted with methylmercury than an untreated plant (left).

A fungus often found in soil heavily polluted with metals can clean up organic mercury, according to a new study. The researchers also engineered a couple of genetic tweaks into the fungus that supercharged its ability to remediate soil and water (Proc. Natl. Acad. Sci. U.S.A. 2022, DOI: 10.1073/pnas.2214513119).

Weiguo Fang of Zhejiang University and colleagues came upon the cleansing power of Metarhizium robertsii while studying how fungi exchange genes with surrounding organisms to survive under environmental stress. M. robertsii colonizes the roots of many plants, promotes their growth, and helps protect from insects. The researchers identified an enzyme in the fungus that breaks off the methyl group from methylmercury, an organic form of mercury that is neurotoxic and an environmental pollutant, Fang explains. M. robertsii also carries another enzyme that reduces the remaining mercury ions into volatile elemental mercury.

In the lab, the researchers found that maize plants that formed symbiotic associations with M. robertsii in mercury-contaminated soil grew better than plants that were not colonized by the fungus. The researchers then engineered the fungus to overexpress the genes for the two enzymes, significantly boosting the fungus’s capacity to remove mercury from soil. In the presence of the engineered fungus, maize grew nearly as well as in control soil. M. robertsii’s mycelium—the fungal equivalent of roots—also effectively cleared methylmercury from water.

“The approach works well in the lab, but its efficacy in the field remains to be determined,” Fang says. The researchers next plan to conduct field studies to see whether the fungus can reduce mercury accumulation in vegetable crops and boost plants’ ability to grow in contaminated soil.

The fungus is “an excellent bioremediator,” says Priyadarshini Dey, a biotechnologist at the Ramaiah Institute of Technology. She notes that releasing the engineered version into the environment could pose ethical and other issues, but one approach could be use isolated enzymes from the fungus in a bioreactor to remediate soil and water. Fang’s team is conducting safety studies to determine whether the enhanced M. robertsii could be safely released.



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