A University of Maryland researcher and colleagues found that the fungus Metarhizium robertsii removes mercury from the soil around plant roots, and from fresh and saltwater. The researchers also genetically engineered the fungus to amplify its mercury detoxifying effects.
Mercury pollution of soil and water is a worldwide threat to public health. This new work suggests Metarhizium could provide an inexpensive and efficient way to protect crops grown in polluted areas and remediate mercury-laden waterways.
The study, which was conducted by UMD professor of entomology Raymond St. Leger and researchers in the laboratory of his former post-doctoral fellow, Weiguo Fang (now at Zhejiang University in Hangzhou, China), was published in Proceedings of the National Academy of Sciences (PNAS) on November 14, 2022.
"This project, led by Dr. Fang, found that Metarhizium stops plants from taking up mercury," said St. Leger. "Despite being planted in polluted soil, the plant grows normally and is edible. What's more, the fungus alone can quickly clear mercury from both fresh and saltwater."
Metarhizium is a nearly ubiquitous fungi, and previous work by the St. Leger laboratory had shown that it colonizes plant roots and protects them from herbivorous insects. Scientists have known that Metarhizium is often one of the only living things found in soils from toxic sites like mercury mines. But no one had previously determined how the fungus survived in mercury polluted soils, or if that had implications for the plants the fungus normally lives with.
St. Leger and other colleagues had previously sequenced the genome of Metarhizium, and Fang noticed that it contains two genes that are very similar to genes present in a bacterium known to detoxify, or bioremediate, mercury.
For the current study, the researchers ran a variety of laboratory experiments and found that corn infected with Metarhizium grew just as well whether it was planted in clean soil or mercury-laden soil. What's more, no mercury was found in the plant tissues of corn grown in polluted soil.
The researchers then genetically modified the fungi, removing the two genes that were similar to those in mercury remediating bacteria. When they replicated their experiments, modified Metarhizium no longer protected corn plants from mercury-laden soil, and the corn died.
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