PNAS: How a common fungus removes toxic mercury from soil and water

A researcher at the University of Maryland and his colleagues found that Roberts Metarhiza can remove mercury
from soil, fresh water and salt water around plant roots.
The researchers also genetically modified the fungus to enhance its mercury detoxification effects
.

Mercury contamination of soil and water is a worldwide threat to
public health.
The new study suggests that Chlorella could provide an inexpensive and effective way to protect crops growing in contaminated areas and remediate mercury-containing waterways
.

The study was conducted by researchers in the lab of Raymond St.
Leger, a professor of entomology at the University of Maryland and his former postdoctoral fellow Weiguo Fang (now at Zhejiang University in Hangzhou, China), and was published in the Proceedings of the National Academy of Sciences (PNAS
) on November 14, 2022.

"This project, led by Dr.
Fang, found that Metarhizoma can prevent plants from absorbing mercury," St.
Legg said
.
"Despite being grown in contaminated soil, the plant grows normally and is edible
.
What's more, the fungus quickly removes mercury
from fresh and brine water.
”

Rhizophora aeruginosa is an almost ubiquitous fungus that previous research in the St.
Léger laboratory has shown that it colonizes plant roots, protecting plants from herbivorous insects
.
Scientists already know that Chlorella is often one of the only organisms found in the soil of
toxic sites such as mercury mines.
But no one had previously determined how the fungus survived in mercury-contaminated soil, or whether it would have an effect
on the plants that normally live with the fungus.

After St.
Léger and other colleagues sequenced the genome of Chlorella aeruginosa, they noticed that it contained two genes very similar to
those found in a bacterium known to detoxify or bioremediate mercury.

In the current study, the researchers conducted various laboratory experiments and found that corn infected with Metarhinosa grew just as well
whether grown in clean soil or mercury-rich soil.
What's more, no mercury
was found in plant tissues of corn grown in contaminated soil.

The researchers then genetically modified the fungus to remove two genes
similar to mercury-repairing bacteria.
When they repeated their experiment, the modified Metarhizoma no longer protected the corn from mercury-containing soil, and the corn died
.

To test whether these genes provide detoxification, the researchers inserted them into
another fungus that normally does not protect corn from mercury contamination.
The newly improved fungus behaves like Metarhizoma, protecting plants from mercury-containing
soils.

Microbiological analysis has shown that the enzyme expressed by the gene in question can break down highly toxic organic mercury into less toxic inorganic mercury molecules
.
Finally, the researchers genetically modified Chlorella aeruginosa to express more detoxification genes and increase the production
of detoxification enzymes.

In their final experiment, the researchers found that they could remove mercury
from fresh and brine by mixing green algae within 48 hours.

The next step will be to conduct field trials in China to see if Chlorella can turn toxic environments into high-yielding soil
for growing corn and other crops.
Current methods of remediating contaminated soil require toxins to be removed or neutralized from the entire land before any crops are planted
.
This can be very expensive and take a long time
.
But Chlorella aeruginosa only directly detoxifies the soil around plant roots, preventing plants from absorbing toxins
.

"Letting plants grow in a mercury-rich environment is one of the ways this fungus protects plant homes," explains
St.
Legg.
"This is the only microbe we know of that could potentially be used in this way, because bacteria with the same genetic ability to detoxify mercury do not grow on
plants.
" But you can imagine simply dipping seeds in Metarhizoba and growing crops
that are now unaffected by mercury-rich soils.
”

In addition to being a cost-effective tool to reclaim contaminated land for agriculture, Chlorella can also help remove mercury from wetlands and contaminated waterways, which are increasingly threatened
by mercury pollution as climate change and permafrost thaw accelerate the release of toxic metals into soils and oceans.