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  • Triple Helix

Clearing Soil and Water of Mercury… With a Fungus

Written by El Hebert '24

Edited by Lizzy Zhang '24

Three corn plants demonstrate mercury cleaning in action. The “Control” plant on the left was grown normally, while the “Untreated” middle plant was grown in mercury-laced soil. On the right, the “WT” plant had the same polluted soil, but the fungus Metarhizium robertsii was planted with it. Photograph from Wu et al [1].


Mercury is toxic to nearly every living thing on Earth. As a pollutant from human industry, it leaches into the soil and water, sickening plants and the animals that eat them. But some life forms can detoxify it by consuming it and converting it to an inert form. This November, researchers from the University of Maryland (UMD) discovered how one such organism thrives in mercury-contaminated soil, and how it could protect our crops [1].


Few environments, no matter how polluted, can exclude life entirely. Even in the wasteland of mercury runoff sites, such as gold mines, researchers have long noted a steadfast contingent of microbes scraping out their living. Many bacteria form bustling colonies in the toxic soil of these areas and in mercury-spiked Petri dishes in the lab — and as they grow, they clean their environments. In Sulawesi, one study watched a bacterium from the group Pseudomonas remove 75% of the mercury from their dish [2].


But tiny bacteria aren’t the only organisms to survive this way. At UMD, Raymond St. Leger, Weiguo Fang and colleagues focused on a soil-dwelling fungus called Metarhizium robertsii, which is known to crop up at polluted sites. In the wider world, M. robertsii is a far-ranging species, and, unlike the bacteria, it has a special relationship to its plant neighbors. The fungus interweaves with plant roots, sharing nutrients and killing off pest insects [3].


Dr. Fang and his team wondered if M. robertsii could also share its mercury resistance with its host. Contaminated plants, after all, are often where the trouble begins for humans. We depend on our agricultural crops, and at the top of the food chain, we end up eating all the mercury they absorb.


First, the researchers scoured the M. robertsii genome for points of interest, and found a set of oddly familiar genes. Dubbed Mmd and Mir, these genes matched up with the mercury resistance suite already known from the soil-cleaning bacteria that live out their days at mining sites. In fact, the fungal and bacterial genes may be the same: Dr. Fang’s previous research has shown that M. robertsii often swaps DNA with surrounding soil bacteria [4]. In this case, the fungus picked up the genes for two enzymes, one called a demethylase and one called a reductase, which are responsible for detoxifying organic mercury.


To see if Mmd and Mir function in M. robertsii as they do in bacteria, the lab created two sets of mutant fungi, one set with the promising genes deleted, and one with the genes overexpressed — that is, forced into high activity. In an ordinary nutrient broth, the mutants and the wild version all sprouted identical little colonies. Then, when exposed to mercury in their dishes (about 30 milligrams per kilogram, as in heavily polluted soil), a difference emerged. While the wild fungus grew and spread, the mutants without Mmd and Mir failed to germinate, and the overexpressing mutants grew even faster than usual.

The researchers then added the mutant strains to potted corn plants, allowing the fungus to colonize the young roots. Once again, the more active the Mmd and Mir genes, the better the fungi grew in contaminated soil. This time, their plant hosts followed suit. Overexpressing mutants protected the corn so well, it grew to the same size as the uncontaminated control crop. In the soil itself, mercury levels began to drop.


M. robertsii even showed its promise in cleaning water. Within two days, wildtype mutants growing in a 1-liter freshwater tank cut organic mercury levels in half; overexpressing mutants removed 65%. It worked almost as well in seawater, too, and did not require any added nutrients to grow and work.


For the final test of the Mmd gene, Dr. Fang’s team transferred it into two unrelated strains of fungi, including ordinary brewers’ yeast. These genetic hybrids, usually vulnerable to pollution, now grew well in mercury-contaminated media, just like M. robertsii could.


Around the world, mercury pollution is on the rise. Mining, refining, and fossil fuel use leak the potent poison into the ecosystem, and thawing permafrost could release even more [5]. M. robertsii may find itself called into duty soon. It’s a particularly promising subject — widely distributed, and already industrially produced as a safe pesticide alternative — and now, its mercury detoxification ability can be pinpointed and even enhanced. Dr. Fang and his colleagues suggest that farmers facing pollution might coat their seeds with enhanced M. robertsii, allowing the crops to grow up with their protectors.


Like the plants themselves, our species could gain strength and resilience if we team up with M. robertsii , our unassuming, gene-stealing, mercury-digesting neighbor in the dirt.

 

References

[1] Wu C, Tang D, Dai J, Tang X, Bao Y, Ning J, Zhen Q, Song H, St. Leger RJ, Fang W. Bioremediation of mercury-polluted soil and water by the plant symbiotic fungus Metarhizium robertsii. Proceedings of the National Academy of Sciences. 2022 Nov 22 [cited 11 Dec 2022];119(47):e2214513119. DOI: 10.1073/pnas.2214513119.


[2] Kepel B. Potential of Organic Mercury-resistant Bacteria Isolated from Mercury Contaminated Sites for Organic Mercury Remediation. Pakistan Journal of Biological Sciences: PJBS. 2019 Jan 1 [cited 11 Dec 2022];22(1):45-50. DOI: 10.3923/pjbs.2019.45.50.


[3] Suryanarayanan TS. Endophyte research: going beyond isolation and metabolite documentation. Fungal ecology. 2013 Dec 1 [cited 11 Dec 2022];6(6):561-8. DOI: 10.1016/j.funeco.2013.09.007.


[4] Zhang Q, Chen X, Xu C, Zhao H, Zhang X, Zeng G, Qian Y, Liu R, Guo N, Mi W, Meng Y. Horizontal gene transfer allowed the emergence of broad host range entomopathogens. Proceedings of the National Academy of Sciences. 2019 Apr 16 [cited 11 Dec 2022];116(16):7982-9. DOI: 10.1073/pnas.1816430116.


[5] Schuster PF, Schaefer KM, Aiken GR, Antweiler RC, Dewild JF, Gryziec JD, Gusmeroli A, Hugelius G, Jafarov E, Krabbenhoft DP, Liu L. Permafrost stores a globally significant amount of mercury. Geophysical Research Letters. 2018 Feb 16 [cited 11 Dec 2022];45(3):1463-71. DOI: 10.1002/2017GL075571.

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