We’re all rightly concerned about greenhouse gases, but another facet of a warming planet that sometimes gets overlooked is the increasing accumulation of acutely toxic forms of mercury in our environment and in the food chain.
Mercury occurs naturally in the Earth’s crust, where it is safely sequestered away and doesn’t bother anyone. While it can be released into the air by volcanic activity, that hasn’t been much of a problem for life on Earth. But burning fossil fuels — especially coal — has increased the rate of mercury emissions into the air quite significantly, and this continues to be a mounting problem.
Metallic mercury (quicksilver, the stuff in old thermometers) is fairly toxic already, but when it gets into the environment, bacteria that live in oxygen-free environments like sediment convert it to methylmercury, which increases its toxicity to animals, including people, by a factor of at least 100.
Here are some of the serious dangers of methylmercury for humans:
Methylmercury easily reaches the bloodstream and is distributed to all tissues, and can cross the normally protective blood-brain barrier and enter the brain. It can also readily move through the placenta to developing fetuses and their developing brains, and is therefore a particular concern to pregnant women and women of childbearing age. Low-level exposure is linked to learning disabilities in children and interference with reproduction in fish-eating animals. Mercury exposure in humans can also lead to a variety of other negative health effects, including neurological, kidney, gastrointestinal, genetic, cardiovascular and developmental disorders, and even death.
Methylmercury from sediment gets ingested by small fish, who are eaten by larger fish, and so on. Larger and longer-lived predators keep accumulating it, because organisms like fish and people are very slow to clear it from their systems.
Warming waters actually accelerate this accumulation for reasons that go beyond mercury emissions; even though coal burning, and thus mercury emissions, have finally begun to decrease recently, methylmercury levels in fish continue to increase anyway. Of course that’s bad for people who eat fish, but it’s no picnic for the fish either:
Flagfish larvae incubated for one month in the levels of methylmercury shown
Methylmercury also makes its way into soil, where it can persist for many decades. Plants that grow in such soil will absorb methylmercury, not only decreasing crop yields, but leading to accumulation of methylmercury in people who eat those plants.
OK, enough of the depressing part! Now we need to turn things around.
As researchers at Zhejiang University (Hangzhou, China) and the University of Maryland have recently found, there’s a natural way to protect plants — and even some fish — from methylmercury accumulation. A fungus called Metarhizium robertsii that has a symbiotic relationship with many crops not only can increase their yields and protect them from insect invasions, but it can also absorb and detoxify methylmercury, allowing the plants to grow with minimal mercury accumulation even in polluted soils. The fungus can even remove methylmercury from freshwater and saltwater.
Not only that, but they also identified the fungal genes responsible, and when they added another copy, the fungus became even more effective at protecting plants and cleaning up water. We’ll learn all about these new findings in the upcoming November 22 issue of the Proceedings of the National Academy of Sciences. I thank Dr. Weiguo Fang for sending me a reprint of the article.
It’s been known for some time that Metarhizium fungi can help plants ward off insect invasions. The plants provide sugar to the fungus while it hangs out among the roots, and the fungus helps the plant by providing nutrients and killing insects. Let’s ask this cockroach what it thinks of the effectiveness of this arrangement:
Metarhizium anisopliae-infected (and quite dead) cockroach
But as Dr. Fang and colleagues discovered, this fungus appears to have imported two bacterial genes, on two separate occasions during the course of its evolution, that look suspiciously like genes that non-soil bacteria can use to detoxify methylmercury. We know that in bacteria, the two genes encode these two enzymes:
Methylmercury demethylase (MMD)
methylmercury (CH3-Hg) → mercuric cation (Hg2+) + methane (CH4)
Mercury ion reductase (MIR)
mercuric cation (Hg2+) → metallic mercury (Hg0)
So it seemed these genes might give Metarhizium fungus the ability to convert methylmercury to plain old metallic mercury. MMD is pretty much the reverse of what the anaerobic bacteria in sediments do to create methylmercury in the first place, although they use different enzymes to do it.
Since Metarhizium fungus grows on roots naturally, the authors were curious to see whether this relationship might help plants do better and/or accumulate less mercury from contaminated soil. So they grew plants in soil with levels of methylmercury or Hg2+ that are typical of harmfully contaminated sites.
In the first two columns below, we can see how much trouble plants have when exposed to these levels of mercury. But in the third column we see the positive effect of merely adding Metarhizium fungus to the soil, and in the last column it gets even better because the researchers put additional copies of the two key genes into the fungus. The plants look normal again!
This is only a subset of the growth experiments, but it’s a pretty important subset! Note that methylmercury is so potent that it achieves the same toxicity as Hg2+ at 1/2000 the concentration
As you would expect, the bigger and healthier the plants got, the less mercury they accumulated in their roots and leaves. The natural fungus did well in that respect, but the engineered fungus did even better.
Not everyone is a fan of genetic engineering, but that’s OK — it should be easy to naturally breed fungus with more-active methylmercury detoxification because we have such a strong selection to use; we just keep growing these fungi in the presence of more methylmercury, and the most resistant ones will grow fastest and win out. They already have the genes, so no GMOs needed.
We can add this fungus to the soil, or we can even treat seeds with it directly before planting, and the fungus will naturally colonize the roots. We end up with plain old metallic mercury, which is a lot less harmful to us, and thus we complete the natural cycle. We can use activated charcoal at our site to sequester the mercury and return it to the Earth in its original form again. So we clean up the site naturally, but we can also grow plants on it in the meantime, through several cycles of this. We can go with non-food crops at first until the site is OK, then grow whatever we want again.
Corn seeds are treated before planting with spores of Metarhizium robertsii fungus, then roots in the field show the green color characteristic of fungal colonization
Another very cool thing the authors showed is that this fungus can detoxify mercury from freshwater or saltwater, and unlike bacteria that can detoxify methylmercury, they don’t even need added nutrients to enable them to do it. One gram of engineered fungus completely removed 2 mg of methylmercury from a liter of water in 48 hours. That’s a pollutant level 1,000-fold higher than the EPA limit! The non-engineered fungus wasn’t too shabby either, doing 75% as well. Other kinds of fungi that don’t contain these genes — like brewer’s yeast — didn’t do anything, even if you gave them nutrients, so we see how critical these two genes are. Once the fungi are done with the cleanup, we can remove them from the water and pack them away where their mercury won’t be released into the air again.
So the authors sure hit a lot of things at once here, didn’t they? They figured out what genes are responsible for methylmercury detoxification in fungi, showed that these fungi could actually do it for real, showed that they protect plants from mercury naturally, gave us a way to remediate an agricultural site even as we still grow plants on it, and also gave us a benign and low-cost way to get methylmercury out of bodies of water! All by observing nature and being smart enough to learn from it.
Man, I think this calls for some corn on the cob!
