There’s more to these mushrooms than meets the eye Smith, Bruhn & Anderson

A huge underground fungus that is one of the largest living organisms on the planet has turned out to be both bigger and older than thought. It may have been quietly spreading through the soil of Michigan since the end of the last ice age.

James B. Anderson of the University of Toronto in Canada and his colleagues discovered the enormous Armillaria gallica fungus in the late 1980s, while studying fungi that were killing red pines on a Michigan plantation. “We found one genetic individual occupying this site,” says Anderson. It spanned at least 0.37 square kilometres. At the time they estimated it was at least 1500 years old and weighed at least 100,000 kilograms.

They published their findings in 1992. At the time the fungus was a serious contender for the largest living organism, but bigger fungi have since been found. An individual of A. ostoyae in the Malheur National Forest in Oregon spans 9.6 square kilometres. Confusingly, the Michigan and the Oregon fungi are both informally called “the humongous fungus”.


Larger than thought

Anderson and his colleagues have now revisited the fungus, which had been left to its own devices since the early 1990s. They collected 245 samples, far more than before, allowing them to get a better sense of its borders. It turns out the fungus weighs at least 400,000 kg, four times larger than the initial estimate.

The fungus grew from a single individual, so its greater size implies it is also older than thought. “We’re now saying 2500 years based on our estimates of growth rate, and that’s a lower bound,” says Anderson.

If the fungus spent some of the time in stasis rather than growing, it could be much older. The upper limit is the end of the last ice age, about 11,000 years ago, because there were no trees in Michigan when it was covered in ice. “It may go all the way back to post-glaciation when the forest was re-establishing on that site,” says Anderson.

The team was also able to estimate how many mutations the fungus has accumulated over its life, because the parts closer to the edge are younger and carry more mutations than the old parts in the centre.

“The fungus does mutate, but the rate of mutation is almost impossibly low,” says Anderson. It’s not clear how the fungus manages this: it may have unusually good DNA repair mechanisms, or a purposely slow rate of cell division. The low mutation rate may help the fungi live as long as they do, and grow to such enormous sizes.

Journal reference: bioRxiv, DOI: 10.1101/377234

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