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Researchers share lessons from humongous fungus

From left Ryan Dood, Tara Peterson and Gracie Peterson serve up free slices from a 100 square foot mushroom pizza during the 27th annual Humongous Fungus Festival held in Crystal Falls this past summer.

COLUMBIA, Mo. (AP) — It was discovered nearly three decades ago. At the time, it was thought to be heavier than a blue whale, bigger than 23 football fields and more than 1,500 years old. The news of its discovery appeared in almost all the major media outlets and even made David Letterman’s Top 10 list.

In July 2018, in a preprint paper posted on bioRxiv, scientists studying it announced that the so-called “humongous fungus,” an individual of Armillaria gallica that lives in a forest in the Upper Peninsula, is actually four times bigger and around one and a half times older than previously thought.

It’s not just that it’s so big and old. It’s also about what “H.F.”, as retired University of Missouri professor Johann Bruhn refers to it, can teach us. Bruhn has studied the Armillaria species for around 40 years as a research associate professor at Michigan Technological University first, and then at Missouri.

In the autumn, if it’s not too dry, mushrooms pop up near a tree or grow from the stem. A person looking at one may think that there is nothing more to it. But that’s not the case. A mushroom is just a piece of the puzzle — the “tip of the iceberg,” so to speak, of a fungus.

The main “body” of most fungi — the part we don’t usually see — is called the mycelium. One example is a fibrous or cottony growth that appears on fruit as it rots. Mycelium is composed of microscopic filaments called hyphae that look like tiny threads woven together, and in forests it mostly stays underground or within decaying wood.

So it lives unseen, until the time when it has enough nutrients and the weather is right for development of fruiting structures: mushrooms. As the mycelium exhausts nutrients from one food source, it grows outward seeking new ones. It usually expands as a ring that is known as a fairy circle. That’s how separate mushrooms a couple of feet away may actually belong to the same fungus.

Fungi belonging to genus Armillaria act primarily as decomposers of roots of trees that are already under stress, as well as their stumps and fallen stems. Infected roots are no longer able to absorb water or nutrients, and so the tree gradually dies. By killing and decomposing stressed trees, the fungus makes room and nutrients available to healthier trees, thus serving as a sort of “gatekeeper.”

Wood decayed by these species is white, and often spongy and wet. The mycelium of Armillaria is bioluminescent, especially in actively decaying wood. The bright white glow can be seen from a considerable distance and is commonly known as a fox fire. From one victim to the next, the fungus spreads by long black cords called rhizomorphs that develop from mycelium. These bootlace-like strands can travel great distances in search of a tree to infect and may form an extensive network. Shielded underground and in decaying wood, rhizomorphs are protected from high temperatures and drought.

That’s not the case with mushrooms of Armillaria gallica. They have a pinkish-brown to reddish-brown cap and usually grow in late summer or during the fall. They are edible, but have to be cooked.

Armillaria gallica lives throughout much of forested North America, including in Missouri. Here, it may

reveal to scientists how climate change could weaken even the most resilient trees. This could have implications for the economy and wildlife.

“So the average person walking through the woods would see clumps of mushrooms here, and they’re separated by quite a distance often,” Bruhn said. “And they think of it as being a small discrete organism with individual representatives scattered through the woods. And what we found was that, in fact, the Armillaria individuals in the forest floor are older than the trees. They’re larger than the trees and they have incredible longevity.”

One of the reasons for that longevity is its low rate of mutation. That, potentially, could improve understanding of the human genome and such diseases as cancer.

The discovery of the “humongous fungus” was a side product of a U.S. Navy effort called Project ELF.

The work began in 1982. The plan was to build miles-long antennae and bury their ends in the ground in Wisconsin and Upper Michigan. The antennae would send signals of extremely low frequency through bedrock to underwater craft around the world.

The public was concerned about possible environmental impact, so the Navy established an elaborate environmental monitoring program to assess the effect of the electrical signals on wildlife, trees, etc. Bruhn was one of the participating scientists.

“So what we concluded was that what we had to do was we had to establish small plantations of a fast-growing tree species that was very genetically homogeneous so that all the trees would be the same age,” Bruhn told the Columbia Missourian . That would enable scientists to measure the trees and build a better statistical model of the effects of the electrical fields on their growth.

The researchers cut patches of mixed hardwood-conifer forest and planted red pines. As he expected, Bruhn soon noticed that two species of Armillaria were killing trees in the research plantations.

“The pine pathogen was focused on the pines, but there was this other species that came eventually known as the ‘humongous fungus’ that was decomposing the hardwood stumps,” Bruhn said.

He observed that the Armillaria species seemed to occupy enormous territory. He wanted to understand how many individual organisms of each species were involved, and how they were spatially distributed in the forest.

He dug up the dead seedlings and cut a window through the bark to expose the fungus growing underneath and then sampled a piece of the fungus and grew it in culture. Then he mapped where the seedlings had been growing. Over about eight years, he ended up with a collection of hundreds of mapped cultures of the Armillaria species.

“One of the interesting things for me is that I cut the back of my finger, and I’ve got 10 stitches in it and the skin is going to grow back together because the skin on one side of the cut recognizes the skin on the other side of the cut as being compatible,” Bruhn said. “Well, this mushroom can do the same thing.”

When two cultures put in a petri dish belong to the same genetic individual they will grow together without leaving a sign that they were once separated. In distinct individuals, there would be a dark brown line between them. But to prove that different samples came from the same individual some molecular tests had to be done.

So Bruhn asked his friend Jim Anderson from the University of Toronto to help. Anderson had the necessary expertise in molecular biology and had a graduate student, Myron Smith, who just happened to be looking for a graduate project to work on.

They analyzed the samples, studied the rate at which the rhizomorphs (long black cords specific to Armillaria and related species that develop from mycelium — the main “body” of most fungi) grow and published a paper in Nature with the results in 1992. Soon, it got the name “humongous fungus.”

At that time, they determined that it occupied at least 37 acres, weighed more than 100 tons and by conservative estimates was more than 1,500 years old. The researchers were sure it was even larger than that. Starting in 2015, they took samples annually, mapped them, did compatibility tests to determine if they had reached the limit, and extended the geographic area of the study.

“We could hardly believe that it was as big as it turned out to be and we can only handle so many samples in the laboratory each year,” Bruhn said. “This is a large perimeter to be making collections.”

This year, they reached the edge. It turned out the humongous fungus weighed more than 400 tons and is more than 2,500 years old.

“We say 2,500 years, but that’s a conservative guess. (It) could be almost as old as the forest that replaced the glaciers,” Bruhn said. “What we wanted to understand was how the individual could get so large and so old and still retain its identity. For a human being, we would eventually contract cancer or something. People don’t live to 2,500 years old. We wouldn’t look like much.”

The team also wanted to understand was where it originated. Typically, fungi start from a single point and grow outward in a radial pattern, but obstacles and the development of new food bases would cause them to change direction. By gathering the samples and analyzing the number of mutations, researchers understood more about both the longevity and the growth of the humongous fungus.

“It’s a great follow-up on the work that they did quite a long time ago,” said Thomas Volk, professor of biology at the University of Wisconsin-La Crosse. “The methods are much more sophisticated now, and I think they have been able to glean a lot more interesting information from the current study.”

The team found that the fungus had very few mutations, and this could be the key to understanding how it has maintained its identity for such a long time.

“Cancers involve huge rates of specific mutations and Armillaria essentially represents the opposite end of the spectrum with just almost unbelievable genetic stability,” Bruhn said.

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