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The largest living organism (Armillaria ostoyae) covers over 2,385 acres and produces honey mushrooms (pictured) in the fall. Wikimedia Commons

It's not a dinosaur, whale, or giant deep-sea creature. In fact, the world's largest known living organism is an underground fungus that most people wouldn't notice even if it was right beneath their feet.

The gargantuan fungus, Armillaria ostoyae (sometimes called Armillaria solidipes), covers more than 3.4 square miles (8.8 km2) in Oregon's Malheur National Forest and is more than 2,400 years old.

Most of the year it exists as a network of interjoined underground fungus filaments, called rhizomorphs (root-like structures that look like black shoelaces.) But each fall its fruiting bodies rise above the surface in the form of edible honey mushrooms, suddenly showing the extent of its impressive domain (over 2,385 acres.)

Its slow growth of just 1 meter per year (on average) makes the large expanses it covers that much more impressive. And as you'll learn, its slow-moving progression is made possible by the sacrifices of those living around it.

The Humongous Fungus is more than 2,400 years old and covers more than 3.4 square miles (8.8 km2) in Oregon's Malheur National Forest. Creative Commons

Serial Tree Killer

Over hundreds and thousands of years, the world's largest living organism has slowly infected, killed, eaten, and engulfed countless trees and shrubs unlucky enough to be in its path. Honey fungus is, after all, an infamous killer in the forest world. Its black rhizomorphs are like highways that allow white rot to travel from host to host. The "white rot" associated with Armillaria infects trees and shrubs by encircling, attacking, and ultimately killing their roots. While this is happening, rhizomorphs continue onward, always seeking another host.

While many parasites found in nature require a living host, Armillaria is a facultative saprophyte, so it can survive and live off of its hosts long after it's killed them. This allows for almost limitless expansion without the need for self-regulation that's required by parasites that depend on a living host.

Over months or years the attacked host dies. The Armillaria is especially pathogenic to softwoods like Douglas-fir (Pseudotsuga menziesii), true firs (Abies spp.), and Western Hemlock (Tsuga heterophylla.)

The fungus' progress can be tracked by the expanding areas of dead and dying trees. Infected and newly-dead trees sprout honey mushrooms in the fall and are easy to detect. The Humongous Fungus was discovered by taking samples from all the known infected trees in the park and comparing their fungal DNA.

When scientists discovered it was all the same DNA, they suddenly realized they were looking at the world's largest known living thing. A parasitic fungus!

Black rhizomorphs invade the host, allowing white rot to piggyback and attack the host. 1 / 4

Ninjas of the Fungus World

In an eye-opening study, biologists compared the genomes of Armillaria with other related species of fungus. Apparently Armillaria ostoyae has evolved devious genetic ways by which to sneak up on unsuspecting trees and hosts.

For example, the fungus can reabsorb chemical markers that would alert trees to its presence. This allows the rhizomorphs to "sneak up" on unsuspecting trees, bypassing the tree's natural defenses. They've also developed extra proteins for killing cells and eating the cellulose "glue" that holds plant cell walls together, gobbling them up without so much as a thought.

The fungus can come in the back door and begin feeding on and killing the host long before other parasitic competitors arrive. And even when the competitors arrive, the Armillaria ostoyae can create such a toxic chemical environment that they have to turn tail and run before they succumb themselves.

Infected trees sprout honey mushrooms in the fall. Creative Commons

Diversity is Key to Biological Success

According to Cardiff University biosciences professor Lynne Boddy, “Fungi are the garbage disposal agents of the natural world. They break down dead, organic matter, and by doing that they release nutrients. Those nutrients are then made available for plants to carry on growing.”

In diverse forests, fungi kill and feed on only the weakest trees. But what can happen with a monoculture of trees (all the same types of tree planted together in large areas) is that disease or weather can weaken all of them at the same time. When this happens it allows fungi like Armillaria to take out entire forests at once.

The more diversity nature has, the less likely a disease or fungus will be able to wipe out huge areas. This is one reason why monoculture farming is dangerous, and why many choose to plant diversity into their fields with a healthy polyculture theme.

By studying the Humongous Fungus in Oregon, scientists are hoping to better understand (and be able to control) Armillaria infestation in other affected areas around the globe. As a side project it appears to be yet another reminder that diversity and variety are key to ongoing biological success.

Sources and Further Reading

1. Andrew, E. (2018, March 20). Meet the World's Largest Living Organism. Retrieved October 17, 2018, from https://www.iflscience.com/plants-and-animals/meet-worlds-largest-living-organism

2. Armillaria. (2018, October 16). Retrieved October 17, 2018, from https://en.wikipedia.org/wiki/Armillaria

3. Armillaria ostoyae. (2018, October 05). Retrieved October 17, 2018, from https://en.wikipedia.org/wiki/Armillaria_ostoyae

4. Ferguson, B. A., Dreisbach, T. A., Parks, C. G., Philip, G. M., & Schmitt, C. L. (2003, April). Coarse-Scale Population Structure of Pathogenic Armillaria Species in a Mixed-Conifer Forest in the Blue Mountains of Northeast Oregon. Retrieved October 17, 2018, from http://www.nrcresearchpress.com/doi/10.1139/x03-065#.W8fENGhKiM-

5. Fleming, N. (2014, November 19). The Largest Living Thing on Earth Is a Humongous Fungus. Retrieved October 17, 2018, from http://www.bbc.com/earth/story/20141114-the-biggest-organism-in-the-world

6. Klein, J. (2017, November 03). The Humongous Fungus and the Genes That Made It That Way. Retrieved October 17, 2018, from https://www.nytimes.com/2017/11/03/science/humongous-fungus-armillaria-genes.html

7. Largest Living Thing. (2015). Retrieved October 17, 2018, from http://www.extremescience.com/biggest-living-thing.htm

8. Morris, L. (2017, May 16). The World's Largest Living Organism. Retrieved October 17, 2018, from https://www.nationalgeographic.com.au/nature/the-worlds-largest-living-organism.aspx

9. Patton, V. (2018, February 11). Oregon Humongous Fungus Sets Record as Largest Single Living Organism on Earth. Retrieved October 17, 2018, from https://www.opb.org/television/programs/ofg/segment/oregon-humongous-fungus/

10. Riggs, K. (2012, November 15). A Fungus Among Us—Author Names Oregon's Humongous Fungus as One of the Grossest Places on Earth. Retrieved October 17, 2018, from https://www.usda.gov/media/blog/2012/11/15/fungus-among-us-author-names-oregons-humongous-fungus-one-grossest-places

11. Sipos, G., Prasanna, A. N., Walter, M. C., O’Connor, E., Bálint, B., Krizsán, K., Nagy, L. G., et al (2017, October 30). Genome Expansion and Lineage-Specific Genetic Innovations in the Forest Pathogenic Fungi Armillaria. Retrieved October 17, 2018, from https://www.nature.com/articles/s41559-017-0347-8

© 2018 Kate P