Wolf lichen (Letharia vulpine), a lichen species studied by a team of University of Montana researchers, show that some of the world's most common lichen species actually are composed of three partners, not the widely recognized two. (Tim Wheeler)
Wolf lichen (Letharia vulpine), a lichen species studied by a team of University of Montana researchers, shows that some of the world's most common lichen species actually are composed of three partners, not the widely recognized two. (Tim Wheeler)

Traditionally, scientists have likened lichen to a married couple: The crusty growths found on trees and rocks are actually composite organisms, formed by the symbiotic partnership between an algae and a single fungus. But a new study throws a wrench into that 150-year-old belief, suggesting that a third partner has been lurking in the mix. A second fungus — this one a type of yeast — makes the synergy possible.

The filaments of a fungus provide the structure and support for lichen organisms, while algae or cyanobacteria swoop in to entangle themselves in the safety net, producing food for their new fungal home by way of photosynthesis. But according to a study published Thursday in Science, single-celled basidiomycete yeasts provide even more support. They're probably lurking in many lichens — so far, they've been spotted in 52 genera.

"This discovery overturns our longstanding assumptions about the best-studied symbiotic relationship on the planet," Purdue mycologist M. Catherine Aime, who co-authored the study, said in a statement. "These yeasts comprise a whole lineage that no one knew existed, and yet they are in a variety of lichens on every continent as a third symbiotic partner. This is an excellent example of how things can be hidden right under our eyes and why it is crucial that we keep studying the microbial world."

Toby Spribille, the University of Montana postdoc who led the study, says he had to spend a long time convincing himself that he wasn't working with contaminated samples: He was trying to figure out why two species of lichen called Bryoria tortuosa and B. fremontii were so different, despite having the same fungal and algal components. The former is yellow and produces toxic acid, but the latter is brown and benign. Instead of minute genetic differences between their algae, he found an entirely new kind of fungus.

Ironically, these long-hidden yeasts actually coat the other two members of their triad. They may provide structural support in addition to producing protective chemicals — the toxic Bryoria tortuosa has more of these organisms than its dull cousin fremontii, suggesting that the basidiomycete yeasts might produce compounds that keep the lichen safe from its own acid.

It will take more research to figure out exactly how many of the world's most famous symbiotic organisms are actually threesomes — let alone what purpose the yeasts actually serve — but it seems as if biology textbooks might need a little revision.

"There is a long, venerable history of scientists and natural historians who have peered at lichens through microscopes since the 1800s," Spribille said in a statement. "The yeasts were always there, and somehow we got to crack it open. It gives me goose bumps."

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