Scientists Discover That Lichens Are Symbiotic Threesomes

When’s the last time you really looked at lichen? If you’re like most people, the answer is probably “never.” That’s a shame, because these unassuming little organisms are useful, amazing, and, scientists now say, a lot more complicated than we previously realized. They published their findings in the journal Science. 

You might not know it from the way we treat these organisms today, but humans and lichens have a long and close relationship. Cultures around the world have used lichens for thousands of years as medicine, dye, and food. Some myths say the first lichens were really scabs scraped onto a rock from the buttocks of a hero, while others say they're lizard sperm. (Do we have your attention yet?)   

So—putting aside the lizard sperm theory—what is lichen, exactly? That depends on who, and when, you ask. Naturalists in the first half of the 19th century were quite confident that they were dealing with a plant, and were scandalized by Swiss botanist Simon Schwendener’s 1868 announcement that lichens were cooperative organisms composed of one type of fungus and one type of microscopic alga. Eventually, Schwendener won over his critics. His findings entered the scientific canon, and there they stayed … until last week, when researchers in Montana and elsewhere said they’d identified a third member of the lichen household.

Microbiologist Toby Spribille of the University of Montana was part of the team shaking up the lichen establishment. “There’s been over 140 years of microscopy,” Spribille told Ed Yong at The Atlantic. “The idea that there’s something so fundamental that people have been missing is stunning.”  

Spribille and his colleagues might have missed it too, had they not taken an interest in a local pair of lichens called Bryoria tortuosa and Bryoria fremontii. The two are closely related but easy enough to tell apart—B. tortuosa is yellow, while B. fremontii is brown—and that’s really good, because B. tortuosa is toxic, while B. fremontii was historically used as food. 

B. tortuosa. Image credit: Tim Wheeler

A few years ago, researchers decided to take a look at the two species’ genes to see where they diverged. Conclusion? They didn’t. According to their genetic barcode analysis, toxic B. tortuosa and edible B. fremontii were just two different names for the same species. 

That didn’t sit right with Spribille and his colleagues, who had begun using advanced genomic techniques to understand cooperative relationships between insects and other organisms. They decided to take another crack at the puzzle using the latest sequencing technology. 

The team went out into the wilds of Montana and collected samples of both lichen types, then ground them up and sequenced their RNA. What they found contradicted not only the single-species theory, but also the one-fungus/one-photosynthesizing-microbe definition of lichen-hood. Each lichen’s genetic code contained not one, but two different types of fungus—and that second fungus was far more prevalent in deadly B. tortuosa.

To make sure they understood what they were seeing, the team then analyzed the DNA of lichen species from all over the world. Most of those species—from 52 lichen genera on six continents—also included that sneaky second fungus, a basidiomycete yeast. Of the 52 genera, all but 10 are in a single lichen family: Parmeliaceae.  

“We found it in everything,” Spribille told Gizmodo. “From Alaska to Ethiopia to Antarctica, it always was there.” 

He notes that overlooking the presence of the silent fungus partner may be the reason scientists have historically had so much trouble growing lichens in the lab.

Whether toxic or edible, he says, every lichen is remarkable. “One thing that sets lichen apart from all other symbioses is that all the components are microbes. But when they come together, they form something self-replicating and beautiful that you can hold in your hand.”

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