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Quote: At first, the duo figured that a basidiomycete fungus was growing on the lichens. Perhaps it was just a contaminant, a speck of microbial fluff that had landed on the specimens. Or it might have been a pathogen, a fungus that was infecting the lichens and causing disease. It might simply have been a false alarm. (Such things happen: genetic algorithms have misidentified plague bacteria on the New York subway, platypuses in Virginia tomato fields, and seals in Vietnamese forests.)
But when Spribille removed all the basidiomycete genes from his data, everything that related to the presence of vulpinic acid also disappeared. “That was the eureka moment,” he says. “That’s when I leaned back in my chair.” That’s when he began to suspect that the basidiomycete was actually part of the lichens—present in both types, but especially abundant in the yellow toxic one.
And not just in these two types, either. Throughout his career, Spribille had collected some 45,000 samples of lichens. He began screening these, from many different lineages and continents. And in almost all the macrolichens—the world’s most species-rich group—he found the genes of basidiomycete fungi. They were everywhere. Now, he needed to see them with his own eyes.
Down a microscope, a lichen looks like a loaf of ciabatta: it has a stiff, dense crust surrounding a spongy, loose interior. The alga is embedded in the thick crust. The familiar ascomycete fungus is there too, but it branches inwards, creating the spongy interior. And the basidiomycetes? They’re in the outermost part of the crust, surrounding the other two partners. “They’re everywhere in that outer layer,” says Spribille.
Despite their seemingly obvious location, it took around five years to find them. They’re embedded in a matrix of sugars, as if someone had plastered over them. To see them, Spribille bought laundry detergent from Wal-Mart and used it to very carefully strip that matrix away.
And even when the basidiomycetes were exposed, they weren’t easy to identify. They look exactly like a cross-section from one of the ascomycete branches. Unless you know what you’re looking for, there’s no reason why you’d think there are two fungi there, rather than one—which is why no one realised for 150 years. Spribille only worked out what was happening by labeling each of the three partners with different fluorescent molecules, which glowed red, green, and blue respectively. Only then did the trinity become clear.
“The findings overthrow the two-organism paradigm,” says Sarah Watkinson from the University of Oxford. “Textbook definitions of lichens may have to be revised.”
“It makes lichens all the more remarkable,” adds Nick Talbot from the University of Exeter. “We now see that they require two different kinds of fungi and an algal species. If the right combination meet together on a rock or twig, then a lichen will form, and this will result in the large and complex plant-like organisms that we see on trees and rocks very commonly. The mechanism by which this symbiotic association occurs is completely unknown and remains a real mystery.”
Based on the locations of the two fungi, it’s possible that the basidiomycete influences the growth of the other fungus, inducing it to create the lichen’s stiff crust. Perhaps by using all three partners, lichenologists will finally be able to grow these organisms in the lab.
In the Montana lichens that Spribille studied, the basidiomycete obviously goes hand-in-hand with vulpinic acid. But is it eating the acid, manufacturing it, or unlocking the ability to make it in the other fungus? If it’s the latter, “the implications go beyond lichenology,” says Watkinson. Lichens are alluring targets for ‘bioprospectors’, who scour nature for substances that might be medically useful to us. And new basidiomycetes are part of an entirely new group, separated from their closest known relatives by 200 million years ago. All kinds of beneficial chemicals might lie within their cells.