In lichen's symbiotic pairing, a third partner?
A type of yeast might be the third partner in most lichen, upending a nearly 150-year-old theory.
It’s an archetype of symbiosis, the marriage of two species to form one organism. A fungus provides minerals, water, and shelter, while a photosynthesizing alga provides nutrients, forming lichen.
But a discovery by a researcher at the University of Graz in Austria who grew up in a trailer park in Montana, homeschooled in what he now calls a “fundamentalist cult,” may have toppled this model.
According to a study published Thursday in Science, a second fungus — a type of yeast — might be the third symbiotic partner in most lichens.
The discovery has forced a shift in thought not just in our understanding of lichens, but also in symbiosis. The explanation as to how coral reefs are formed, how cows digest food, and even how mitochondria power the cells of eukaryotic organisms, symbiosis is a concept that was born out of lichens. With the nearly 150-year-old hypothesis in question, the discovery is a “game-changer not only for lichen research, but also for the field of symbiosis more generally,” Toby Kiers, an evolutionary biologist at Vrije University Amsterdam, said in an article that will appear in Science July 29.
"This is a pretty fundamental shake-up of what we thought we knew about the lichen symbiosis," Tony Spribille, who discovered the yeast, adds in a statement. "It forces a reassessment of basic assumptions about how lichens are formed and who does what in the symbiosis."
The prevailing theory of lichen symbiosis was first revealed in 1868 by Simon Schwendener. The Swiss botanist found lichen was not a plant, but rather a composite organism of fungi and cyanobacteria, or microscopic algae. Through photosynthesis, the alga produces nutrients for the fungus, while the fungus provides the alga minerals, water, and shelter.
“This kind of mutually beneficial relationship was unheard of, and required a new word,” writes The Atlantic’s Ed Yong, about Dr. Schwendener’s discovery. “Two Germans, Albert Frank, and Anton de Bary, provided the perfect one — symbiosis, from the Greek for ‘together’ and ‘living.”
But Dr. Spribille, in collaboration with John McCutcheon, a professor of microbiology at the University of Montana, found a previously unknown, second fungus is the third species that forms lichen. The basidiomycete yeast appears to be as much a part of the lichen as the other two species. It forms the outermost crust of the lichen organism.
A romance of two fungi and algae, Spribille’s discovery is as much about them as it is about the coming together of Spribille, a lichen aficionado, and Dr. McCutcheon, who relies on genetic sequencing to study insect-microbial partnerships. In 2011, when Spribille returned from the University of Gottingen in Germany to his native Montana for postdoctoral research, he wondered why two types of Montana lichens known to consist of the same fungus and algae, appear different, according to The New York Times. One of the lichens appears yellow, and produces vulpinic acid. The other lichen is dark brown, and lacks the toxin.
Spribille brought the riddle to McCutcheon. When they performed genetic sequencing, they found basidiomycete yeast in one of lichens, but not in the other. Spribille presumed the yeast was a contaminant or a pathogen. But, when he removed all the basidiomycete genes, everything that related to the presence of vulpinic acid also disappeared.
“That was the eureka moment,” he told The Atlantic. “That was when I leaned back in my chair.”
Spribille then screened the 45,000 samples of lichens he had collected over his career. In almost all of the macrolichens — bushy or leaf-like lichen — he found the basidiomycete genes.
Spribille reached out to Swedish colleagues. A genomic analysis of the chartreuse lichen they were studying found it, too, had a yeast partner. All told, 52 lichen genera from six continents showed a yeast partner in their genes.
“This is not a single oddity,” said Dr. Kiers of Vrije University Amsterdam. “We’re looking at a global phenomenon.”
The exact role of the yeast in lichen remains uncertain, according Science. And there are those who wonder if the second fungus is truly a symbiotic partner. One critic is Thorsten Lumbsch, a mycologist at the Field Museum of Natural History in Chicago. In Science, Dr. Lumbsch said it must be established whether the yeast exchanges nutrients or interacts with the other fungus and the alga in other ways. Other lichens are made up of more than two organisms.
Regardless, the discovery has turned the definitions of lichen and symbiosis inside out.
"The word symbiosis in part comes from the study of lichens," said McCutcheon, in a statement. "The textbook definition of lichen has always been restricted to one fungus and one fungus only. Our work shows that this definition doesn't seem to be correct."