Symbiotic relationships, where very different species collaborate for mutual benefit, are a hugely important feature of ecology. Biologists spend a lot of time exploring these relationships, but are used to exploring the relationship between just two species. Now they have discovered that a remarkable three-way symbiosis has been under their noses, undetected for over a century, in the form of lichen.

The identification of lichen as a composite organism of cyanobacteria or algae and fungi dates back 150 years, representing the first identification of such intimate symbiosis. It is only recently that a third species was proposed as a crucial part of lichen's widespread success.

Dr Toby Spribille of the University of Montana has confirmed this theory in Science, showing that many lichens also incorporate yeast species into their mix. The work explains two things that had puzzled lichenologists: why two lichens look different and have contrasting chemistry, but reveal the same genetics when sequenced, and why attempts to synthesize lichen have seldom proven effective.

"This is a pretty fundamental shake-up of what we thought we knew about the lichen symbiosis," Spribille said in a statement. "It forces a reassessment of basic assumptions about how lichens are formed and who does what in the symbiosis."

Lichens have been occupying rocks and any other suitable substrates for at least 400 million years. What at first looked like a single species, possibly a moss, was found in 1867 to represent either algae or cyanobacteria thriving among the filaments of a fungus. The filaments protect the photosynthesizer from the wider environment and keep it moist.

In turn, the algae or cyanobacteria produce organic carbon, and sometimes fixed nitrogen, from the atmosphere, on which the fungi also get to feed.

This combination has proven so successful that lichens have succeeded in colonizing places on the planet where nothing else lives, as well as competing for space in more hospitable environments. Many fungi species can only partner with a single photosynthesizer, making finding each other an impressive feat.

Yet Spribille has shown this only scratches the surface.

Spribille was alerted to the possibility from observations of two types of lichen, Bryoria fremontii and B. tortuosa. The vulpinic acid tortuosa produces is toxic to mammals, while fremontii is edible. They can be told apart by their respective brown and yellow colors, yet genetic tests show no consistent differences.

^{Bryoria fremontii, known as Wila, is a popular food in some areas. Millifolium - Own work, CC BY-SA 3.0} 

When Spribille ground up lichen samples and extracted the RNA he found evidence of two fungal species in each sample. On closer exploration, the second fungus was revealed as a previously unknown species of yeast. Although the yeast exists in fremontii, it is far more common in tortuosa, producing enough vulpinic acid to both change the color and make the plant toxic to grazing animals.

Spribille teamed up with other researchers and found relatives of this yeast thrive in lichen colonies worldwide. "It's everywhere," said co-author Dr John McCutcheon. "This thing has basically been hidden in plain sight for more than 100 years. People were probably looking right at it, and they thought they knew what they were seeing, but they were actually seeing something else."

Some researchers have hypothesized a third component to lichen, but had not anticipated a yeast as the missing ingredient.