It looks like a single species, was thought to be two, but turns out to be a three-way symbiosis. ason Hollinger - Mushroom Observer, CC BY-SA 3.0

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.

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