For over 140 years it was believed that lichen was the result of symbiosis between algae and a single fungus. A new study has discovered that a third component, a particular type of yeast, is involved in the formation of lichen.

There is a kind of lichen that grows as a thin layer on trees and rocks, and another one that is larger, called “macrolichen,” which grows leaves and branches. A group of Canadian researchers led by Toby Spribille found in samples of the latter from six different continents the presence of yeast, a fungus called a Basidiomycete in the outer layer. “These yeasts are sort of hidden just below the surface,” said John McCutcheon, a research fellow at the Canadian Institute for Advanced Research and co-author of the study.

A lichen is a composite organism that arises from algae or cyanobacteria (or both) living among filaments of a fungus in a symbiotic relationship. Credit: Lichen Portal/Wikipedia

With fungus providing the structure and algae producing the food, lichen was seen as the perfect symbiotic relationship. According to the findings of the new study, the presence of basidiomycete yeasts provides, even more, support and changes any previous conceptions and assumptions about lichens. The yeasts have been found in 56 different species of lichen from all around the world.

The yeast was discovered unintentionally

Toby Spribille, the researcher from the University of Montana who led the study, found the yeast while studying two types of lichen. He was trying to find out why the Bryoria tortuosa and Bryoria fremontii were so different.

While both types of lichen are composed of the same fungus and algae, the B. tortuosa is yellow because of the presence of vulpinic acid, the B. fremontii is brown and has no toxic acid. Spribille was focusing on the genetic structures of the algae and remained confused after a genetic analysis showed that they were genetically identical in both types.

He contacted John McCutcheon, a microbiologist at the University of Montana to apply more sophisticated genetic techniques to try to solve the mystery. McCutcheon then found the other fungus. “The yeasts were always there, and somehow we got to crack it open. It gives me goose bumps,” Spribille shared in a statement published in EurekAlert.

“These yeasts comprise a whole lineage that no one knew existed, and yet they are in a variety of lichens on every continent as a third symbiotic partner,” M. Catherine Aime, co-author of the study, said in the statement.

The yeasts grow over the other fungus and the algae, suggesting scientists that they provide an additional support of the organic structure. For example, the basidiomycete yeasts may protect the B. tortuosa lichen from its acid. After studying over 50 samples from different locations, researchers found a distinct variety of the yeast in each one of them.

The new findings helped scientists understand why it is so difficult to grow macro lichens in the laboratory. While they thought they only needed fungus and algae, they were missing the yeast as a third component.

There is still a lot of research to do around the newly found component, including how many of the lichens species have it, and what its function may be. The discovery broke years of belief and shocked scientists.

The paper was published Friday in Science.

Source: The Washington Post