The unusual biology of lichens (12/3/2007)

Imke Schmitt is investigating the chemical compounds that give lichens such as this one their bright orange, red, or yellow colors. - Photo Credit: Imke Schmitt

Lichens are extraordinary organisms, at once commonplace and exotic. They grow all around us on ordinary trees and rocks and stone walls. But as symbionts that can survive the Earth's most desolate frontiers, they also seem the stuff of science fiction. In reality, lichens are complex organisms comprising a fungus and an alga living in symbiosis.

For biologist Imke Schmitt, the Bell Museum's new curator of lichens, these peculiar organisms have long been a source of fascination and a subject of study. Schmitt, who comes to the Bell Museum from the Field Museum in Chicago, has worked for many years on an international project aimed at finding out how different groups of lichens are related to each other. Using DNA samples, she has reconstructed the evolutionary history of several groups of lichens and is now researching the Pertusariales, a group with worldwide distribution.

"Lichens are not really a different group of organisms, but they are fungi that get their nutrients from their algal partners," explains Schmitt. Scientists refer to lichens as "lichenized fungi," and classify them according to the fungal partner, which gives the lichen its distinct form--crusty or leafy or shrub-like.

The fungi require a symbiont--an alga or, in some cases, a cyanobacterium--to survive. Algae can perform photosynthesis, and provide the fungal partner with essential nutrients. What the algae receive in return is less clear. It's likely that the symbiosis allows both partners to occupy a broader range of environments, but only the algae are capable of living on their own. Scientists have tried, with limited success, to replicate this complex symbiosis in the lab.

Most lichens cannot be cultivated; they exist only in nature. But they exist everywhere in nature. Lichens are most abundant in the tropics, but they can eke out a living even in the least hospitable conditions on Earth: the McMurdo dry valleys of Antarctica, where extreme cold temperatures and lack of precipitation are thought to resemble conditions on Mars.

"Lichens are better adapted than most other organisms to the harshest places," says Schmitt. Their survival skills include the ability to go dormant for decades or more when under stress.

Because they require no soil for nutrition, have no roots, and glean minerals and moisture from dust and rain, lichens can thrive where most other plants cannot. They are considered pioneers, among the first organisms to colonize new territories such as volcanic islands.

Though their geographic expansion is unsurpassed, individual lichens grow extremely slowly--some species by only a millimeter a year. "Historic photos of Arctic expeditions show individual lichens that can still be found today," says Schmitt. "They're easily identifiable, because their forms have changed so little over time."

Having helped to establish a solid family tree for fungi, Schmitt is now turning her attention to the evolution of certain traits in lichens--in particular, their unique chemistry. Scientists have identified more than 1,000 chemical compounds produced by lichens. "Some of these molecules haven't been found in any other organism," says Schmitt.

The compounds serve many functions. Some produce the brilliant red, orange, and yellow hues found in lichens. Others are thought to aid in symbiosis, repel water, or function as a sunscreen. Some produce a bitter taste that may repel grazing animals. And others might prove useful to humans for their antibiotic properties.

Lichens have long been used to fight infection, but because they can't be cultivated, it is currently impossible to obtain large amounts of lichen compounds for testing and research. Schmitt, however, is working to identify the genes that produce various compounds, including those that may have antibiotic activity.

"If you know the genes," she says, "you could eventually put them into bacteria to produce a large amount of the compound, which may be useful in medicine."

New lichen samples must, as always, be obtained from nature, and in her new position at the Bell Museum Schmitt will continue to collect specimens and build the world-class lichenized fungi collection established by her predecessor, Cliff Wetmore.

"Lichens are difficult to study," Schmitt says, "but they have such an interesting biology and there is so much to be learned."

Note: This story has been adapted from a news release issued by the University of Minnesota

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