Lichens: ancient partners with modern significance

By Gary Raham
North Forty News

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As a young teacher, new to Colorado, I built rock gardens and even a fireplace out of rock covered with lichens. A freshly minted biologist, I knew that lichens represented a unique union of two kingdoms of living things: algae (pond scum to some) and fungi (think bread mold and mushrooms). That unusual marriage fascinated me.

My very first published article, "The Lichen Liaison," appeared in The American Biology Teacher in 1978 and described various ways teachers could highlight this relationship for students.

In the last 32 years, scientists have learned to appreciate lichens even more. They represent a 600 million-year-old association that underpins planetary ecology in significant ways. And if a curious person knows how to "read" lichens properly, he or she can use lichens for everything from telling time to measuring the health of the world's diverse living systems.

Some scientists, tongues firmly cheek-bound, describe lichens as fungi that have discovered agriculture. Fungi can't make their own food like green plants do. Molds, mildews and mushrooms serve as the planet's clean-up crews, digesting away garbage, excrement and the last mortal remains of...pretty much everything. But lichen fungi learned how to surround and house single-celled algae and bacteria possessing the ability to create sugars from sunlight and carbon dioxide.

In the process, something new sprang into being: lichens. Their 20,000-plus species latch onto rocks as patches and dribbles of intense color, cover soils with a water-saving and nutritious crust, and hang from tree limbs like stiff, green hair. They clothe bare rocks with rainbow hues of life – but they do it very slowly, often growing just a fraction of an inch per year.

Timekeepers

Currently, I'm helping to map an old dinosaur quarry west of Denver. One of the techniques they might employ to confirm the age of certain blocks of apparently quarried sandstone is lichenometry – timekeeping by lichens.

Scientists have focused on brilliant yellow lichens framed in black called map lichens (Rhizocarpon geographicum) for this purpose. They grow at a rate of about 0.2 inch per year on silicate rocks. I didn't see this particular species on site, but other leaf-like species might work as well.

Typically, lichens have been used to date the rate of retreat of glaciers, measure how frequently earthquakes occur in certain areas, and date the age of manmade monuments, like the famous stone sculptures of Easter Island. (Sculptors may have crafted the last head about 400 years ago.)

Rock destroyers

Lichens can literally break rocks apart, slowly converting them to soil, but in some cases they may actually protect surfaces from weathering by frost, wind and air pollution.

Slender fungal filaments called hyphae insinuate themselves into minute cracks. The turgor pressure created as those hyphae expand with water can be immense. Also, lichens produce unique acids and compounds called depsides that form metal complexes. The actions of lichen substances mine rocks for minerals the lichen needs, but they also contribute to their dissolution.

However, lichens that colonize monuments and building stones don't appear to cause significant damage – at least within human lifetimes. They add a certain rustic charm. If you spray dilute manure on surfaces it's possible to encourage the growth of bright yellow Xanthoria spp.

Behind the scenes

Lichens perform underappreciated roles in a variety of habitats. Vegetation dominated by lichens covers nearly 8 percent of the Earth, largely in the Arctic tundra. The lichens suck carbon dioxide from the air, thus serving as carbon sinks and playing an important role in climate control. They provide food for a sizable suite of creatures ranging from invertebrates to caribou and deer. When they decay, they enrich the soil with nitrogen (anywhere between 2.2 and 88 pounds of nitrogen per hectare per year).

Lichens survive nearly everywhere except the deep sea. They cling to rocks below tide level and at the tops of the tallest mountains. They colonize trees, bare ground, mosses, other lichens and occasionally the leaves of flowering plants. In the Antarctic, they nestle within rocks beneath lenses of quartz where they can harvest sunlight and yet stay relatively warm and sheltered.

Writer Gordon Grice, while admiring the scenery in Arkansas, had an epiphany about lichens: "I thought I was taking in the subtle hues of the cliffs along the White River, but I was really admiring the layer of life covering them. Lichens stipple landscapes of granite, blanket the ground like snow, and drip from tree branches as if they were tinsel."

Whistle blowers

Lichens can display a pioneering hardiness coupled with surprising vulnerability. They shrivel quickly under the assault of pollutants released by industry, because lichens tend to concentrate substances from their environments. This trait, coupled with their ubiquity on a global scale, their longevity and perennial growth habit, makes them excellent biomonitors – canaries in the minefields of polluted urban smog.

Air pollution monitoring by lichens has become mandatory in some European countries such as Germany, Italy and Switzerland. The health of lichens in the Arctic can even be tracked by Landsat technology (photography by satellite) because usnic acid, unique to lichens, absorbs and transmits light differently than other plants. Scientists have tracked declines in lichens due to increased emissions of sulfur dioxide and other pollutants.

Yet the lichen partnership of fungus and algae most likely helped crumble the bare rocks of earth into the first soils. They painted the earth with the first welcome mats of terrestrial life and will likely be the creatures left behind to write "adiós" on the door when the party is over. Don't look past them as you admire the mountain views.