Search

Rewilding Route 1: Likin’ lichens

Posted on: January 28, 2025

BY RICK BORCHELT

Let’s imagine a fungus and an alga, deep in the recesses of time, millions of years ago, sizing each other up in an inhospitable pile of rocks along an ancient sea until one of them says something like Humphrey Bogart’s immortal “Casablanca” line, “I think this is the beginning of a beautiful friendship.”

The wispy strands of old man’s beard lichens (Usnea), shown here with foliose lichens, often find their way into bird nests.
Courtesy of Rick Borchelt

The partners in this case weren’t pairing up to run an undercover gambling enterprise, like Bogart’s character Rick Blaine was hinting at, but to create an entirely new life form — one we know today as a lichen.

Late winter, with the leaves gone, is a great time to look in local woods, on rocks or soil, or on branches blown down in ice or snow for these remarkably diverse organisms. Many are brightly colored, some are twisted into fantastical shapes, and still others form dinner-plate-sized shields on bark or rock.  

The red tops of the fruticose lichens called British soldiers (Cladonia) are often mistaken for small wildflowers.
Courtesy of EPA

How this partnership originally came about is still shrouded in mystery, but it’s been a successful one: Today, there are more than 20,000 lichen species worldwide, and they occur in practically every habitat on the planet. Indeed, by some estimates, lichens occupy about 7%  of Earth’s surface.

What we do know is that lichens are relative newcomers to the photosynthesis game, the process of turning sunlight into food energy and producing oxygen in the process. Algae and cyanobacteria, not a true bacteria at all, were the first photosynthesizers, living mostly in or near water. Fungi diverged from these ancient organisms about 1.5 billion years ago; as they did, they lost the ability to photosynthesize. Lacking that energy-producing process, they became either parasites of living plants or saprophytes, organisms which feed on decaying plant and animal matter. Early fungi were probably aquatic.

By about 400 million years ago, things we would recognize today as plants had begun to evolve, with obvious roots and leaves. Ferns, mosses, and other spore-bearing plants dominated the land, and fungi were also common. It wasn’t until about 250 million years ago that that secret handshake between fungi and algae gave us our first lichens. If not embarking on a gambling venture, what did these partners stand to gain from their relationship?

For the fungus partner, or mycobiont component of the lichen, it’s an easy answer: It gains food energy from photosynthesis, allowing it to reproduce and colonize areas of bare rock, soil, bark or other substrates where ready-made food wasn’t available. For the alga, or photobiont component of the lichen, the tough “skin” of fungi kept it moist and allowed it to exploit those same inhospitable, dry environments.  

Today’s lichens are layered, with fungus scattered throughout. There’s usually a hard upper surface (the cortex) of very dense, fibrous fungal cells, covering a layer of softer fungus cells. The lower surface has a similar layer of soft fungus cells and usually lacks the hard cortex. Sandwiched in between these two mycobiont layers are loose strands of fungus intermeshed with photosynthesizing algae. The fungi on the surface layers absorb water and some nutrients, including nitrogen, directly from the environment around them, and funnel them down to the photobiont layer.

In 1877, the German scientist Albert Bernhard Frank coined the term symbiosis to describe the relationship between fungi and algae; it comes straight from the ancient Greek word meaning living together or companion. The scientific community adopted the term to describe close, long-term biological interactions between two or more species.  

Some birds, like this blue-gray gnatcatcher (Polioptila caerulea), camouflage their nests with lichens.
Courtesy of Daniel Schlaepfer

In recent years, researchers have discovered that lichens may have other partners in their relationships, too. A single lichen, for example, might have three or even more kinds of fungus growing as part of the same organism. The photobiont layer might also include cyanobacteria (sometimes called blue-green algae, although they are neither algae nor always blue-green), regular bacteria or yeasts. The complexity of these villages, with very distantly related community members, is one of the reasons why it’s so difficult to grow lichens in the laboratory — or get them to grow where you want them in the garden. 

In our area, we have three major physical types of lichens: foliose, crustose and fruticose. Foliose lichens, which most often grow on bark or twigs, are what most people think of when they say the word lichen. They typically resemble leafy lettuce, although some closely resemble wispy strands of hair. Crustose lichens are flat and closely appressed to rocks or brick or other hard surfaces, and can be dull gray or white or parti-colored reds, yellows, and oranges. In the arid Southwest, entire canyon walls and rock faces can be painted with a kaleidoscope of crustose lichens. Fruticose lichens are usually found on soil and look like miniature shrubs, often with a brightly colored top that holds reproductive cells. 

A selection of foliose lichens on branches in Laurel.
Courtesy of Rick Borchelt

Lichens play a vital role in Earth’s ecosystems, especially after landslides, floods, volcanic eruptions or other geologic activities expose raw rock and soil. Lichens are among the very first organisms to colonize this new territory. Over centuries the lichens trap blowing dust and detritus — and decay themselves — to create the first soil for plants to then move in.  

Crustose lichens, here surrounded by a few green mosses, are most often found on smooth surfaces like stone and brick.
Courtesy of Rick Borchelt

Some lichens are of more utilitarian use. Because they absorb air pollutants, they can serve as effective biological pollution monitors. Lichen collections from Plummers Island in the Potomac, for example, show dramatic increases in lead pollution from the early 1900s to the present day, peaking during the early 1960s, when the American Legion Bridge was built adjacent to the island. Lichen lead levels have dropped considerably on Plummers Island since lead was phased out of gasoline.

Early spring is a good time to look for lichens on branches that have fallen during winter storms.
Courtesy of Rick Borchelt

But many lichen species are also very sensitive to some of the pollutants they absorb. Most of the lichen species that used to be on Plummers Island have died out, victims of other kinds of urban air toxins. While the ancient partnership between fungus and algae has been strong for hundreds of millions of years, human alteration of the environment — for lichens as for many other living organisms — might be too rapid and too drastic even for these tough, complex communities to survive. 

______________________________

Have questions for Rick about the world of nature in and around the Maryland suburbs or suggestions for future columns? Drop him a note at rborchelt@gmail.com.

Share:

Facebook
Threads
Twitter

The Streetcar Suburbs Spotlight

Local news and events straight to your inbox

Free! Cancel anytime.

Have a tip?

Send us tips/photos/videos

Related Posts

By AGNES PASCO CONATY Nature plays such a vital role in my life, and I so often write about our natural world, right here at...

BY RICK BORCHELT Our unseasonably warm December weather brought the first waves of what will be many robin reports in the local listservs, blogs, Facebook...

By RICK BORCHELT “Not a creature was stirring, not even a mouse.” Wherever he was when he wrote his famous poem, “A Visit From St....

Get Local News in Your Inbox

Sign up here for the Streetcar Suburbs Newsletter!
 
Loading...
';