Living Lichen

I dare you to look down. Not down at your screen but further, to where a bundle of color blooms flat on a rock. Or on a tree, or paving stone. Welcome to the world of lichen.

Here, several millimeters over a barren surface, something thrives. Lichen is not a blink-and-you’ll-miss-it kind of thing. Lichen is not any one thing at all. Unlike mosses, which are an ancient phylum of plants, lichens don’t fit neatly into any category we have for life forms because lichen is not one thing but instead a symbiotic interaction.

Lichen encompasses the relationship between a fungus and a photosynthetic roommate called a photobiont, which can be either algae or cyanobacteria or both. The thallus structure that you see is a fungal frame encompassing a layer of algae. The pair are engaged in an obligate mutualism: they quite literally can’t live without one another. The photobiont gets protection, covered by the fungal mass, and the fungus gets the carbon product of photosynthesis. The result: this pair, acting as one, can survive.

On every continent, lichen makes a life for itself needing just a stable surface. Creeping along rock faces at a millimeter a year, large lichen colonies you encounter are quite likely to be older than you. Lichens vary greatly depending on the species of the fungus and the species of the photobiont, the pair hanging in spirals from trees or laminating itself along a rock face. There are three basic forms of lichen: crustose, foliose, and fruticose. Crustose lichens grow in a thin, paint-like layer barely above the surface where foliose lichens grow three-dimensional leaf-like folds. Fruticose lichens are raised and shrubby and are the most sensitive to pollutants.

Lichen is a pioneer species, the first to settle an inhospitable environment, dissolving tiny crevices into rock faces and enriching substrate for future neighbors. Lichen doesn’t depend on the soil for nutrients, instead getting them straight from the atmosphere. This absorbency is a strength in clean air but a fatal weakness in the presence of pollutants. The strategy that thrived for millions of years of evolution put some lichens on the brink today. Die-offs of fragile varieties of lichens correspond to lead in the environment or soot in the air. Cities started to become lichen deserts, losing the miraculous urban resident before most people even noticed it was there. Where air pollution has been mitigated, slowly but surely returning are the lichens, a glimmer of hope, a canary returning to the coal mine.

Lichen endures drought via acceptance. Lichen, in its infinite patience, waits for rain. And when the rain comes, the paper-thin upper cortex of fungus goes translucent. With the fungal color out of the way, lichens that all appeared the same drag greyish-green become an explosion of turquoise and orange and emerald, revealing distinct phytogroups hidden just below the surface.

Symbiosis is all around us, globally and internally. In your gut at this moment are roughly 38 trillion microbes. The mutualisms we recognize most are removed from us: the coral-algae relationship is conducted underwater and mycorrhizal root-fungal networks interact underground. Lichen is on our level. Growing underfoot and across our buildings, lichen has been waiting to be noticed. Lichen urges us to be patient, to take in what nutrients we can, and most importantly, to depend on the relationships that sustain us.

The world is symbiotic.

Lichens are just visible…

but only if you pay attention.

All photos and illustrations are by author, Ava Kocher.

Special thanks to lichenologist Dr. Francois Lutzoni for sharing his endless insight into the world of lichens.

Rachel Carson National Environmental Leadership Fellow Ava Kocher

Ava Kocher is an undergraduate student at Duke University studying Biology and Marine Biology. From Massachusetts, Ava’s childhood spent exploring the woods and Charles River sparked her passion for environmental stewardship. She channels this through taking care of animals and ecosystems. At Duke, she works at the Duke Lemur Center and does research on sea urchins under climate stress. She also leads trips in the outdoors to foster connections between Duke students and the natural world.