Lichens come in some fabulous colors. They decorate our rocks and trees and in some places they dominate their substrate so heavily that we don’t even see the rock underneath. Only seeing the lichens. The wonderful flatirons of Boulder are a great example. You can see the Pleopsidium flavum from miles away1! Depending on who you ask, lichens cover between 5-8% of the Earth’s land surface2.
The brilliant reds and oranges of the family Teloschistaceae are particularly stunning. The red color comes from a series of different compounds called anthraquinones, in part. The color is so distinctive to the family that is it nearly diagnostic (don’t worry, lichen lovers, exceptions abound).
One fun thing about this group of chemicals is that they turn a deep purple when exposed to potassium hydroxide, called a K test if you want to look cool around other lichen enthusiasts.
Another important lichen color to learn is a rather unique yellow/green (left). This color comes from usnic acid. It is a very common lichen substance. The term lichen substance is a very unimaginative term used for the unique chemical compounds that lichens produce. Lichen substances are very important in taxonomy and often identification.
Lichens in the family Physciaceae are a wonderful blue grey color (below) that is a great test of your photography skills. I, at least, never get the color to show up true.
Once you are done ogling over some lovely lichen thalli, ponder a simple question. Why are lichens colorful? Lichens live an odd life. They grow extremely slowly and never move, just sitting there soaking in the sun, while their photobiont friends make them sugars.
The other photosynthetic organisms lichen hunters encounter frequently are a good start for what color a lichen should be. I’m talking about plants (hardcore lichen finders only acknowledge plants as a potential substrate for lichens to grow on; skip this paragraph if it’s upsetting). Plants are green because they want their photosynthetic cells to get as much light as possible and the chlorophyll they use is a bright green. In other words, no other colors are in the way of absorbing precious light. Our good friend Peltigera aphthosa (right) is green (when wet) for a similar reason3. But it is by far the exception; most other lichens need to be cut into to reveal the photobiont. This means the lichen is sacrificing optimal light penetration.
So why are lichens so beautiful? The resources I own spend very little time on the subject. In Brodo’s Lichens of North America we only get two pages. One possibility is that bright colors are protective against ultraviolet radiation. This appears to be very true with our Rusavskia elegans (I promise a lichens in space post soon and, yes, its related). White colors can scatter light, which is protective for species that are in exposed habitat. The lichens can’t afford for their photobiont friends to bake or they won’t live long.
I hope to come back to this question in a new series of posts. Get ready to break out your old biology textbooks, and, far more frighteningly, maybe even a little organic chemistry.
1 Okay, some squinting required, but if you see any lemon yellow up there it’s very likely this species.
2 Asplund, J., & Wardle, D. A. (2016). How lichens impact on terrestrial community and Ecosystem Properties. Biological Reviews, 92(3), 1720–1738. https://doi.org/10.1111/brv.12305
3 While I couldn’t specifically find an answer, the photobiont in Peltigera aphthosa is a green algae in the genus Coccomyxa. Like plants it uses chlorophyll a, but may have different concentrations of other chlorophylls. There are three CO Peltigera that share this character, Peltigera venosa, leucophlebia, and aphthosa. The remaining Peltigera have Nostoc, a blue-green cyanobacteria, as a photobiont. They also are bright green only when wet and brown/green when dry. I’d love to try and figure out why this rather major difference does not put the green algae species in their own genus. Check out this post for some more information about Petigera leucophlebia.