Lichen Colors

Xanthomendoza trachyphillaBrowns Park NWR, Moffat County, CO.

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.

Usnea hirta Lyons, CO

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.

Physcia sp. – Lyons, CO

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.

Peltigera aphthosa – Boulder, CO

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. I’ll also take this time to state that most Peltigera have Nostoc, a blue-green cyanobacteria, as a photobiont. They also are bright green only when wet and brown/green when dry.

A Surprise from the West Slope

Rhizoplaca novomexicana – Slick Rock, San Miguel County, CO. On sandstone.

Unfortunately it’s been a bit since my last post, but fortunately some of that time was spent staring intently at rocks, trees and dirt on the west slope of Colorado. Since I had not spent much time at all in that part of the state after my infatuation with lichens began, I did not know what to expect. What I certainly did not expect was for Rhizoplaca novomexicana to be by far the most conspicuous lichen in my travels.

Called New Mexico Rim-Lichen by some authors, there are specimen records for most of the mountain west from the Mexico border just reaching Canada. In Colorado, it can be found just about anywhere including the plains and the high tundra, rocks required. Along the Front Range it is rather local on sunny rocks and more common above timberline. But judging from four days spent near the Utah border it can be found anywhere out west, I found some just about anywhere I looked, especially on sandstone. As a taxonomic note this species was long in the huge Lecanora genus, that is before modern taxonomists carved it up into a dozen or so new genera. Within the genus Rhizoplaca it stands out by being truly crustose. It’s close relative Rhizoplaca chrysoleuca is attached at single point, like a very short tree trunk, or umbilicate in lichen terms.

Rhizoplaca novomexicana – Near Lyons, CO.

In identifying this species, narrow down your search criteria first by a high level description. It’s crustose, it has lobate margins, and it has plenty of apothecia. This lichen can be sterile but usually a fertile specimen is nearby. Next up note the rather unique yellow-green color of the overall thallus. Also a nice clue, is that this species is often missing older parts of the center of the thallus as in the lichen to the right.

The key I first used to identify this species, Brodo’s Lichens of North America, makes a big point of the shape of the margins. They are distinctly raised vs flat and instead of overlapping form tiny winding canyons. This is also a important characteristic of the most common lobate crust along the front range, Protoparmeliopsis garovaglii. To close out the ID check out the color of the apothecia. They should nearly black to a kind of bluish grey; there always seems to be a hint of cobalt color if you look carefully enough. Often, but not always, the apothecia are dusted in a yellow pruina like the individual below.

Rhizoplaca novomexicana – Near Lyons, CO

I am excited to share some of the other tales from exploring the dusty canyons of western Colorado. So hopefully a quick introduction to a new fungus whetted your appetite.