Painted Rocks: Yellow-Green

     In this post I would like to give some attention to two of the more salient yellow or yellow-green lichens that populate rocks in Colorado. As with the orange lichens that live here, the yellow lichens are fun to learn to identify because of their brilliant coloration that sets them apart from their rocky substrate, as well as their relative ease of identification even without magnification. The two species I have in mind are Pleopsidium flavum and Candelariella rosulans. As always with these lichen posts, feel free to bone up on your lichen terminology from this previous introductory post, as the language used when describing lichens can be quite unfamiliar. 

Pleopsidium flavum, honorable mention to Pleopsidium chlorophanum

     When I first took an interest in lichens, I noticed that many of the rocks around the foothills were not just red, gray, or brown as I might expect, but actually the chartreuse color of a glow stick or perhaps the radioactive goo from the Teenage Mutant Ninja Turtles cartoons. The organism responsible for this unexpected color shift is Pleopsidium flavum, a lichen that can grow to cover whole cliffs and which to my knowledge has no colloquial name. 

Pike's Peak granite providing a home to P. flavum and a Xanthoria species (orange). P. flavum's rosetted habit can be broken up by a bumpy substrate. I mostly included this photo as I felt it did credit to the intensity of this lichen's coloration.

     Confusingly, the only field guide-like treatment of Colorado lichens lists this species as P. chlorophanum(1). However a newer, more thorough review by Nash, Ryan, Gries, & Bungartz (2001)(2) maintains that while P. chlorophanum is assumed to occur at high altitudes in North America, it is quite rare (only one collection has been made in North America), and most Pleopsidium species that one encounters at lower altitudes are P. flavum(3). P. chlorophanum is a fascinating extremophilic lichen which thrives in Antarctica and has been shown to be able to actively photosynthesize and grow in simulated Martian conditions(4). The implications of this seem obvious:

Food for thought. The genus Pleopsidium could really use an Oscar win, anyhow.

However, until I get an angry missive from William A. Weber (author of A Colorado Lichen Primer) putting me straight, I am operating on the assumption that I have only ever seen the less researched but much more common P. flavum here in CO. P. flavum has a shiny, yellow-green thallus which prefers more acidic rock surfaces such as granite. Its areoles (distinct islands of thallus tissue) are around .5-1 mm in diameter, while the lobes around the thallus' margin are 1.5 to 2.3 mm across(2). Apothecia are the same color as the thallus, sunken, and >1 mm across. These measurements may be helpful for differentiating it from the highly similar aforementioned P. chlorophanum (if you manage to find it), but the only field mark you really need to identify this lichen is its intense coloration. No other lichen this color in our region is really shaped like P. flavum, so a mistaken identification is unlikely. Another oddity that is specific to P. flavum is its preference for growing on vertical surfaces, eschewing similar conditions on flatter rocks(1). The specific epithet flavum simply means "yellow," as apparently Latin lacks an adjective for "violently chartreuse."

P. flavum growing in a rosette on granite in the foothills near Boulder, CO. This photo provides a good reference for the relative sizes lobes, areoles, and apothecia. Apothecia are subtly visible towards the center of the thallus as circular darker patches ringed with lighter thallus tissue. 

The chartreuse coloration is due to the presence of rhizocarpic acid, which also lends its color to species such as Acarospora schleicheri and the ever-popular Rhizocarpon geographicum. Rhizocarpic acid is an interesting chemical in its own right, as it serves to absorb destructive UVA and UVB (ultraviolet) radiation, then re-radiate it on a spectrum usable for photosynthesis by P. flavum's photosymbiont, the alga Trebouxia(5). In this way, rhizocarpic acid serves to both shelter and nurture the photosymbiont needed for P. flavum to thrive and grow. This is a beautiful illustration of the symbiotic co-evolution of lichenized fungi and their photosynthetic partners, as rhizocarpic acid and other secondary metabolites in lichens are produced by the fungus, not the alga(6).

Large expanses of vertical stone can become overrun with P. flavum, which in this photo is sharing volcanic basalt with a Lecanora lichen, possibly Lecanora garovaglii. This photo was taken on the aptly-named Lichen Summit of North Table Mountain near Golden, which has a well-established population of P. flavum.

Candelariella rosulans and Friends 

Candelariella ssp. on granite in the foothills near Boulder, where C. rosulans was discovered. The "roses" of the apothecia are clearly visible but tiny even in this macro photo (click to enlarge). 

     There are a number of egg yolk-colored, crustose, tiny lichens that inhabit the rocks of Colorado at virtually every altitude. These are the Candelariellas, and they pepper communities of larger crustose lichens with their unmistakable color. C. rosulans is noteworthy because it was discovered in the foothills immediately outside of Boulder(1). It is the only Candelariella species that Corbridge and Weber bothered to highlight in their field guide, making it a sort of flagship for this genus in our area. The specific epithet means "little rose(7)" and likely refers to the apothecia, which have a crenulated, rose-like aesthetic when viewed under magnification. So too do all other rock-dwelling Candelariella species in our region, of which there are at least five(8), which also tend to have exactly the same coloration, preference for acidic rocks in sun, similarly sized squamules/areoles up to 1mm across, and similar chemistry (although not identical) as delineated by spot tests(2). The chemical that colors all Candelariellas the same bright yellow is calycin, which serves the same photoprotective and nurturing role as rhizocarpic acid does for P. flavum(5). Calycin-bearing lichens have also been used to dye wool yellow in Sweden(9), although I do not know if they have been used for this purpose in America. 

     Put simply, there is no good way to differentiate the species of this genus in the field that I am aware of, although microscopes and chemical testing can evidently parse them out. I find them rewarding to be able ID as a genus, however, as they are a distinctive, attractive, and relatively easy to parse down to the genus level. Nothing else in our region really looks like the Candelariellas. A truly intrepid amateur lichenologist who wishes to master this genus in spite of these challenges may find a list of Candelariella species in Colorado here. The lichens in the photos below may well all be C. rosulans simply because I took many of these pictures relatively close to the Boulder foothills. 

Candelariella ssp. growing on sandstone near Fort Collins, cohabitating with a handsome Caloplaca trachyphylla (orange at left). I have yet to find a crustose lichen community on stone that has not had some sort of Candelariella hiding between its larger lichen neighbors. Due to their tiny size, many Candelariellas are only apparent upon very close inspection, or even when examining macro photos after the fact. 

A tiny Candelariella lichen ekes out an existence between crystals of quartz at around 12,000 ft. elevation on Mount Evans. None of the quartz crystals in this photo are larger than .5 cm. 

     Thank you for taking the time to learn a little bit about yellow and yellow-green lichens! I find these a joy to behold when hiking as they contrast strongly with their surroundings and remind me that a complex ecological system is always humming along right next to me. 

References 

1. Corbridge, J. N., & Weber, W. A. (1998). A Colorado Lichen Primer. Niwot, CO: University Press of Colorado.
2. Nash, T.H., Ryan, B.D., Gries, C., & Bungartz, F.(eds.)( 2001). Lichen Flora of the Greater Sonoran Desert Region. Vol 2. Tempe, AZ. Retrieved from http://nhc.asu.edu/lherbarium/flora/index.php
3. Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2001. Lichen Flora of the Greater Sonoran Desert Region. Vol 3. Tempe, AZ. Retrieved from http://lichenportal.org/portal/taxa/index.php?taxauthid=1&taxon=53030&cl=1202
4. de Vera, J., Schulze-Makuch, D., Khan, A., Lorek, A., Koncz, A., Möhlmann, D., & Spohn, T. (2014). Adaptation of an Antarctic lichen to Martian niche conditions can occur within 34 days. Planetary & Space Science, 98182-190. doi:10.1016/j.pss.2013.07.014
5. Hidalgo, M. E., Fernández, E., Ponce, M., Rubio, C., & Quilhot, W. (2002). Photophysical, photochemical, and thermodynamic properties of shikimic acid derivatives: calycin and rhizocarpic acid (lichens). Journal Of Photochemistry And Photobiology. B, Biology, 66(3), 213-217.
6. Molnár, K., & Farkas, E. (2010). Current results on biological activities of lichen secondary metabolites: a review.
7. Numen, the Latin Lexicon (n.d.) Rosulans [entry in online dictionary]. Retrieved from http://latinlexicon.org/definition.php?p1=2051833
8. Consortium of North American Lichen Herbaria. (n.d.). Colorado [list in online database]. Retrieved from http://lichenportal.org/portal/checklists/checklist.php?cl=1202&pid=510  
9. Ethnolichenology of the World. (n.d.). Candelariella vitellina. Retrieved from http://web.uvic.ca/~stucraw/part2AM.html