Painted Rocks: The Orange Lichens

     One of the more striking experiences with lichens anyone is likely to have in Colorado is encountering splashes of DayGlo orange or chartreuse on an otherwise brown, black, and green lichenized rock face. This post explores tricks for identifying down to the genus level what species of shockingly orange lichen you may have encountered, and some interesting facts about them as well. The genera of interest are Xanthoria/Xanthomendoza and Caloplaca. 

Xanthoria/Xanthomendoza (orange) species, possibly X. elegans, growing on Pike's Peak granite. The violent coloration can be missed due to the small size of the thalli: the larger crystals in this picture are not more than 1cm across. The brilliantly yellow-green lichen is Pleopsidium flavum, which I will look at in a future post.

Xanthoria/Xanthomendoza vs. Caloplaca 

     Orange lichens that you encounter in Colorado belong one of two (now three) genera. The genera of Xanthoria/Xanthomendoza discussed below represent eight species, while Caloplaca weighs in at a whopping 54 species. Not all species in these genera are orange, but all orange species in Colorado belong to these genera, and all of them owe their orange coloration to the same remarkable pigment: parietin(1). Parietin functions to protect the lichen from UV radiation(2) and to chemically beat down competing microorganisms on the rock surface(3). It has also demonstrated some weak anti-cancer properties in being able to induce apoptosis in certain cell lines, but not as well as some other lichen secondary metabolites(4). Parietin holds its color well after the death of the lichen, and some people use it for dying(5). From a lichen identification perspective, a spot test using a solution of potassium hydroxide will yield a brilliant purple color in the presence of parietin(1). This doesn't help to differentiate between any of Colorado's orange genera as they all have the same primary pigment, but it can be handy for other genera. A good photo of a KOH spot-test can be found here (scroll down for image). 

     Note that I have been separating Xanthoria and Xanthomendoza genera from each other. All Xanthomendozae used to be classified under Xanthoria, until it was noticed that the two bear different conidia, and that those which became Xanthomendoza have either rhizenes or other holdfasts while Xanthoria do not(6). Other than those differences, the two genera look essentially the same. 

     If color is of no use, how then to differentiate Xanthoria/Xanthomendoza ssp. from Caloplaca ssp.? All three genera belong to the same family, Teloschistaceae(7). All three genera have many species which tend to be orange, and may grow on rocks or on bark. The species with orange thalli all have rounded, cup-shaped lecanorine apothecia(6,8). The best feature to distinguish between Caloplaca and Xanthoria/Xanthomendoza ssp. appears to be the habit of the thallus: Xanthoria and Xanthomendoza ssp. tend to have foliose thalli(9), while Caloplaca ssp. tend to be crustose or occasionally fruticose(9,8). A wikipedia page which does not cite its source for the information also notes that Xanthoria ssp. may be squamulose(9). 

     In practice this means that an orange lichen which can be cleanly separated from its substrate with a knife blade probably belongs to Xanthoria or Xanthomendoza, while orange lichens which cling tenaciously to their substrates and do not have a lower cortex (which may be a different color than the thallus) are more likely to be Caloplaca ssp. This is hardly a foolproof test, however, since squamulose lichens also do not have a lower cortex, and since some Caloplaca ssp. such as Caloplaca coralloides have such tightly bound fruticose structures as to appear crustose(8). Also, based on my own observations of photos of each genus, it seems that Xanthoria/Xanthomendoza lichens tend to have a more flowing, consistent thallus while the thalli of Caloplaca ssp. tend to be more warty, fissured, or grainy. Below are some pictures of lichens that I'm pretty sure I have identified correctly from each genus. Click image to enlarge. 

Genus Caloplaca

Caloplaca trachyphylla growing on sandstone in the Utah desert. This lichen almost always grows on sandstone, and forms beautiful, prominent rosettes like this one. The crustose habit and "warty" thallus help to separate it from Xanthoria elegans. 

A close-up of C. trachyphylla. Note how the lecanorine apothecia become more flattened and even convex towards the thallus' center. The white growth toward the top center-left may be the beginnings of a colony of Acarospora stapfiana, another lichen which is parasitic on C. tracyphylla.

Caloplaca arenaria, also growing on sandstone. The salient parietin-bearing structures in this picture are actually the apothecia. This species often has a near non-existent thallus, which if present is gray(12). The black, blurry marks at left are millimeters.  Seen on the macro scale this species appears as tiny, blood-red granules.

Genus Xanthoria 

   

Xanthoria elegans growing on Pike's Peak granite with an unidentified gray foliose lichen. X. elegans may form more rosette-like structures, but this admittedly blurry photo does a nice job showing its more flowing structure.  

If you see vertical streaks of orange like this on rocks, odds are you're dealing with X. elegans. It enjoys drainage channels and seeps that come off of nitrogen-rich cracks in rocks where rodents or birds habitually poop(1). The purple flowers barely visible in this photo are the relatively rare wildflower Boykinia jamesii, which grows in abundance in the Crags area on the West side of Pike's Peak.

 

Genus Xanthomendoza 

A magnified view of a Xanthomendoza species I found on the wooden fence in my back yard. I'm not certain on the ID, but it appears to be either Xanthomendoza ulophyllodes or Xanthomendoza fulva. The presence of apothecia seems to indicate Xanthomendoza ulophyllodes.

Xanthomendoza montana growing on an exposed rock face on Mount Evans. As best I can tell, X. montana is best differentiated from Xanthoria elegans by its more lumpy, mounded habit. 

Xanthomendoza montana growing in luxuriant lumps near the summit of Mount Evans. The thallus can hardly be seen through the prolific apothecia. 

Xanthomendoza montana growing on granite at approx. 12.000 ft. The white flowers are Minuartia macrantha, the alpine sandwort. 

     Thank you for reading about orange lichens! Think of them as you enjoy your orange Thanksgiving foods such as Ipomoea batatas, although in that case the orange pigments are carotenoids, not parietins. 

References

1. Corbridge, J. N., & Weber, W. A. (1998). A Colorado Lichen Primer. Niwot, CO: University Press of Colorado.
2. Nybakken, L., Solhaug, K. A., Bilger, W., & Gauslaa, Y. (2004). The lichens Xanthoria elegans and Cetraria islandica maintain a high protection against UV-B radiation in Arctic habitats. Oecologia, 140(2), 211-216. doi:10.1007/s00442-004-1583-6
3. Gazzano, C., Favero-Longo, S. E., Iacomussi, P., & Piervittori, R. (2013). Biocidal effect of lichen secondary metabolites against rock-dwelling microcolonial fungi, cyanobacteria and green algae. International Biodeterioration & Biodegradation, 84300-306. doi:10.1016/j.ibiod.2012.05.033
4. Bačkorová, M., Jendželovský, R., Kello, M., Bačkor, M., Mikeš, J., & Fedoročko, P. (2012). Lichen secondary metabolites are responsible for induction of apoptosis in HT-29 and A2780 human cancer cell lines. Toxicology In Vitro, 26(3), 462-468. doi:10.1016/j.tiv.2012.01.017
5. Mycopigments (2013). Lichen dyes [web log post]. Retrieved from http://mycopigments.com/lichen-dyes/
6. Sharnoff, S. (2014). Field Guide to California Lichens. New Haven, CT: Yale University Press.
7. KARNEFELT, I. (1989). MORPHOLOGY AND PHYLOGENY IN THE TELOSCHISTALES. Cryptogamic Botany, 1(2), 147-203.
8. Kelso, M. (2005). Caloplaca coralloides. Retrieved from: http://www.mbari.org/staff/conn/botany/lichens/taxonomy.htm
9. Wade, A. E. (1965). The genus Caloplaca th. fr. in the British Isles. The Lichenologist: 3(1), pp. 1-28. DOI: http://dx.doi.org/10.1017/S0024282965000038  
10. Xanthoria (n.d.) [article entry in wiki]. Retrieved from: https://en.wikipedia.org/wiki/Xanthoria
11. Consortium of North American Lichen Herbaria [CNALH](n.d.) Colorado. Retrieved from: http://lichenportal.org/portal/checklists/checklist.php?pagenumber=2&cl=1202&dynclid=0&pid=510&searchsynonyms=1
12. 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=56217&cl=1202