Monday, June 27, 2011

Lichens are extremely unique organisms. Although we tend to think of any given lichen as a single species, they are actually the composite of a fungus and an organism capable of photosynthesis: algae or cyanobacteria. The fungus and its photosynthetic symbiont (shortened to “photobiont”) are mutually symbiotic; the photobiont produces energy in the form of carbohydrates which is absorbed by the fungus. In return, the fungus provides structure and moisture to the photobiont and partially shields it from light, all things which it requires. This relationship has previously been considered to be a prime example of mutually beneficial symbiosis. However, recently it has been reevaluated. As it turns out, lichens—an incredibly diverse group—cover all possible relationships from “mild parasitism” of the photobiont, to a “rampant, photobiont-destroying disease.”

Lichen morphology

 The relationship between specific species of fungus and specific species of lichen is similarly interesting. A single “species” of lichen (named for the fungus involved) always involves the same species of lichen-forming fungus, and, usually, the same species of photobiont. However, there are numerous instances of the same lichen-forming fungus associating with different species of algae throughout the lichen’s geographic range. There are even cases of a single fungus associating at some times with an algae photobiont and at other times with cyanobacteria. When the same fungus associates with different photobionts, the results can appear either almost indistinguishable, or make the lichen look quite different. Although the photobiont does play an indispensible role, it is the fungus component that generally controls the appearance of the lichen. However, in studies where the two components of the lichen (the fungus and the photobiont) have been grown separately in the laboratory, both components change drastically. The algal cells tend to form small colonies composed of cells that bear little resemblance to those within lichen, while the fungus component produces a “characterless heap of hyphae.” However, when those two separate components are recombined under the right conditions, they form a new association that reforms into the original lichen.   

Lichens do not compose a specific taxonomic group, either among fungus or among the photobionts. The fungi that form lichens are a large and diverse group, mostly composed of Ascomycetes (sac fungi), but including a few Basidiomycetes (mushroom-producing fungi). The only way that these lichen-forming fungi are connected is by their means of nutrition: the photobiont. The photobionts themselves are similarly unrelated. Green algae, golden algae, brown algae, and cyanobacteria all serve as photobionts, a group which spans not only genera but domains.

Lichens can be classified into three main categories, depending on their physical structure. Foliose lichens are relatively flat, with distinct upper and lower surfaces (our example, Umbilicaria mammulata.) Fruticose lichens can be erect or pendant (our Usnea and Caldina), and have no obvious upper or lower surfaces. Lastly, Crustose lichens appear to form crusts over their substrates.


Southern Soldiers

Cladonia didyma

            This lichen is recognizable because of its striking podetia (fruiting bodies). The “stalks” are a pale greenish-grey, and can branch towards the tips. The podetia can be 10-30 mm tall, are usually fertile, and have red “caps” (apothecia). The primary squamules (the small, flat bodies adhered to the substrate) are lobed. This lichen tends to grow on sandy soil or rotting wood, and inhabits most of the eastern U.S, as far north as Michigan. Although it may appear very similar to its close relatives, Cladonia cristatella (British soldiers) and Cladonia floerkeana (Gritty British soldiers), C. didyma lacks the cortex, or outer layer of the podetia. As a result, under close examination, the podetia of C. didyma appear “brownish to translucent,” whereas the podetia of C. cristatella and C. floerkeana will appear smooth and “with a continuous cortex.”

            This specimen was collected by Nikki in Pond Drain.


Smooth Rock Tripe

Umbilicaria mammulata

This foliose lichen has the potential to be one of the largest lichens in the world in terms of diameter and mass. One specimen found in Tennessee has exceeded 63 cm (over 2 feet) in diameter! This lichen has a thallus (main body) that is generally smooth and is usually 4-15 cm in diameter. The upper surface can be red-brown to grey-brown, and the lower surface is pitch-black. It generally grows on boulders and rocks in the forest, or occasionally in the open near a lakeshore. Its fruiting bodies are rarely seen. It can be distinguished from other, related species by the following characteristics: it possesses a thin, brownish, and not at all shiny thallus. It is edible, though not entirely desirable.

This specimen was collected on a boulder along Hedwig Trail.


Grey Reindeer Lichen

Cladina (aff.) rangiferina

            This lichen is white to silver-grey in color. It is mainly characterized by its branching, tree-like stems which are typically “combed” down at the tips. This species is unusual and identifiable among the Cladina genus by the fact that it is grey, instead of green or yellow-green, as is common in related species. This species has a wide range that covers much of Canada, part of the north-western coast of the U.S, and most of the eastern half of the country. It can typically be found on thin soil, over rocks, or in sandy areas. It can also be found over wet soil. Together with other closely related species, C. rangiferina constitutes the principal winter food of caribou in North America and reindeer in Europe. Furthermore, Native Americans in regions near the Great Lakes were known to practice the peculiar tradition of bathing new-born babies in a decoction of reindeer lichen. It has also been used as a food among native tribes, and to make tea.

This specimen was collected along Hedwig Trail.


Fence-Rail Cladonia

Cladonia parasitica

This species, similar in appearance to the Southern Soldiers, is also a member of the Cladonia genus. In this species, also, the thallus is described as greenish-grey. The podetia are often very sparse, although this is clearly not one of those cases. It is notable among the Cladonia genus for the absence of large, contiguous, primary squamules; instead, they are finely divided and granular. These squamules tend to form a granular “crust” over the substrate of the lichen (commonly rotting wood, bark or old fence rails). They even cover their own podetia, virtually obscuring them. However, in places not covered by the squamules, the brownish-translucent stereome (the inner layer of the stalk—this species as well lacks a cortex) shows through. The podetia are tipped with reddish-brown apothecia. Their very presence sets them apart from other closely related species, most of which have at least a few bright red apothecia.  


Usnea sp.

            Usnea  is a broad genus, encompassing both some shrubby and many pendant (hanging) lichens. This lichen is clearly one of the pendant lichens, but beyond that, classification is difficult. This whole genus is very commercially important because of its production of usnic acid, an antibiotic. Thousands of kilograms of Usnea are harvested every year by pharmaceutical firms for extraction of that acid, which is typically used in antibacterial ointments and in cosmetics as a deodorant (by controlling of bacteria). Also, some species are used in Chinese medicine, homeopathic medicine, and traditional remedies in many areas of the globe. Usnea species have also been used in dye-making and beer-making. Furthermore, they serve as a food source for wildlife and a major source of nesting material for some birds.

This specimen was collected along Hedwig Trail, growing on the side of a rotten tree.

Articles by Hazel Galloway

  • Brodo, Irwin M. Lichens of North America. New Haven: Yale University Press, 2001.