Fast Facts: Fun Things to Know about Lichens
- There may be as many as 17,000 known species of lichens.
- Lichenized fungi occur in 16 orders of ascomycetes, and five of these fungal orders have no free-living species.
- A few lichens are formed with a basidiomycete as the fungal partner; these produce mushroom-shaped fruiting structures.
- The lichenization process has evolved independently several times.
- Some lichens live more than 1,000 years, and ephemeral lichens are rare.
- More than 50% percent of lichen species contain substances with antibiotic properties; lichens have been (and still are!) widely used medicinally, both internally and externally applied.
- The "manna" described in the Bible, which kept the Jews from starving while in the wilderness, is thought to be a lichen, Lecanora esculenta.
- Some lichens survive where fog or dew is their only source of water.
- In polar and subpolar ecosystems, lichens are often the dominant autotrophs.
- Lichens are excellent bioindicators of air pollution, and also accumulate metals and organic pollutants, enabling us to document deposition of such substances by analyzing lichen composition.
The Lichen Symbiosis
Today most people are familiar with the concept that lichens are symbiotic organisms, combining alga and fungus, but little more than 100 years ago, that was a radical and suspect idea. As symbionts, lichens are unique: they may combine representatives of two biotic kingdoms, sometimes even three, and they are the only combination organisms that do not resemble either partner.
In the photo left, note that edges of this bright green Peltigera aphthosa are beginning to dry to a pale gray-green (underside is whitish). Dark spots on the thallus surface, called cephalodia, contain cyanobacteria. Sharp eyes will also spot a bit of Vulpicida tilesii on a spruce twig in the upper right corner.
When combined, lichenized fungi create a whole new body form, called a thallus. Some have suggested that the fungal partner is engaging in a controlled parasitism, taking advantage of algal cells that are virtually slaves. Judging by the success of their partnership, it seems clear that the algae also benefit from the arrangement. Their fungal "protector" enables them to succeed in places where they cannot survive alone.
Some researchers consider lichens to be fungi who have discovered agriculture. The interaction between the symbionts, and other associated organisms, can be so complex that lichens have been called miniature ecosystems. Here are some reasons why (from Nash, ed., Lichen Biology):
- The mycobiont (fungal partner, heterotroph/consumer) obtains its carbon nutrition from the photobiont (algal partner, autotroph/producer).
- Carbohydrates are transferred to fungi from green algae as polyols, from blue-greens (cyanobacteria) as glucose. The cyanobacteria also provide a nitrogen source.
- A fungus may associate simultaneously with cyanobacteria and green algae, producing different structures even within the same "individual" thallus, as in the photo above.
- The photobiont's cell walls are more permeable to carbohydrate loss when lichenized than when free living.
- Some fungi produce haustoria that physically penetrate the resident algal cells.
- Lichens produce a variety of secondary products that occur as extracellular crystals within the thallus. Most of these chemicals are unknown in free-living fungi or other organisms, and are unique to the symbiosis.
- The fungal partner may enhance water uptake for the algae, and often provides protection from high light intensities.
- As a partnership of unlike organisms, lichens can only reproduce asexually. Sexual spores are produced only by the fungus. These germinate into fungal hyphae which then must seek out compatible algal cells in order to survive.
Lichen Uses
Lichens have also been used by humans in many ways, including
- food and seasonings (one is used today, in an Indian spice mixture called kabul masala),
- dyestuffs (traditionally used to create the soft colors of Harris tweeds, for example),
- poisons (species of Letharia and Vulpicida have been used to poison wolves and foxes),
- perfume base (a secret ingredient in perfume formulas, also used in deodorants for their antibacterial properties),
- tinder and fuel,
- paints,
- fiber (stuffing, textiles, and absorbents),
- decorations (for example, on tribal masks; also as shrubs in model railroad layouts)
- charms (religious, magic, rain, love, and good luck),
- cosmetics,
- log chinking,
- fermenting and brewing,
- animal feed (e.g., caribou in Lapland),
- medicinal (to cure syphilis, rabies, and snake bite, among others)
For a more exhaustive-- and exhausting!-- list of lichen use by humans and other animals, visit the definitive website www.lichen.com.
Last Updated: September 2007. Images copyright by S.L. White unless otherwise credited.
