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 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 pinastri 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., 1996 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 that then must seek out compatible algal cells in order to survive.
Primary Reference: Nash, Thomas H. III, ed. 1996 Lichen Biology. Cambridge University Press