| What Are Lichens? A Symbiotic Union A lichen is a specific combination of a fungus and one or two algae. The organisms resulting from these symbiotic unions are completely different from those of either the alga or the fungus growing alone, as are the physiological conditions under which the lichen can survive. You can think of lichens as fungi that have discovered farming. Instead of invading or scavenging for a living like other fungi (moulds, mildews, mushrooms), lichen fungi cultivate algae within a fabric of fungal threads of which the lichens are composed. As photosynthesizers, algae are able to manufacture their own carbohydrates, proteins and vitamins. Some of these foodstuffs are "harvested" by fungi for their own nourishment. In return, the fungus appears to provide the alga with protection from the elements. A lichen is simply the physical manifestation of this symbiotic relationship. The algae found in lichens commonly occur as free-living species. One species of alga, Trebouxia, is rarely found outside the lichenized state. The lichen fungi are generally found only in the lichens. For these reasons, lichen are generally classified according to the fungus species with which they are associated or are considered to be the fungus growing in a "lichenized" state. Most lichen fungi cultivate green algae, though in some cases the phycobiont is a cyanobacterium ("blue-green alga"). In a few lichen species, both types of algae are present; the paler green algal cells are scattered throughout the lichen thallus, whereas the darker blue-green algal cells are localized in tiny colonies called cephalodia. Hardy Pioneers Lichens are widespread in nature; they occur from arid desert regions to the arctic, and grow on bare soil, tree trunks, sun baked rocks, fence post, and windswept alpine peaks all over the world. They are often the first colonist of bare areas of rock and mineral soil. In some desolate areas, such as the arctic and mountain tops, they are nearly the only type of vegetation hardy enough to survive. In such regions, they can be the principal type of vegetation. There are about 17,000 species of lichens in the world. More than 1500 species are expected to occur in Alberta. Only a couple hundred are conspicuous and commonly found in the Calgary area (i.e., within 100 km of the city centre). Since lichens do not need an organic food source, as do their component fungi, and unlike free-living algae, they can remain alive even when desiccated. They require only light, air, a few minerals and a temperature above freezing. They apparently absorb and concentrate minerals from their substrate, rainwater, and airborne dust which falls on their surface. In the absence of rain or snow, they can absorb moisture in the form of water vapour directly from the air. Pollution Sensitivity Ironically, these hardy organisms are at the same time, among the species most sensitive to the toxic effects of air pollutants; especially sulphur dixoide. They are particularly susceptible and sensitive to airborne toxic compounds. The biodiversity of lichen communities is a sensitive index of air pollution levels. This ability of lichens to concentrate minerals from their environment makes them valuable indicators for biogeochemical prospecting. Because lichens rapidly absorb substances from water and concentrate them in their tissues, they are useful as bioaccumulators and can reveal a lot of information about which elements are present in their environment. Many shapes sizes and colours Lichens come in many different shapes, sizes and colours. However, they never have leaves or roots like other plants. Lichen growth forms are artificially grouped into a number of growth forms. The principal growth forms are crustose (crust-like), foliose (leaf-like), and fruticose ( hair-like or twig-like). These groups can be usefully subdivided into 6 "life" forms (crust, scale, leaf, club, shrub, and hair-like). Reproduction Lichens reproduce in several differenct ways. Sometimes the fungal partner produces spores in saucer-shaped furuiting bodies called apothecia. These spores may find a suitable algal cells in the environment and capture them in order to form a new lichen. In some species, the inner tissue (medulla) of the lichen is exposed at the thallus surface and forms a powdery masses (soralia) of bundles (soredia) of alga cells tied up with strands of fungal mycelium. In other species, tiny coral-like growths (isidia) form on the surface of the lichen. These also contain packaged algae and are easily broken off. If soredia or isidia end up on a suitable substrate they can develop into a new lichen thallus. |
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