Lichens

Lichens are stable and self-supporting composites of a fungus and photosynthetic Algae. Because of this dualism, they are difficult to put into a specific kingdom. The algae are either a green alga or a blue-green alga (Cyanobacterium), both of which contain chlorophyll. The lichen’s body (thallus) benefits from the interaction between the fungal and algal components. The fungus gives structural support and protection to the algal cells, which by photosynthesis produce sugars and carbohydrates; the fungus utilizes some of these to obtain energy for growth and reproduction. This relationship enables both components to live in extreme environments normally hostile to their survival, so that lichens have a cosmopolitan distribution. The scientific name given to lichen refers to the fungal component; the algal component has its own name.

Lichens differ from what we normally understand as fungi. The greater part of the thallus is usually above ground; it varies in structure and remains visible throughout the year, bearing small fruiting bodies (ascomata). Some lichens are leaf-like (foliose), with upper and lower cortical fungal cells. The algal cells are between these surface cells and loosely arranged central fungal cells (medulla). The lower layer often has tiny structures (rhizinae) or a central holdfast (umbilicum) for attachment to the substratum. Other forms are upright and bushy, or pendant from Trees (fruticose), or appearing as a crust (crustose) on trees or racks; they are often conspicuously coloured. These lichens differ slightly in their internal structure from foliose lichens.

Lichens have no protective cuticle and dessicate rapidly, becoming metabolically inactive for long periods of Drought but regaining their physiological functions by re-moistening of the lichen body. Moisture is therefore essential for lichen growth, and lichens obtain moisture from the atmosphere in the form of dew, fog or rain. However, beside beneficial nutrients such moisture can contain pollutants such as sulfur dioxide, sulfuric acid, nitric acid, and metals, especially in cities and industrialized areas. Because lichens are very sensitive to these toxins, which can seriously affect the delicate balance between the fungus and the algae, they are known worldwide as sensitive bio-indicators for assessing atmospheric pollution and health.

Lichens can reproduce in various ways: sexually by fruiting bodies containing spores, which upon germination need to join compatible algal cells to form a new lichen (lichenization); or vegetatively, whereby the lichen produces on the thallus small structures which contain algal cells surrounded by fungal strands, forming complete miniature lichen bodies. These are easily dispersed and able to form a new lichen upon a suitable substratum. A more successful form of vegetative reproduction is by fragmentation of the lichen body.

There are over 400 species of lichen known from Saskatchewan, with additional species to be discovered. Lichens are used as biochemical indicators for the dating of surfaces (lichenometry), and can be valuable indicators of climate change. They have economic uses as dyes, base ingredients for perfumes, cosmetics, medicines, and food for indigenous peoples. Because of their sheer mass, certain lichens in northern regions are a vital winter nutrient for barren-ground and woodland caribou.

Bernard de Vries

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Further Reading

Brodo, Irwin M., S. Duran Sharnoff and S. Sharnoff. 2001. Lichens of North America. New Haven: Yale University Press.