What Are Lichens

Lichens are made up of two, and some times three, different organisms from three different Kingdoms, which form a symbiotic relationship with each other for their mutual survival. The dominant member is a ascomycetous fungus (Kingdom Fungi), which is capable of making it's own food. The fungus forms the visible portion of lichen inside of which, and protected by them, are cells of an algae (kingdom Protista) or some times cyanobacteria (Kingdom Monera), once known as blue-green algae. Some lichen can consist of all three organisms at once. The algae provide nutrients, as they contain the pigment chlorophyll, which it uses during photosynthesis to produce carbohydrates the same way as green plants do. Thus the fungus obtains nutrients from the algae, the fungal tissue in turn provides shelter for the algae allowing it to grow in harsh conditions such as rock surfaces where it would otherwise be destroyed.

Stereocaulon ramulosum which has two different alga partners lichen soredia

How they Reproduce

Lichen-forming fungi can reproduce sexually or asexually thus they have a number of different methods of reproduction. Asexual Reproduction Most lichens are very brittle when dry, some simply relying on breakage's of the thallus to produce fragments that are dispersed by wind, rain, or insects and birds. Others make non-sexual reproductive packages known as soredia (tufts of a few algal cells wrapped in hyphae), or isidia (cylindrical, finger-like projections from the upper surface) or lobules (miniature lobes developing along the margins) that break off and are dispersed as described above. Sexual Reproduction Ascomycetous lichens produce fruiting bodies called apothecia, which are typically disc-shaped from which are produced spore. These are then dispersed by wind and rain etc. After the spore germinates they need to find a new algae partner to form into lichen. Some are able to steal them from other lichens, others by luck just happen upon suitable algae.

Apothecia of Ramalina celastri magnified. Growing on rock

Where do lichens grow ?

Lichens are remarkable in that they can tolerate the most extreme environments, thus they can live in hot dry places as well as arctic conditions and the wettest of rain forest. Although they can tolerate salt spry and immersion in water they are not aquatic. Lichens can live on soil, woody debris, rocks, tree bark, tree leaves, other lichens, desert sand, animal bones, and rusty metal. For this reason they are nature's pioneers. Been first to colonise the most inhospitable places from there they begin the slow process of creating the foundation for other habitation.

Growing on tree bark

How They survive ?

To assist their survival in such inhospitable conditions, lichens are able to shut down metabolically during periods of unfavourable conditions then with the appropriate amount of light and moisture, clean air, and freedom from competition, lichens will continue to grow. Most lichens grow very slowly, often less than a millimetre per year. Another method that helps with there survival is that lichens can produce an arsenal of more than 500 unique biochemical compounds that serve to control light exposure, repel herbivores, kill attacking microbes, and discourage competition from plants.

Lichens growing rotten log

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