LICHENS

Lichens is a living structure consisting of a partnership between two (occasionally three) organisms, a fungus and photosynthetic partner, either green algae or cyano-bacteria. When the relationship is of benefit to both participants it is known as mutualism or symbiosis and this is thought to be usually the case.

The body (thallus) of most lichens is different from those of either the fungus or alga growing separately. The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall, forming penetration pegs or haustoria similar to those produced by pathogenic fungi. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the substrate.

The algal or cyanobacterial cells are photosynthetic, and as in plants they reduce atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. If a cyanobacterium is present, as a primary partner or another symbiont in addition to green alga (as in certain tripartite lichens), they can fix atmospheric nitrogen, complementing the activities of the green alga.

Some lichens have the aspect of leaves (foliose lichens); others cover the substrate like a crust (crustose lichens) others adopt shrubby forms (fruticose lichens) and a few are gelatinous. Lichens are capable of surviving extremely low levels of water content.

The reproduction of lichens may be vegetative in a process of fragmentation or the formation of tiny dust –like particles (diaspores) containing small amounts of both partner organisms are shed and dispersed by the wind. Many lichen fungi produce sexual structures in the manner typical of fungi but these spores don’t contain a photosynthetic partner and the spores will not develop into another lichens unless they meet with the appropriate alga or bacterium – so it could be viewed as an ‘escape’ from the relationship.

Lichens occur in some of the most extreme environments on Earth—arctic tundra, hot deserts, rocky coasts, and toxic slag heaps. However, they are also abundant as epiphytes on leaves and branches in rain forests and temperate woodland, on bare rock, including walls and gravestones, and on exposed soil surfaces. The roofs of many buildings have lichens growing on them. Lichens are widespread and may be long-lived, although many are also vulnerable to environmental disturbance, and may be useful to scientists as a biomonitor, aiding in the assessment of air pollution, ozone depletion, and metal contamination. (Because lichens do not possess roots, their primary source of most elements is the air).

The European Space Agency has proven that lichens can even survive unprotected in space.

When growing on mineral surfaces, some lichens slowly decompose their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of weathering by which rocks are gradually turned into soil. While this contribution to weathering is usually benign, it can cause problems for artificial stone structures. For example, there is an ongoing lichen growth problem on Mount Rushmore National Memorial that requires the employment of mountain-climbing conservators to clean the monument.

(source: Wikipedia)