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Physical Description:
Lichens have been described as "dual organisms" because they are symbiotic associations between two entirely different types of microorganism:
-- a fungus (termed the mycobiont)
-- A green alga or a cyanobacterium (termed the photobiont).
The fungus provides support and nutrients and the algae in turn provide carbohydrates produced in photosynthesis.
There are many examples of symbiosis in nature, but lichens are unique because they look and behave quite differently from their component organisms. The process of lichenization in nature has only been demonstrated a few times Scientifics have asked all of this time: how do two or more microorganisms interact at the cellular, genetical and biochemical levels to produce a unique, hybrid organism? They don’t know yet the answer that is why lichens still forms one of the greatest mysteries of biology,
Lichens are regarded as organisms in their own right and are given generic and species names. However, for taxonomic purposes lichens are regarded as a special group of fungi - the lichenised fungi
Global Distribution:
There are about 15000 to 20000 species of lichen, they can grow almost everywhere in the world. They can stand extreme weathers they are abundant in mountain cold forests and in the areas influenced by fog and oceanic climates. It is estimated that lichens are the dominant vegetation on 8% of the earth's terrestrial surface. Lichens form vast continuous mats in the northern boreal forest.
Habitat
They live on rock rarely on bark or wood. They can be exposed to sheltered rocks in a wide variety of habitats; alpine to the sea level including on rocks in the ocean spray zone. The essential thing for grow lichen is moisture
Feeding:
The dominant partner is a fungus. Fungi are incapable of making their own food. They usually provide for themselves as parasites or decomposers. The lichen fungi (kingdom Fungi) cultivate partners that manufacture food by photosynthesis. Sometimes the partners are algae (kingdom Protista), other times cyanobacteria (kingdom Monera), formerly called blue-green algae. Some enterprising fungi exploit both at once.
Predators:
They are a vital winter food source for animals like deer and caribou. But they are not consumed too much. They don’t have a big gamma of predators.
The most serious threat to the continued health of lichens is not predation, but the increased pollution of this century. Several studies have shown serious impacts on the growth and health of lichens resulting from factory and urban air pollution. Because some lichens are so sensitive, they are now being used to quickly and cheaply assess levels of air toxins in Europe and North America.
Reproduction:
Grey Lichen reproduces in two basic ways. Firstly it may produce soredia, or a cluster of algal cells wrapped in fungal filaments. These may disperse and form new lichens. A second way to reproduce itself is through isidia, which are much like soredia except that isidia are enclosed within a layer of protective cortex tissue. An isidium is much more like a miniature lichen.
Interesting facts:
&Mac183; The first fact interesting is the one that does not have answer until now: how do these organisms can coexist in such perfect way.
&Mac183; Most lichens grow very, very slowly, often less than a millimeter per year, and some lichens are thought to be among the oldest living things on Earth.
&Mac183; Lichens with known, slow growth rates have been used to estimate the dates of geological events such as the retreat of glaciers. Geologists of the University of Arizona have been using lichen measurements to establish dates for huge rockslides in the Sierra Nevada and other places that were caused by earthquakes.
&Mac183; Lichens 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.
&Mac183; The damage also that the human have caused to the lichen is huge : “ If a single footprint can bring a local nitrogen cycle almost to a halt, the impact of a century's work of livestock hoofprints can easily be imagined." -- Thomas L. Fleishner , Ecological costs of livestock grazing in western North America. 1994. Conservation Biology 8 (3): 633.
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References:
Mosses, Lichens and Ferns of Northwest North America by Vitt, Marsh and Bobey; 1998; pages 156- 253
http://www.earthlife.net/lichens/intro.html
http://www.ucmp.berkeley.edu/fungi/lichens/lichens.html
http://helios.bto.ed.ac.uk/bto/microbes/lichen.htm
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