Lichens are composite organisms consisting of a symbiotic association of a fungus with a photosynthetic partner, usually either a green algae or cyanobacterium. The morphology, physiology and biochemistry of lichens are very different to that of isolated fungus and algae. Lichens occur in some of the most extreme environments on Earth, such as arctic tundra, hot deserts, rocky coasts and toxic slag heaps.

They are abundant on leaves and branches in rain forests and temperate woodland, on bare rock, including walls and gravestones, and on exposed soil surfaces in otherwise harsh habitats. Lichens are widespread and long-lived, however, many species are also vulnerable to environmental disturbance, and may be useful to scientists in assessing the effects of air pollution, ozone depletion, and metal contamination. Lichens have also been used in making dyes and perfumes, as well as in traditional medicines.

Lichens must compete with plants for access to sunlight, but because of their small size and slow growth, they thrive in places where higher plants have difficulty growing. Lichens are often the first to settle in places lacking soil, constituting the sole vegetation in some extreme environments such as those found at high mountain elevations and cooler latitudes. Some survive in the tough conditions of deserts, and others on frozen soil of the Arctic regions. Recent research shows that lichen can even endure extended exposure to space.

The form of most lichens is quite different from those of either the fungus or alga growing separately, and may strikingly resemble simple plants in form and growth. 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 nodes or roots similar to those produced by pathogenic fungi.

The algal or cyanobacterial cells are photosynthetic, and as in higher 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. The fungal partner protects the algae by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the substrate. If a bacteria 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 algae.

Lichens are capable of surviving extremely low levels of water content. However, the reconfiguration of membranes following a period of dehydration requires several minutes at least. During this period a solution of metabolites from both the fungus and plant leaks into the extracellar spaces. This is readily available to both forms to take up essential metabolic products ensuring a perfect level of mutualism. Other epiphytic organisms may also benefit from this nutrient rich base. This phenomenon also points to a possible explanation of lichen evolution from its original components with its subsequent migration from an aquatic environment to dry land. Thus, during repeated periods of dehydration in an algae and the resultant leakage of beneficial metabolites to an adjacent aquatic fungus, the mutalistic patterns slowly became constant.

Lichens may be eaten by some animals, such as reindeer living in arctic regions. The larvae of a surprising number of Lepidoptera moth and butterfly species feed exclusively on lichens. These include Common Footman and Marbled Beauty. Lichens are also used by the Northern Flying Squirrel for nesting, food, and a water source during winter.

Extracts from many lichen species were used to treat wounds in Russia in the mid-twentieth century. The lichen Umbilicaria esculenta is collected from cliffs for use in a variety of traditional Korean and Japanese foods.

Many lichens produce secondary compounds, including pigments that reduce harmful amounts of sunlight and powerful toxins that reduce herbivory or kill bacteria. These compounds are very useful for lichen identification, and have had economic importance as dyes such as cudbear or primitive antibiotics.

There are reports dating almost 2000 years of lichens being used to extract purple and red colors. Of great historical and commercial significance are lichens belonging to the family Roccellaceae, commonly called orchella weed. The pH indicator litmus is a dye extracted from the lichen genus Rocella tinctoria by boiling.

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