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An Introduction to Lichens
by Salvatore De Santis
Herbarium Intern
1999

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It is debated whether the relationship in a lichen is mutualistic or part of a controlled parasitism. On one hand, the fungus and the photobiont seem to be in a mutualistic relationship because when they are combined, they have the ability to deal with ecological conditions that neither part would be able to handle on its own. It also seems that neither partner is damaged by the other. Upon taking a closer look at a lichen, some might say that the photobiont is a captive of the mycobiont, not a partner. The fungal partner "enslaves" the photobiont to feed from the photobiont’s photosynthesis.

In a lichen, the mycobiont produces a thallus, which houses the photobiont. There are three major morphological types of thalli: foliose, crustose, and fruticose.

Foliose lichens are leaflike in both appearance and structure. They adhere to their substrate loosely. See Figure 1.

Crustose lichens are "crust-like." They are tightly attached to or embedded in their substrate, and have no lower cortex. Crustose lichens consist of about 75 percent of all lichens on earth. See Figure 2.

Fruticose lichens have no distinct top and bottom and are often round in cross-section. Their thalli may be upright, shrubby, or of pendulous strands. See Figure 3.

Lichens can survive severe conditions because they can withstand drying. The lack of water interrupts photosynthesis. In this suspended state, some lichens are able to withstand great extremes in temperature. The slow growth in lichens could possibly be attributed to their slow rate of photosynthesis (since they are usually dry).

A lichen absorbs most of its mineral nutrients from the air and rainfall. Pollution in the atmosphere can be especially dangerous to lichens because they retain, and can accumulate, deadly amounts of heavy metals, sulfur, radioactive elements, NO₂, and ozone. Sulfur dioxide (SO₂) is especially lethal to lichens because it lowers pH and deteriorates chlorophyll, which causes photosynthesis to cease. Anti-sulfur dioxide legislation in the last 25 years is allowing lichens to return to formerly polluted areas. Lichens have been used to monitor the amount of pollutants in an environment. This is done by observing the condition of lichens as well as their chemical composition.

Reproduction in lichens may be either sexual or vegetative (asexual). In asexual reproduction, fragments of the thallus containing both the photobiont and the mycobiont separate and form into a new lichen. This may happen when a piece of the thallus is accidentally broken off, but specialized structures that have evolved in lichens, namely isidia and soredia, usually carry out this type of reproduction.

In most lichens undergoing sexual reproduction, tiny spores are produced within an ascus. The asci form inside of structures called ascomata. The most common type of ascoma, called an apothecium, is shaped like an open disc. In sexual reproduction, only the fungal partner is reproduced. The spores that germinate must find the appropriate photobiont in order to form a new lichen. Since this is an undependable type of reproduction, vegetative reproduction is very important.

As mentioned, lichens can be used as pollution indicators. Lichenometry is a technique used in dating rock surfaces on which certain lichens grow by means of their rate of growth. Lichens produce many secondary compounds which play an important role in distiguishing species in the laboratory. A practical use for these same compounds is seen in medicine as well as natural dyes and as a component in perfume. In nature these compounds may serve as a defense against herbivores and may also help break down rock substrates. Lichens colonize places that have not had any previous growth, such as rocks. Lichens having cyanobacteria as a photobiont provide fixed nitrogen to their environment. Ecologically, they are also important in providing food and shelter to wildlife including deer, moose, and elk, as well as cetain species of squirrels, mice, and bats. Lichens contribute to the background diversity of the natural landscape and can be appreciated by all, even if only subliminally to the untrained eye.
 

• How are lichens identified?