Lichens

Lichens

Symbiotic associations of fungi (mycobionts) and photosynthetic partners (photobionts). These associations always result in a distinct morphological body termed a thallus that may adhere tightly to the substrate or be leafy, stalked, or hanging. A thallus consists of layers, that is, a cortex and medulla made up of the fungus, and a photosynthetic layer of algal or cyanobacterial cells that are closely associated with fungal hyphae. Rhizoids anchor thalli to their substrates. See Cyanobacteria

Lichens are formed from specialized groups of parasitic fungi; this association is one of a controlled parasitism rather than mutualism. Thus, the photobionts that lichen fungi slowly parasitize should be considered victims and not partners. Lichen-forming fungi share two characteristics with fungi that parasitize plants: concentric bodies and specialized branches of hyphae (haustoria) that penetrate host cells and absorb nutrients from them.

Lichens have a worldwide distribution and grow on almost any inanimate object. They are among the hardiest of organisms and thrive in some of the Earth's harshest environments, such as polar regions, deserts, and high mountains.

The name given to a lichen applies only to the mycobiont, while the photobiont has a separate name. Most of the 15,000 lichen-forming fungi are in the fungal class Ascomycotina (ascolichens). Approximately a dozen species of basidiomycetes form lichens. Lichens that do not have sexual reproduction (Lepraria) are placed in the Lichenes Imperfecti.

Photobionts of lichens are either green algae or cyanobacteria. The most common photobiont is Trebouxia. This unicellular green algae has never been found in the free-living state. It is believed that Trebouxia is a lichenized and highly modified form of the filamentous alga Pleurastrum terrestre.

The basic metabolic processes of lichens are photosynthesis, respiration, and nitrogen fixation. Lichens have adapted these processes to different conditions of light, temperature, day length, and water. The mycobiont causes the photobiont to excrete most of the carbon that it fixes during photosynthesis. Only a single type of compound is excreted. The mycobiont absorbs these compounds and converts them to mannitol, its own storage compound. See Plant respiration

Nitrogen-fixing lichens are common and contribute nitrogen to different ecosystems when they decay. In cyanolichens the mycobiont inhibits the nitrogen-assimilating enzymes of the cyanobiont, causing it to release most of the ammonia it produces. The ammonia is absorbed by the mycobiont and used to make proteins and nucleic acids. See Nitrogen fixation

Lichens produce several hundred secondary compounds that accumulate as crystals in the thalli, often at high concentrations. These compounds may protect the slow-growing thalli from harmful bacteria, fungi, and insects and may play a regulatory role in the interactions between bionts. Lichen secondary compounds represent a new class of antibiotics in an age where standard antibiotics such as penicillin are becoming ineffective against antibiotic-resistant microbes. Secondary compounds are used extensively by taxonomists to characterize new taxa of lichens (chemotaxonomy).