Lichen

Lichens are symbiotic organisms made up by the association of microscopic green algae or cyanobacteria and filamentous fungi. There is evidence that lichens might involve a controlled form of parasitism of the algal cells. In laboratory settings, algae grow faster when they are alone rather than when they are part of a lichen. But there is also a mutualism side to the relationship, the fungus part of the lichen provides the alga with water and minerals that the fungus absorbs from whatever the lichen is growing on. As for the alga, it uses the minerals and water to make food for the fungus and itself. Lichens take the external shape of the fungal partner and hence are named based on the fungus. The fungus most commonly forms the majority of a lichen's bulk, though in filamentous and gelatinous lichens this may not always be the case. The lichen fungus is typically a member of the Ascomycota—rarely a member of the Basidiomycota. Some lichen taxonomists place lichens in their own division, the Mycophycophyta, but this practice ignores the fact that the components belong to separate lineages.

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 alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the substratum. 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.

Morphology and structure

Lichens live on various surfaces, such as soil, wood, and rock. They 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 at high latitudes. Some survive in the tough conditions of deserts, and others on frozen soil of the arctic regions. Recent ESA research shows that lichens can even endure extended exposure to space. Some lichens have the aspect of leaves (foliose lichens); others cover the substratum like a crust (crustose lichens); others adopt shrubby forms (fruticose lichens); and there are gelatinous lichens (see lichen forms to the right ->).

Although the form of a lichen is determined by the genetic material of the fungal partner, association with a photobiont is required for the development of that form. When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as an undifferentiated mass of hyphae. If combined with its photobiont under appropriate conditions, the morphogenesis of the lichen occurs and its characteristic form emerges. (Brodo, Sharnoff & Sharnoff, 2001)

There is evidence to suggest that the lichen symbiosis is parasitic rather than mutualistic (Ahmadjian 1993). The photosynthetic partner can exist in nature independently of the fungal partner, but not vice versa. Furthermore, photobiont cells are routinely destroyed in the course of nutrient exchange. The association is able to continue because photobiont cells reproduce faster than they are destroyed. (Ibid.)

When seen under magnification, a section through a typical foliose lichen thallus reveals four layers of interlaced filaments (fungus). The upper layer is formed by densely agglutinated fungal hyphae building a protective outer layer called the cortex. Cyanobacteria may be held in small eruptions of or under the surface, called cephalodia/cephalodium. Beneath the upper cortex is an algal layer composed of algal cells embedded in rather densely interwoven fungal hyphae. Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by an haustorium. Beneath the algal layer is a layer of loosely interwoven fungal hyphae without algal cells. This layer is called the medulla. Beneath the medulla, the bottom surface resembles the upper surface and is called the lower cortex, consisting of densely packed fungal hyphae. The lower cortex often bears structures, such as rhizines or a tomentum, serving to attach the thallus to the substratum on which it grows. Lichens also sometimes contain structures made from fungal metabolites, for example crustose lichens sometimes have a polysaccharide layer in the cortex. Although each lichen thallus generally appears homogeneous, it may consist of several different species of fungus and photobiont living together.

Reproduction

Lichens most frequently reproduce asexually, either by vegetative reproduction or through the dispersal of diaspores containing algal and fungal cells. Soredia (sing. soredium) are small groups of algal cells surrounded by fungal filaments that form in cavities called soralia, which open when the lichen dries or surrounding tissues die and release the soredia to be dispersed by wind. Another form of diaspore are isidia, elongated outgrowths from the thallus that break off for dispersal. Fruticose lichens in particular can easily fragment. Due to the relative lack of differentiation in the thallus, the line between diaspore formation and vegetative reproduction is often blurred. Many lichens break up into fragments when they dry, dispersing themselves to resume growth when moisture returns.

Lichens also reproduce sexually in a manner typical of fungi, forming fungal and algal "propagules" that, following germination, must meet with a compatible partner before a functional lichen can form. This is generally not a common means of reproduction for most lichens, though it is more common in basidiomycetous lichens since they appear to lack structures specifically designed for asexual reproduction. Spores are produced in spore-producing bodies, the three most common spore body types are the apothecia, perithecia and the pycnidia.

Ecology

Lichens are "functionally photosynthetic" symbioses, meaning that, like plants, they derive their energy for growth and reproduction directly from sunlight through the photosynthetic activities of the algal partner. 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.

A major ecophysiological advantage of lichens is that they are poikilohydric (poikilo- variable, hydric- relating to water), meaning that they have little control over their hydration status and can tolerate irregular and extended periods of severe desiccation. Like some mosses, liverworts, ferns, and a few "resurrection plants", upon desiccation, lichens enter a metabolic suspension or stasis (known as cyptobiosis) in which the cells of the lichen symbionts are dehydrated to a degree which halts most biochemical activity. In this cryptobiotic state, lichens can survive wider extremes of temperature, radiation and drought in the harsh environments they often inhabit.

Lichens do not have roots and do not need to tap continuous resevoirs of water like most higher plants, thus they can grow in locations impossible for most plants, such as bare rock, sterile soil or sand, and various man-made structures such as walls, roofs and monuments. Many lichens also grow as epiphytes (epi- on the surface, phyte- plant) on other plants, partiucularly on the trunks and branches of trees. When growing on other plants, lichens are not parasites; they do not consume any part of the plant nor poison it. Some ground-dwelling lichens like members of genus Cladina (reindeer lichens), however, produce chemicals which leach into the soil and inhibit the germination of plant seeds and growth of young plants. Stability (that is, longevity) of their substratum is a major factor of lichen habitats. Most lichens grow on stable rock surfaces or the bark of old trees, but many others grow on soil and sand. In these latter cases, lichens are often an important part of soil stabilization; indeed, in some desert ecosystems, vascular (higher) plant seeds cannot become established except in places where lichen crusts stabilize the sand and help retain water.

Lichens are a part of the food available for many animals, such as reindeer, living in arctic regions. The larvae of a surprising number of Lepidoptera species feed exclusively on lichens. These include Common Footman and Marbled Beauty. However, lichens are very low in protein and high in carbohydrates, making it unsuitable for some animals. Lichens are also used by the Northern Flying squirrel for nesting, food, and a water source during winter. Although lichens typically grow in harsh environments in nature, most lichens, especially epiphytic fruticose species and those containing cyanobacteria, are sensitive to man-made pollutants. Hence, they have been widely used as pollution indicator organisms. When growing on mineral surfaces, some lichens slowly degrade 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 man-made 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.

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 (or had) economic importance as dyes or primitive antibiotics. Extracts from many Usnea species were used to treat wounds in Russia in the mid twentieth century. Orcein and other lichen dyes have largely been replaced by synthetic versions [http://waynesword.palomar.edu/ecoph5.htm.

The European Space Agency has discovered that lichens can survive unprotected in space. In an experiment led by Leopoldo Sancho from the Complutense University of Madrid, two species of lichen – Rhizocarpon geographicum and Xanthoria elegans – were sealed in a capsule and launched on a Russian Soyuz rocket on 31 May 2005. Once in orbit the capsules were opened and the lichens were directly exposed to the vacuum of space with its widely fluctuating temperatures and cosmic radiation. After 15 days the lichens were brought back to earth and were found to be in full health with no discernable damage from their time in orbit. ^*

Growth form

Lichens are informally classified by growth form into:

crustose (paint-like, flat), e.g., Caloplaca flavescens
filamentose (hair-like), e.g., Ephebe lanata
foliose (leafy), e.g., Hypogymnia physodes
fruticose (branched), e.g., Cladina evensii, C. subtenuis, and Usnea australis
leprose (powdery), e.g., Lepraria incana
squamulose (consisting of small scale-like structures, lacking a lower cortex), e.g., Normandina pulchella
gelatinous lichens, in which the cyanobacteria produce a polysaccharide that absorbs and retains water.

References

Ahmadjian, V. 1993. The Lichen Symbiosis. New York: John Wiley & Sons.

Brodo, I.M., S.D. Sharnoff, and S. Sharnoff, 2001. Lichens of North America. Yale University Press, New Haven. http://www.newscientistspace.com/article/dn8297 Hardy lichen shown to survive in space http://www.lichen.com

Purvis, O.W., Coppins, B.J., Hawksworth, D.L., James, P.W. and Moore, D.M. (Editors) The Lichen Flora of Great Britain and Ireland. Natural History Museum, London.

External links

Lichens | Symbiosis | Mycology | cryptogams