Lichen Totally Explained

Everything about Lichen totally explained

Lichens (or /lɪtʃ.ən/) are symbiotic associations of a fungus (the mycobiont) with a photosynthetic partner (the photobiont also known as the phycobiont) that can produce food for the lichen from sunlight. The photobiont is usually either green alga or cyanobacterium. A few lichens are known to contain yellow-green algae or, in one case, a brown alga. Some lichens contain both green algae and cyanobacteria as photobionts; in these cases, the cyanobacteria symbiont component may specialize in fixing atmospheric nitrogen for metabolic use.
The body (thallus) of most lichens is quite different from that of either the fungus or alga growing separately, and may strikingly resemble simple plants in form and growth (Sanders 2001). The fungus surrounds the algal "cells, often enclosing them within complex fungal tissues unique to lichen associations; however, in almost all kinds, the algal cells are never enclosed inside the fungal cells themselves. It has been suggested that the fungus is sometimes penetrated by haustoria by the mycobiont, but with the development of electron microscopy there's little solid evidence of this, and if true, is an isolated occurrence and in any event is entirely unecessesary. Thus lichens are poikilohydric, that is, capable of surviving extremely low levels of water content. However, the re-configuration of membranes following a period of dehydration requires several minutes at least. During this period a "soup" of metabolites from both the mycobiont and phycobiont leaks into the extracellar spaces. This is readily available to both bionts to uptake essential metabolic products ensuring a perfect level of mutualism Definitive data derived from poikilohydric canopy mosses is provided by Coxson (1990)showing leaching from the canopy mosses in Guadaloupe of numerous matabolites immediately following rehydration. Not only do the two bionts profit, but also the all the other epiphytic organisms from the nutrient rich leachate. This fundamental phenomenon also points to a possible explanation of lichen evolution from its original phycobiont and mycobiont componants with its subsequent migration from an aquatic environment to dry land. Thus, during repeated periods of low levels of hydration in an alga and the resultant leakage of beneficial metabolites to an adjacent aquatic fungi, the mutalistic "marriage" slowly became constant.
In the natural environment, lichen "provides" the alga with water and minerals that the fungus absorbs from whatever the lichen is growing on, its substrate. As for the alga, it uses the minerals and water to make food for the fungus and itself. Algal and fungal components of some lichens have been cultured separately under laboratory conditions, but in the natural environment of a lichen, neither can grow and reproduce without a symbiotic partner. Indeed, although strains of cyanobacteria found in various cyanolichens are often closely related to one another, they differ from the most closely related free-living strains (External Link

). The lichen association is a close symbiosis: It extends the ecological range of both partners and is obligatory for their growth and reproduction in natural environoments. Propagules ("diaspores") typically contain cells from both partners, although the fungal components of so-called "fringe species" rely instead on algal cells dispersed by the "core species".
   There has nonetheless been controversy as to whether the lichen combination should be considered an example of mutualism or commensalism or even parasitism. An observation offered in support of this is that cyanobacteria in laboratory settings can grow faster when they're alone rather than when they're part of a lichen. The same, however, might be said of isolated skin cells growing in laboratory culture, which grow more quickly than similar cells that are integrated into a functional tissue. However, from the work of Coxson (see above) mutualism would appear to best summarise our current knowledge.
   Lichens are named based on the fungal component, which plays the primary role in determining the lichens form. The fungus typically comprises the majority of a lichen's bulk, though in filamentous and gelatinous lichens this isn't always the case. The lichen fungus is typically a member of the Ascomycota—rarely a member of the Basidiomycota, and then termed basidiolichens to differentiate them from the more common ascolichens. Formerly, some lichen taxonomists placed lichens in their own division, the Mycophycophyta, but this practice is no longer accepted because the components belong to separate lineages. Neither the ascolichens nor the basidiolichens form monophyletic lineages in their respective fungal phyla, but they do form several major solely or primarily lichen-forming groups within each phylum. Even more unusual than basidiolichens is the fungus Geosiphon pyriforme, a member of the Glomeromycota that's unique in that it encloses a cyanobacterial symbiont inside its cells. Geosiphon isn't usually considered to be a lichen, and its peculiar symbiosis wasn't recognized for many years. The genus is more closely allied to endomycorrhizal genera.
   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 substrate. 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 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 lichen can even endure extended exposure to space. Some lichens have the aspect of leaves (foliose lichens); others cover the substrate like a crust (crustose lichens); others adopt shrubby forms (fruticose lichens); and there are gelatinous lichens (illustration, 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, its characteristic form emerges, in the process called morphogenesis (Brodo, Sharnoff & Sharnoff, 2001). In a few remarkable cases, a single lichen fungus can develop into two very different lichen forms when associating with either a green algal or a cyanobacterial symbiont. Quite naturally, these alternative forms were at first considered to be different species, until they were first found growing in a conjoined manner.
   There is evidence to suggest that the lichen symbiosis is parasitic rather than mutualistic (Ahmadjian 1993). However, this now needs to be re-examined in light of Coxons work. 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're destroyed. (ibid.)
   Under magnification, a section through a typical foliose lichen thallus reveals four layers of interlaced fungal filaments. The uppermost layer is formed by densely agglutinated fungal hyphae building a protective outer layer called the cortex, which can reach several hundred μm in thickness. This cortex may be further topped by an epicortex 0.6-1μm thick in some Parmeliaceae, which may be with or without pores, and is secreted by cells - it isn't itself cellular.

Ecology

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're poikilohydric (poikilo- variable, hydric- relating to water), meaning that though they've little control over the status of their hydration, they 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 cryptobiosis) in which the cells of the lichen symbionts are dehydrated to a degree that 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 don't have roots and don't need to tap continuous reservoirs 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 artificial structures such as walls, roofs and monuments. Many lichens also grow as epiphytes (epi- on the surface, phyte- plant) on other plants, particularly on the trunks and branches of trees. When growing on other plants, lichens are not parasites; they don't consume any part of the plant nor poison it. Some ground-dwelling lichens, such as members of the subgenus 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 substrate 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 can't become established except in places where lichen crusts stabilize the sand and help retain water.
   Lichens may be eaten by some 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 them 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 naturally harsh environments, most lichens, especially epiphytic fruticose species and those containing cyanobacteria, are sensitive to manufactured pollutants. Hence, they've been widely used as pollution indicator organisms. When growing on mineral surfaces, some lichens slowly decompose 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 artificial stone structures. For example, there's 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 had economic importance as dyes or primitive antibiotics. Extracts from many Usnea (External Link

) 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 (External Link

).
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 discernible damage from their time in orbit. (External Link

)

Growth form

Lichens are informally classified by growth form into:

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

Paleontology

The extreme habitats that lichens inhabit are not ordinarily conducive to producing fossils. Though lichens may have been among the first photosynthesizers to colonize land, the oldest fossil lichens in which both symbiotic partners have been recovered date to the Early Devonian Rhynie chert, about 400 million years old. The slightly older fossil Spongiophyton has also been interpreted as a lichen on morphological and isotopic grounds, although the isotopic basis is decidedly shaky. It has been suggested - although not yet proven - that the even older fossil Nematothallus was a lichen. although this claim was met with scepticism and has since been retracted by its author. A lichen-like symbiosis, however, has been observed in marine fossils from the Ediacaran, .

Lichen examples

Iceland moss

Oakmoss

Parmelia (lichen)

Reindeer moss

Rock tripe

Gallery

Image:Lichen_squamulose.jpg|Xanthoparmelia cf. lavicola, a foliose lichen, on basalt. Image:Usnea australis.jpg|Usnea australis, a fruticose form, growing on a tree branch Image:Rhizocarpon geographicum01.jpg|Map lichen (Rhizocarpon geographicum) on rock Image:Hyella caespitosa hypae.jpg|The cyanobacterium Hyella caespitosa with fungal hyphae in the lichen Pyrenocollema halodytes Image:LogLichen.jpg|Physcia millegrana (a foliose lichen), with an unlichenized polypore fungus (bottom right), on a fallen log. Image:caribou_moss.jpg|Reindeer moss (Cladonia rangiferina) Image:Kananakislichen.jpg|Hypogymnia cf. tubulosa with Bryoria sp. and Tuckermannopsis sp. in the Canadian Rockies Image:Lichenlimestone.JPG|Crustose lichens on limestone in Alta Murgia-Southern Italy Image:Plants flowers ice rocks lichens 209.jpg|Cladonia cf. cristatella, a lichen commonly referred to as 'British Soldiers'. Notice the red tips. Image:Plants flowers ice rocks lichens 230.jpg|Foliose lichens on rock growing outward and dying in the center. These lichens are at least several decades old. Image:Fruticose_lichen_branches_blackpine_lake.jpg|Letharia sp. with Bryoria sp. on pine branches near Blackpine Lake, Washington Image:Lichen growing on tree branch - March 2008.JPG|Lichen growing on a tree branch Image:Reddish-colored lichen on volcanic rock.jpg|Reddish-colored lichen on volcanic rock in Craters of the Moon National Monument (Idaho, USA) Further Information

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