|
( Lichen)
Lichens (IPA /'la?k?n/[1] or /l?t?.?n/[2]) 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 a 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 phycobiont is sometimes penetrated by haustoria from the mycobiont, but with the development of electron microscopy there is 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 showing leaching from the canopy mosses in Guadaloupe of numerous metabolites immediately following rehydration. Not only do the two bionts profit, but also 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 components 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 fungus, 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 [1]. 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.”
|
Lichen Subcategories
Lichen Articles
|
|
