Lichens are symbiotic organisms made up by the association of microscopic green algae or Cyanobacteria and filamentous fungi. 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 the 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 cells contain Chlorophyll, permitting them to live in a purely mineral environment by producing their own organic compounds (see Photosynthesis). The fungus protects the alga against drying out and, in some cases, provides it with minerals obtained from the Substratum. If a cyanobacterium, such as in Terricolous Lichens, is present this can fix atmospheric nitrogen, complementing the activities of the green alga.
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. 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 cephalopodia. 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 homogenous, it may consist of several different species of fungus and photobiont living together.
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.
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. Although lichens typically grow in harsh environments in nature, many lichens are sensitive to man-made pollutants. Hence, they have potential as pollution indicator organisms. When growing on mineral surfaces, some lichens slowly degrade their substrate by secreting acids that dissolve 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 employ of mountain climbing conservators to clean the monument.
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. The Lichens were exposed to temperatures ranging from -20°C to 20°C, the vacuum of space and radiation for 15 days. Afterward, they were found to be in the same state as the start of the experiment.