Lichens

Some General Characteristics

• A lichen is a symbiosis between a fungus and an alga and/or cyanobacterium. (In some cases the symbiosis may involve more than one fungus, and in some cases more than one alga.)
• The fungal partner in the symbiosis is referred to as the mycobiont and the photosynthetic partner is called the photobiont.
• The relationship is not considered to be physiologically obligate—most mycobionts and all photobionts studied have been successfully grown in culture without their symbiotic partners (Honegger 1998). However, fungi that are capable of forming lichens are essentially never found in nature in a non-lichenized form, so the symbiosis is often described as ecologically obligate for the fungus. Many photobionts, on the other hand, are known to be free-living in natural environments, but some, such as Trebouxia, are only rarely encountered outside of lichens.
• Lichens are referred to by the scientific name of the fungal partner. The lichen symbiosis itself does not have a scientific name.
• In a lichen symbiosis, the photobiont supplies the mycobiont with organic carbon (carbohydrates), and in some cases usable nitrogen (i.e., if the photobiont is a cyanobacterium). In exchange, the mycobiont provides the photobiont with a protective environment, water, and nutrients.
• A mycobiont grown in culture without its photobiont forms colonies similiar in appearance to those of closely-related non-lichen-forming fungi. When a free-living mycobiont comes in contact with a compatible photobiont, it grows around the photobiont, so that its thallus (the body of the fungus) encloses the photobiont. Some mycobionts, upon acquiring a photobiont, undergo extreme morphological transformations, taking on complex, plant-like forms— appearing leaf-like or even shrub-like. Lichens may have branches, with leaf-like structures containing algal cells, and, in some cases, branching root-like structures that attach to and penetrate a substrate (Sanders 2001).
• The association between mycobiont and photobiont is fairly specific. Mycobionts can't form symbioses with just any available alga or cyanobacterium— they are adapted to accommodate only certain taxa. Most mycobionts only form symbioses with photobionts belonging to a specific group (e.g., a particular genus, or in some instances a single species).
• Photobionts found in lichens include chlorophyte green algae, cyanobacteria, and in rare instances brown algae (Class: Phaeophyceae) or yellow-green algae (Class: Xanthophyceae) (Tschermak-Woess 1988). The majority of lichenized fungi form associations with green algae. Of the species of macrolichens found in New England, 74% contain only a green alga, 21% have only a cyanobacterium, and about 5% have both a green alga and a cyanobacterium (Hinds and Hinds 2007).
• Almost all mycobionts are sac fungi (Division Ascomycota); a small number are club fungi (Division Basidiomycota).
• Lichens may reproduce asexually by producing vegetative propagules (dispersal units) that contain both fungal and algal cells, or by fragmentation of the thallus, which also allows both symbiotic partners to be dispersed as a unit.
• Lichens also reproduce sexually, but it is only the mycobiont that undergoes sexual reproduction. The fungal offspring that are produced must then find compatible photobionts to form new lichens.

Groups of Lichens

Lichens may be divided into two groups: crustose lichens (or microlichens) and macrolichens.

The crustose lichens tend to be tightly attached to a substrate or may be found inside a substrate. Some crustose lichens are microscopic and most require a microscope for identification. Many crustose lichens found in New England have not yet been delimited into species (Hinds and Hinds 2007).

Compared to crustose lichens, macrolichens are typically larger, more conspicuous and easier to identify; consequently, they have been studied to a much greater extent. Macrolichens include foliose, fruticose, umbilicate, large squamulose and filamentous growth forms. A total of 461 species of macrolichens are known to occur in New England (Hinds and Hinds 2007).