Lichens

What are lichens? The term lichen refers to a symbiotic association between a filamentous fungus, the ‘mycobiont’, and at least one photosynthetic organism, the ‘photobiont’, consisting of a micro alga, a cyanobacterium, or both (Figure 1). Although the symbiotic nature of lichens was first revealed in 1867, the development of a lichen thallus is often so integrated (Figure 1D) that they have been perceived and studied as single organisms until quite recently, and have often been referred to as the ultimate example of mutualism. The name of a ‘lichen species’ corresponds to the species name of the fungal partner. Only about 100 photobiont species have been reported to be associated with more than 13,500 lichen-forming fungal species, so that many different mycobiont species share the same photobiont. More than 98% of this highly diverse group of lichenforming fungi is concentrated within the Ascomycota, the largest fungal phylum. The few remaining lichen-forming fungal species are classified within the Basidiomycota, a phylum that includes typical mushrooms. The most common lichen photobionts belong to the green algae Trebouxia and Trentepohlia, and the cyanobacterium Nostoc.