A lichen is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship. Lichens are important actors in nutrient cycling and act as producers which many higher trophic feeders feed on, such as reindeer, gastropods, nematodes, mites, and springtails. Lichens have properties different from those of their component organisms. They come in many colors, sizes, and forms and are sometimes plant-like, but are not plants. They may have tiny, leafless branches (fruticose); flat leaf-like structures (foliose); grow crust-like, adhering tightly to a surface (substrate) like a thick coat of paint (crustose); have a powder-like appearance (leprose); or other growth forms.
The Cladoniaceae are a family of lichenized fungi in the order Lecanorales. It is one of the largest families of lichen-forming fungi, with about 560 species distributed amongst 17 genera. The reindeer moss and cup lichens (Cladonia) belong to this family. The latter genus, which comprises about 500 species, forms a major part of the diet of large mammals in taiga and tundra ecosystems. Many Cladoniaceae lichens grow on soil, but other can use decaying wood, tree trunks, and, in a few instances, rocks as their substrate. They grow in places with high humidity, and cannot tolerate aridity.
The Arthoniales is the second largest order of mainly crustose lichens, but fruticose lichens are present as well. The order contains around 1500 species, while the largest order with lichenized fungi, the Lecanorales, contains more than 14000 species.
The Collemataceae are a lichenized family of fungi in the order Peltigerales. The family contains ten genera and about 325 species. The family has a widespread distribution.
The Ostropomycetidae are a subclass of mostly lichen-forming fungi in the class Lecanoromycetes. The subclass was circumscribed in 2004 by Catherine Reeb, François M. Lutzoni, and Claude Roux. It contains ten orders and 36 families.
The Baeomycetales are an order of mostly lichen-forming fungi in the subclass Ostropomycetidae, in the class Lecanoromycetes. It contains 8 families, 33 genera and about 170 species. As a result of molecular phylogenetics research published in the late 2010s, several orders were folded into the Baeomycetales, resulting in a substantial increase in the number of taxa.
The Graphidaceae are a family of lichen-forming fungi in the order Graphidales. The family contains nearly a hundred genera and more than 2000 species. Although the family has a cosmopolitan distribution, most Graphidaceae species occur in tropical regions, and typically grow on bark.
Verrucariaceae is a family of lichens and a few non-lichenised fungi in the order Verrucariales. The lichens have a wide variety of thallus forms, from crustose (crust-like) to foliose (bushy) and squamulose (scaly). Most of them grow on land, some in freshwater and a few in the sea. Many are free-living but there are some species that are parasites on other lichens, while one marine species always lives together with a leafy green alga.
The Caliciaceae are a family of mostly lichen-forming fungi belonging to the class Lecanoromycetes in the division Ascomycota. Although the family has had its classification changed several times throughout its taxonomic history, the use of modern molecular phylogenetic methods have helped to establish its current placement in the order Caliciales. Caliciaceae contains 36 genera and about 600 species. The largest genus is Buellia, with around 300 species; there are more than a dozen genera that contain only a single species.
Lichen anatomy and physiology is very different from the anatomy and physiology of the fungus and/or algae and/or cyanobacteria that make up the lichen when growing apart from the lichen, either naturally, or in culture. The fungal partner is called the mycobiont. The photosynthetic partner, algae or cyanobacteria, is called the photobiont. The body of a lichens that does not contain reproductive parts of the fungus is called the thallus. The thallus is different from those of either the fungus or alga growing separately. The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall, forming penetration pegs or haustoria similar to those produced by pathogenic fungi. Lichens are capable of surviving extremely low levels of water content (poikilohydric). However, the re-configuration of membranes following a period of dehydration requires several minutes at least.
Symbiosis in lichens is the mutually beneficial symbiotic relationship of green algae and/or blue-green algae (cyanobacteria) living among filaments of a fungus, forming lichen.
Malmideaceae is a family of crustose and corticolous lichens in the order Lecanorales. It contains eight genera and about 70 species.
Schaereria is a genus of lichen-forming fungi. It is the sole genus in the family Schaereriaceae, which itself is the only family in the Schaereriales, an order in the subclass Ostropomycetidae of the class Lecanoromycetes.
Leprocaulaceae is a family of mostly lichen-forming fungi. It is the single family in the monotypic order Leprocaulales. Leprocaulaceae contains three genera and about 33 species.
Megalospora is a genus of lichen-forming fungi in the family Megalosporaceae.
Mastodia is a genus of lichen-forming fungi in the family Verrucariaceae. It has six species. The genus was circumscribed in 1847 by Joseph Dalton Hooker and William Henry Harvey. The type species, Mastodia tessellata, is a bipolar, coastal lichen. It forms a symbiotic association with the macroscopic genus Prasiola; this is the only known lichen symbiosis involving a foliose green alga. Studies suggest that throughout its geographic range, the lichen comprises two fungal species and three algal lineages that associate.
The following outline provides an overview of and topical guide to lichens.
Sagiolechiaceae is a small family of lichen-forming fungi in the order Ostropales. It contains two genera, Rhexophiale, and Sagiolechia, the type genus. The family was circumscribed in 2010 by lichenologists Elisabeth Baloch, Robert Lücking, H. Thorsten Lumbsch, and Mats Wedin. Molecular phylogenetic analysis showed that the two genera formed a distinct clade in Ostropales. Four species were included in the original circumscription of the family.
Hydropunctaria rheitrophila is a species of freshwater, saxicolous (rock-dwelling), crustose lichen in the family Verrucariaceae. It was formally described as a new species in 1922 by German lichenologist Georg Hermann Zschacke as a species of Verrucaria. Christine Keller, Cécile Gueidan, and Holger Thüs transferred it to the newly circumscribed genus Hydropunctaria in 2009. It is one of several aquatic lichens that are in this genus. The photobiont partner of Hydropunctaria rheitrophila is a yellow-green alga.
Wahlenbergiella mucosa is a species of saxicolous (rock-dwelling), crustose lichen in the family Verrucariaceae. It is a marine species that grows in the littoral zone, and therefore remains immersed in seawater for extended periods. Its photobiont partner is the green alga Paulbroadya petersii.
