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Lichens are emergent symbiotic community of organisms that are comprised of two or three primary organisms and potentially hundreds of other internal and external community members. While we can culture these organisms (or at least some of them) individually, they look very different. It is only in they union that the forms and functions of the lichen system emerge.

The Fungi

• Mycobiont– The majority of the lichen body, or thallus (pl. thalli), is comprised of the mycelium of a single fungus, from which the name of the community it determined. This mycobiont shapes the home for all the residents, while assisting in the production of an array of chemical compounds. There are 14,000 known lichenizing fungi (mycobionts), with 3,000 or so thought to be yet undescribed. Around 98% of the names species are Ascomycotan, while a few lichenized basidiomycotan fungi are found in boreal, temperate, and tropical regions. Most mycobionts seem to have evolved to be dependent on their photobiont partner.
• Endolichenic fungi – These fungi live inside the lichen and are poorly studied though have already been found to offer many unique bioactive compounds.
• Lichenicolous fungi – These fungi live on the surface of lichens and can be parasitic, often in relation to specific host lichens.
• Jelly Fungi Yeasts (?!) – Recently it was discovered the the yeast state of basidiomycotan jelly fungi in the genus Tremella are common in a wide number of lichens, seemingly playing a critical role in the community’s structure and functioning.

The Photobionts

There are only around 100 species, most of which are free-living and are not dependent on a specific mycobiont group.

• Cyanobacteria – Nostoc species are among the most common. They can be the primary photobiont, share a thallus with an algae, or live in specialized cephalodia where they fix nitrogen for the thallus.
• Algae ­– The genera Trebouxia and Myrmecia are the most common photobionts and are rarely found free living
• Red and brown algae – A small number of lichen species in the Verrucariaceae host red or brown algae as their primary photobiont.

Other Organisms

• Epibiont bacteria – These organisms live on the surface of lichens and may assist in nitrogen fixation. These bacteria can often be distinctly different in community structure than the bacteria in surrounding soils.
• Inner, nitrogen-fixing bacteriaare also thought to be a third player in algal lichens.
• Tardigrades
• Viruses

Lichen Morphology and Reproduction

Lichens Defined

The lichen thallus is comprised of an outer cortex that is like a skin, within which is the medulla of hyphae that houses the photobiont and other organisms. In algal lichens stratification is often apparent and measurable, while many jelly cyanolichens have a more mixed interior that is not as well defined.

Some thalli host cyanobacteria in specialized structures known as cephalodia that are akin to the nodules on the roots of leguminous plants, wherein nitrogen is fixed for the entire thallus. Larger, lobe-like versions of cephalodia are known as photosymbiodemes.

Lichens are typically first described by their general form. Fruticose species are highly branched and well-equipped to obtain moisture from the air. Foliose species are akin to solar panels and are most often found in temperate forests, often attached to their substrate via rhizines. Crustose species form layered areolas as a thin thallus on rock and bark, where they can penetrate to a light depth to establish and obtain some nutrition.

Thallus reproduction is done by self-differentiation through the dispersal of isidia diaspores (projections), soredia diaspores (eruptions), or small lobules. The mycobiont may release asexual conidiospores from sunken structures known as pycnidia. Or it may release sexual ascospores from perithecia (sunken chambers) or apothecia (cups). Basiomycotan lichenenizing fungi that produce fruit bodies will reproduce by releasing basidiospores.

The reproduction of the photobionts in lichens seems to be suppressed by the mycobiont as sexual production is rarely seen in photobionts within lichens. It is though this suppression keeps the photobiont at an ideal state for engaging with the mycobiont.

Lichen Chemistry and Resilience

Lichens Defined

Lichens exemplify the notion of chemical mastery among fungi as over 1,000 secondary metabolites have been identified in lichens to date, many of which are exclusive to these mini-ecosystems. The most abundant classes of lichen secondary metabolites are phenolic compounds, dibenzofurans, depsides, and depsidones (e.g. norstitic acid). Various chemical tests are used to elicit a color change in lichen tissue that indicates the presence of these and other compounds – such tests are often need for identifying some genera to species.

These compounds might help the light reflect or absorb light, defend against predation, or provide antibiotic support to humans. Some compounds are hydrophobic and help create openings in the thallus that allows for gas exchange. The ecological significance – let along human application – of the compounds produced by lichens is barely understood, though will likely be of greater and greater value in the coming years.

It is likely these compounds that are to thank for the incredible resilience found in lichens on the whole, with some species able to survive temperatures ranging -196ºC (-320ºF) to 90ºC (194ºF), as well as the vacuum and radiation of outer space!

In Forests

Lichens as Habitat (Indicators)

In forested systems lichens perform a number of roles, including:

• Thalli fix nitrogen while also taking in a variety of minerals and nutrients from the air. When they are rained on or fall and decompose, these lichen-derived nutrients are added to the soil.
• White, green, and blue crustose lichens protect the bark of hardwood saplings from herbivory.
• Thalli help trap dew and moisture that adds to the cool and cooling climate of the forest interior, and may have a slight influence on downwind precipitation patterns.
• Thallis serve as food for insects and small and large mammals. Some species, such as Horsehair Lichens (Bryoria spp.) are the primary winter forage of keystone species, including woodland caribou. While in Alaska, mountain goats are known to eat protein-rich Lobaria Up to 93% of the winter and spring diet of the northern flying squirrel is made of Bryoria and Usnea
• Some squirrels and birds use lichens as nesting material. At least fifty bird species are known to use fruticose lichens in nesting material, including hawks, chickadees, thrushes, and warblers (e.g. the parula warbler). Hummingbirds will use Parmelia sulcata to camouflage their nests.
• Some insects such as Lacewings use lichens as camouflage, but there are likely many more intersections to be discovered.
• In boreal forests, Stereocaulon ground cover helps increase moisture retention, while adding organic matter and trapping seeds and water.

Surveying Lichens

Lichens as Habitat (Indicators)

Alongside the many questions of lichen formation, ecology, chemistry, and historical importance still to be further investigated, there are several area of lichen research with more immediate and hands on applications for professional and amateur researchers.

As indicators of habitat health, lichens can be surveyed in several ways to determine the impacts of air pollution (whether from industry, automobiles, or agriculture) on air quality. This is done is by monitoring morphology or diversity changes in populations over several years, or by sampling thalli periodically for their accumulation of pollutants, such as heavy metals. This work was pioneered in 1970 by Hawksworth and Rose, and was practiced during the 1970s and 1980s across England, Scotland, and Ireland. The USDA and US Forest Service continue to use similar practices for monitoring environmental health. Some of the most sensitive lichens are filamentous, fruticose species, such as those in the genera Usnea, Ramalina, and Teloschistes, as well as cyano-foliose species in the genera Lobaria, Pannaria, and Nephroma.

Lichenometry compares the growth rates of lichens to determine the amount of time a rock surface has been exposed, as in the retreat of glaciers, lake level changes, seismic events associated with the rockfalls, and changes in permafrost and snow cover levels. Rhizocarpon species are most commonly employed in this practice.

Key Texts

• Monitoring Lichens as Indicators of Pollution – Purvis, O. and Minis, P. (2005)
• Biological monitoring: lichens as bioindicators of air pollution assessment – A review – Conti, M. and Cecchetti, G. (2001)
• Biological Soil Crust Rehabilitation in Theory andPractice: An Underexploited Opportunity – Bowker (2007)
• Radical Mycology – Chapter 5 (written by lichenologist Nastassja Noell)
• Two Basidiomycete Fungi in the Cortex of Wolf Lichens – Tuovinen, V. et al. (2019)
• Bacterial communities associated with the lichen symbiosis – Bates, S. et al. (2010)
• Lichens their biological and economic significance – Perez-Llano, G. (1944)
• Utilization of lichens in the arctic and subarctic – Llano, G. (1956)
• Lichen Diversity and Stand Continuity in the Northern Hardwoods and Spruce-Fir Forests of Northern New England and Western New Brunswick – Selva, S. (1994)
• The 12 essays at the Ways of Enlichenment by Trevor Goward
• Endolichenic fungi: the lesser known fungal associates of lichens – Suryanarayanan, T. (2017)
• The Role of Lichens in Boreal Tundra Transition Areas – Kershaw, K. (1978)
• Survival of Antarctic Cryptoendolithic Fungi in Simulated Martian Conditions On Board the International Space Station – Onofri, S., et al. (2015)
• Response of desert biological soil crusts to alterationsin precipitation frequency – Belnap, J. (2003)
• Nitrogen fixation in biological soil crusts from southeast Utah, USA – Belnap, J. (2002)
• Biological Soil Crusts: Ecology and Management
• Biocrust science and global change – Reed, S. (2019)
• Novel hallucinogens from eastern Ecuador

Lichens as Habitat Indicators

• Lichens as Bioindicators of Air Quality – USDA
• Lichens as rapid bioindicators of pollution and habitat disturbances in the tropics – Hawksworth, D. et al.
• Epiphytic Lichens: Their Usefulness as Bio-indicators of Air Pollution – Sett, R. and Kundu. M. (2016)
• Lichens as Indicators of Air Pollution – Nash, T. and Gries, C.
• Qualitative Scale for estimating Sulphur Dioxide Air Pollution in England and Wales using Epiphytic Lichens – Hawksworth, D. (1970)
• Lichens and pollution monitoring – Richardson, D., et al. (1981)
• Air-quality.org

Online Lichen Identification Resources

• A Cumulative Checklist for the Lichen-forming, Lichenicolous and
  Allied Fungi of the Continental United States and Canada
• Air Pollution Sensitivity Ratings for Macrolichens in the US Pacific NW
• The lichens of the coastal douglas-fir dry subzone of British Columbia – Noble, J (1982)
• An extensive list from Harvard
• USDA Lichen Illustration and Photograph Databases
• Ways of Enlichenment
• Sharnoff’s Lichens
• U.S. Bureau of Land Management’s Lichen conservation recommendations