Lichens

Lichens are not a single organism but a stable association between a fungus (mycobiont) and a photosynthetic partner (photobiont), usually a green alga or a cyanobacterium. The fungus provides structure, water retention, and protection, while the photobiont produces sugars via photosynthesis. This mutualistic relationship allows lichens to colonize harsh environments where neither partner could survive alone.¹

Recent genomic studies have revealed that lichens often harbor additional microorganisms, including basidiomycete yeasts, which may contribute to the production of secondary metabolites and influence the lichen's phenotype.²

Lichens exhibit three main growth forms: crustose (crust-like, tightly attached to substrate), foliose (leafy with distinct lobes), and fruticose (shrubby or pendulous). These forms are not taxonomic but ecological, reflecting adaptations to moisture and light availability.

Internally, lichens have a layered structure: an upper cortex, a photobiont layer, a medulla, and often a lower cortex. Some lichens have a gelatinous appearance when wet due to cyanobacteria that swell with water.

Lichens reproduce both sexually and asexually. Sexual reproduction involves the fungal partner producing spores that must encounter a compatible photobiont to form a new lichen. Asexual reproduction occurs via soredia (clusters of fungal hyphae and photobiont cells) or isidia (outgrowths containing both symbionts), which are dispersed by wind or animals.³

Some lichens also produce vegetative propagules called lobules or produce conidia that may act as spermatia in sexual reproduction.

Lichens are pioneer species that colonize bare rock, soil, and bark, contributing to soil formation by physically and chemically weathering substrates. They are particularly abundant in extreme environments such as arctic tundra, alpine peaks, and deserts, where they dominate the vegetation.⁴

Lichens are sensitive to air pollution, especially sulfur dioxide, making them valuable bioindicators of air quality. Their presence or absence can signal environmental health.

Lichens have been used for dyes, traditional medicines, and food. For example, litmus, a pH indicator, is derived from lichens such as Roccella tinctoria. In Nordic countries, reindeer lichen (Cladonia rangiferina) is a crucial winter forage for reindeer.⁵

Lichens produce unique secondary metabolites, such as usnic acid, which have antibiotic and UV-protective properties, and are being explored for pharmaceutical applications.

Lichens have been used in forensic science to estimate the time of death by analyzing the growth of lichens on remains. They also serve as proxies for climate change, as their growth rings can indicate past environmental conditions.

Some lichens are capable of surviving in space: experiments on the International Space Station showed that lichens can withstand exposure to cosmic radiation and vacuum, suggesting potential for panspermia.⁶

Lichens have a rich history in heraldry and art; the green pigment in some medieval manuscripts was derived from lichens. Additionally, the study of lichens, lichenology, has a dedicated community and journals.

Mycobiont

The fungal partner in a lichen symbiosis.

Photobiont

The photosynthetic partner (alga or cyanobacterium) in a lichen.

Crustose

A lichen growth form that is crust-like and tightly attached to the substrate.

Foliose

A lichen growth form with leafy lobes.

Fruticose

A lichen growth form that is shrubby or pendulous.

Soredia

Asexual reproductive structures consisting of fungal hyphae and photobiont cells.

Isidia

Asexual reproductive outgrowths containing both symbionts.