Lichens

An Introduction to: the Lichens

There are around 20,000 species of lichen worldwide, and 1,800 recorded in Britain. Lichens are a combination of two different organisms, a fungus and an algae or cyanobacteria, working together symbiotically for the mutual benefits of survival. The fungi partner (mycobiont) of lichen require carbon as a food source, and this is provided by the algae or cyanobacteria (photobiont), that are photosynthetic. The lichen symbiosis is thought to be a mutualism, since both the fungi and the photosynthetic partners benefit. One in five of all known fungi form lichens.

Typically the fungal element of lichen makes up around 80% or more of the bulk of the mass or body of the lichen called the thallus, and the remaining algal constituents, the cells, are buried within it. Lichen can take on many varied forms, and can thrive on many different kinds of substrate to which it attaches itself. In natural environments lichen can grow on the trunks and branches of living trees or dead trees, on rocks and stones, on moss and other lichens, bone, shells, and earth. In urban environments they thrive on brick walls, mortar, roof tiles, concrete, paving slabs and tarmac, on gravestones and monuments, timber structures such as gates, fences, posts, benches and sheds, old rusted metal items, textiles, and even glass and plastics.

The different forms of lichen help lichenologists and those with a keen interest in the subject of lichenology identify species. Lichens can take on many different forms, and below is an overview of three of the main forms:

Crustose lichens have a crust-like or flattened structure tightly adhering to the substrate surface, such as rocks, bark, or soil. They often form a thin, continuous layer that is closely appressed to the substrate. Crustose lichens can vary in texture from smooth to rough or granular.
Foliose lichens have a leafy or lobed structure, with the thallus (lichen body) divided into distinct lobes or leaf-like structures. These lobes are often loosely attached to the substrate, allowing air and moisture to circulate beneath them. Foliose lichens can be found on tree bark, rocks, and soil, and they come in a variety of colours and textures.
Fruticose lichens have a branching or bushy growth form, with the thallus forming three-dimensional structures resembling miniature shrubs or tufts. Fruticose lichens can be upright or pendant, and they may grow on a wide range of substrates, including bark, soil, and rocks. They are often more flexible and mobile than other lichen forms.

Lichen structure is fairly complex. The thallus serves as the main body of the lichen, comprising fungal hyphae intertwined with photosynthetic algae or cyanobacteria. It varies in shape and form across species, appearing as crust-like (crustose), leafy (foliose), shrub-like (fruticose), or gelatinous (gelatinous). The prothallus is a specialized structure found in certain lichen species during their early developmental stages. It serves as the initial stage of lichen colonization and establishment on a substrate, providing a foundation for fungal and algal colonization. The prothallus is typically a thin, inconspicuous layer that forms on the substrate surface, secreting enzymes and compounds to break down organic matter and prepare the substrate for colonization by fungal hyphae and photosynthetic algae or cyanobacteria. As the prothallus matures, it evolves into the characteristic thallus structure of the lichen, contributing to its growth and development. Surrounding the thallus is the cortex, a protective outer layer made of densely packed fungal hyphae, shielding the lichen from environmental stressors like desiccation and UV radiation. Beneath the cortex lies the medulla, an inner layer of loosely arranged fungal hyphae that support and protect the photosynthetic partner, also potentially storing water and nutrients. Rhizines, specialized structures on the lower surface of foliose and fruticose lichens, anchor the lichen to the substrate, providing stability. Reproductive structures called apothecia, typically found in some lichen species, facilitate sexual reproduction by producing spores. Finally, soredia, asexual reproductive structures consisting of fungal hyphae and photosynthetic cells, disperse from the thallus and germinate to form new lichen individuals, common in foliose and crustose lichens. These structures collaborate to enable lichens to thrive in diverse habitats, fulfilling various ecological roles by combining the resources and capabilities of both fungal and photosynthetic partners.

Of note, lichenologists and lichen enthusiasts tend to use chemical spot-tests to analyse and help in identifying some species of lichen.

Classification

Kingdom:
Phylum:
Subphylum:
Class:

Fungi
Ascomycota (Ascomycete Fungi)
Pezizomycotina (Higher Ascomycetes)
Lecanoromycetes (Common Lichens)

Family Cladoniaceae