Phylum

The phylum Ascomycota contains the vast majority of lichen-forming fungi, accounting for approximately 98–99% of all known lichens. In lichen symbioses, ascomycetes act as the mycobiont, forming the structural and protective framework that houses the photosynthetic partner. They are characterised by the production of sexual spores within sac-like structures known as asci, typically contained in fruiting bodies such as apothecia or perithecia.

Ascomycetous lichens exhibit an extraordinary range of growth forms, including crustose, foliose, and fruticose types, and colonise diverse substrates such as bark, stone, soil, wood, and historic materials. Major lichen-forming classes within Ascomycota include Lecanoromycetes, Arthoniomycetes, Eurotiomycetes, and Dothideomycetes. Members of this phylum play key ecological roles in primary succession, nutrient cycling, and environmental monitoring. Their sensitivity to air quality, moisture, and substrate chemistry makes ascomycete lichens especially valuable as bioindicators and important organisms in biodiversity conservation and heritage science.

The phylum Basidiomycota includes a small but evolutionarily significant group of lichen-forming fungi, representing approximately 1–2% of all known lichens. In these lichens, the basidiomycete fungus acts as the mycobiont and forms a symbiosis most commonly with cyanobacteria, particularly Nostoc, although green algal partners also occur.

Basidiolichens differ from the more common ascomycete lichens by producing sexual spores on basidia rather than in asci. They are mainly found in the classes Agaricomycetes and Pucciniomycetes, and often exhibit filamentous, gelatinous, or clavarioid growth forms. Well-known genera include Cora, Dictyonema, Lichenomphalia, and Multiclavula, many of which are distributed in tropical and subtropical regions.

Although less diverse, basidiolichens are of high scientific importance, offering key insights into the evolution of lichen symbiosis, fungal diversification, and alternative pathways of lichenisation within the fungal kingdom.