Lichenization

Lichenization is the biological process by which a fungus forms a lichen through a symbiotic association with photosynthetic organisms, typically green algae or cyanobacteria. In most lichens, the fungal partner, called the mycobiont, provides the structure, protection, and moisture retention needed for survival in exposed environments, while the photosynthetic partner, the photobiont, supplies sugars produced through photosynthesis. The resulting composite organism is a lichen, consisting of the mycobiont and photobiont living in a close, interdependent relationship.

Photobionts are commonly single-celled green algae from genera such as Trebouxia or cyanobacteria such as Nostoc. The lichen thallus, the visible body of the organism, can take on crustose, foliose, or fruticose forms, often enabling colonization on rock, bark, soil, or other substrates in harsh or nutrient-poor environments.

The degree of mutual dependence in lichenization can vary; many researchers describe the relationship as mutualistic, though the exact balance of benefits may differ among pairs. The fungus generally provides a protected microhabitat and supplies minerals and water, while the photobiont contributes carbohydrates via photosynthesis. Reproduction in lichens can occur sexually by fungal spores that must locate a compatible photobiont, or asexually through structures such as soredia or isidia that contain both partners.

Lichens are widespread and ecologically important, often pioneering bare substrates and contributing to soil formation, nutrient cycling, and bioindication of air quality. The study of lichenization continues to illuminate the complexity of symbiotic life in fungi.