Lichen

Lichen is a composite organism arising from a mutualistic relationship between a fungus (the mycobiont) and a photosynthetic partner (the photobiont), usually green algae or cyanobacteria. In most lichens the fungal partner provides structure and protection, while the photobiont supplies carbohydrates produced by photosynthesis. The result is a stable, resilient thallus that can inhabit environments too harsh for other organisms.

Lichen thalli come in several growth forms: crustose (crust-like, tightly attached to substrate), foliose (leaf-like), and fruticose (shrub-like). The thallus consists of a cortex, a photosynthetic layer, and a stringy or granular medulla, with the fungal network trapping moisture and nutrients.

Habitats: found on rocks, tree bark, soil, and other surfaces worldwide, from alpine to arctic to deserts. They are often among the first colonizers of bare rock and can contribute to soil formation by weathering. Many lichens are sensitive to air pollution and are used as bioindicators of environmental quality.

Reproduction: lichens can reproduce sexually via fungal spores; for establishment, these spores must encounter a compatible photobiont. Asexual forms reproduce vegetatively by structures such as soredia or isidia that contain both partners.

Ecology and uses: lichens contribute to nutrient cycles, provide food for some animals, and host diverse microbiota. They produce secondary metabolites that have ecological roles and are used in dyes and traditional medicines; and they are studied for insights into symbiosis and adaptation.