Mycobionts

Mycobionts are the fungal partners in lichens, forming a mutualistic association with a photosynthetic partner known as the photobiont (typically green algae or cyanobacteria). In most lichens the mycobiont is an Ascomycete, though Basidiomycetes are involved in a few cases. The fungal partner provides the protective body of the lichen, forming the thallus, and creates the hydrated microenvironment in which the photobiont can photosynthesize. It also contributes enzymes and minerals needed for nutrient exchange with the photobiont and helps the organism adhere to substrates in often extreme habitats.

The photobiont supplies carbohydrates produced by photosynthesis, which the mycobiont uses for growth and maintenance. In lichens that include cyanobacteria as photobionts, nitrogen fixation may also occur, enriching the symbiotic unit. The two partners exchange nutrients in a highly integrated, though not always uniform, relationship. The mycobiont largely controls the external morphology of the lichen, while the photobiont's photosynthetic capacity influences the thallus’s color and productivity.

Reproduction can involve the fungal partner alone, producing sexual spores that must re-establish a new photobiont partner in the environment, or through vegetative propagules such as soredia or isidia that contain both mycobiont and photobiont, enabling immediate colonization of new substrates.

Ecologically, mycobionts enable lichens to inhabit a wide range of substrates, from bare rock to tree bark, often serving as indicators of air quality and contributing to soil formation and nutrient cycling in various ecosystems.