Overview

Lichens are composite organisms that arise from a long-term association between a fungus and one or more photosynthetic partners. They grow on a wide range of surfaces — rock, soil, bark, roofs and even man-made materials — and often appear plant-like, although they are not plants. The visible body of a lichen is called the thallus and represents the integrated result of the partners living together.

Partners and relationship

The dominant partner is usually a fungus (the mycobiont) while the photosynthetic partner (the photobiont) may be a green alga or a cyanobacterium. In many lichens the two partners exchange nutrients: the photobiont supplies carbohydrates from photosynthesis, while the fungus provides mineral nutrients, water retention and physical protection. Scientists commonly discuss the fungal partner and the photosynthetic partner when describing this relationship.

Forms and structure

Lichen thalli come in several basic growth forms, which are useful for identification and ecological description:

  • Crustose — flat crusts tightly attached to the substrate.
  • Foliose — leaf-like, with an upper and lower surface and loose attachment.
  • Fruticose — bushy or shrubby, often three-dimensional and branched.

Reproduction and lifecycle

Lichens reproduce in multiple ways. The fungal partner may reproduce sexually, producing spores, while lichens also disperse via asexual propagules such as soredia or isidia that contain both partners. Successful establishment requires recolonization of both partners at a site or dispersal of joint propagules that already include the photobiont.

Ecology, uses and importance

Lichens are ecologically important as pioneers on bare rock, contributing to soil formation by physical and chemical weathering. They are sensitive to air pollution and are widely used as bioindicators of atmospheric quality. Humans have used lichens for dyes, traditional medicines, and fragrance fixatives; some species also have research interest for antimicrobial compounds.

History and notable facts

Historically lichens were thought to be single plants; in the 19th century scientists proposed their dual nature and that idea gradually gained acceptance. Lichens demonstrate complex symbioses that can involve multiple microbial partners beyond the classic fungal and algal components, making them a subject of ongoing ecological and microbiological research.