Lichen

A lichen is not one species or a single organism in the ordinary sense, but a stable association whose central partners are a fungus and a photobiont. The photobiont is usually a green alga, a cyanobacterium, or both. Through photosynthesis it produces carbohydrates that the fungal partner uses. The fungus forms much of the lichen's body, called a thallus, and helps create the conditions in which the association lives. Additional yeasts and bacteria have also been found in some lichens, so the full community can be more complex than a two-part partnership. Scientists continue to study how benefits and costs are shared among the partners; describing every lichen relationship as a simple, equally beneficial mutualism can oversimplify this diversity.

Lichens can grow on surfaces such as bark, rock, or soil. Three commonly described thallus forms are crustose, which forms a crust closely attached to a surface; foliose, which is leaf-like; and fruticose, which is branched, shrubby, or hanging. Lichens do not have the roots and vascular tissues of plants. They take up water and nutrients from their surroundings, including air and rain, and many can tolerate drying before resuming activity when moisture returns.

The fungal partner can produce spores, but a spore alone does not contain the photosynthetic partner needed to establish the same lichen association. It must encounter a suitable photobiont. Lichens also disperse vegetatively: fragments of the thallus, or specialized structures such as soredia and isidia, can carry fungal and photosynthetic partners together and grow into new thalli when conditions are suitable.

Lichens occupy a wide range of habitats and can colonize exposed surfaces. Some contribute to weathering and soil development, provide food or habitat, or participate in nutrient cycling; the details depend on species and ecosystem. Because lichen species and communities can respond to pollutants and accumulate elements from their surroundings, researchers use them in biomonitoring and air-quality studies. Their value as indicators depends on species, location, and sampling methods, so measurements require careful interpretation.