Liquen

Liquen is a composite organism resulting from a symbiotic relationship between a fungus (the mycobiont) and a photosynthetic partner (the photobiont), usually green algae or cyanobacteria. The fungal component provides the thallus structure and protection, while the photobiont supplies carbohydrates through photosynthesis. The association is tightly integrated, and most lichenized fungi depend on their photobiont for energy.

Although often treated as a single organism, a lichen is a complex community with several growth forms: crustose (crust-like), foliose (leaf-like), and fruticose (shrub-like). The thallus architecture allows lichens to colonize harsh substrates such as bare rock, bark, and soil, often in extreme environments where few other organisms can survive.

Life cycle and reproduction: lichens reproduce sexually by fungal spores that must encounter a compatible photobiont to form a new lichen, or asexually by fragmentation, or by dispersal units called soredia or isidia that contain both partners. Growth is typically slow, and many species accumulate minerals that contribute to rock weathering and soil formation.

Ecological and human relevance: lichens are important indicators of air quality because they are sensitive to atmospheric pollutants. They contribute to nutrient cycles in ecosystems and provide food and habitat for some invertebrates. Humans use lichens for dyes, perfumes, medicines, and traditional practices; lichenometry is a dating method in geology and archaeology that uses lichen growth to estimate surface exposure time.

Conservation: due to their sensitivity to environmental change, many lichen species face threats from habitat loss, pollution, and climate change. They are protected in some regions, and monitoring programs track their distribution as a measure of ecosystem health.