Lichens: Fungus-Algae Symbiosis
Lichens are fascinating composite organisms formed by a stable symbiosis between a fungus (the mycobiont) and a photosynthetic partner—a green alga or cyanobacterium (the photobiont). This association produces an organism capable of colonizing environments where neither partner could survive alone.
Key Points
- Lichens are fungus-algae (or cyanobacteria) symbioses
- About 20% of fungal species form lichens
- They colonize environments where partners couldn't survive alone
- Excellent indicators of air quality
A Complex Symbiosis
In the lichen, the fungus provides protective structure and retains moisture, while the alga or cyanobacterium produces sugars through photosynthesis. Recent research has revealed that many lichens also contain yeasts, adding a third partner to this symbiosis [1].
The Three Growth Forms
Lichens are generally classified by their physical appearance (morphology):
Crustose
Forms a crust tightly attached to the substrate (rock, bark). Cannot be removed without damage.
Foliose
Leaf-like with distinct lobes. Has a different upper and lower surface.
Fruticose
Forms small upright or hanging bushes. Branches are often cylindrical or flattened.
Environmental Indicators
Lichens are very sensitive to air pollution, especially sulfur dioxide. Their presence or absence is used as an indicator of air quality. In polluted urban areas, a marked decrease in lichen diversity is often observed.