The dual nature of lichens
A fungus that has caught an alga to suck out its nutrients – this is one way lichens could be described. Those two very different organisms constitute the body of lichens, but the fungus has the best of it. Lichens are mainly composed of hyphae (fungus strings) forming different layers inside the lichen.
The fungus itself cannot acquire energy from the sun through photosynthesis, but has specialised in using small algae, usually green algae, for that purpose. Those are hidden among the hyphae making sugar for the fungus. There are not many species of algae involved in the lichen symbiosis. The same algal species is found in different species of lichens. Some lichen species have nitrogen-fixating cyanobacteria instead of algae.
Lichens have different appearances; they can be fruticose (like little bushes), foliose (leaf like) or crustose. Their appearance is decided by the fungus and lichen together. At Uppsala University the different fungi involved in lichens have been cultivated in a laboratory. By studying them in detail scientists can figure out how they are related to each other and to other fungi.