A lichen (also called lichen) is a fungus that lives in mutualistic symbiosis with an alga or with a blue algae, or rarely with both. The fungal component usually belongs to the Ascomycetes, only rarely to the Basidiomycetes.
Together they have a well-defined morphological structure and form a physiological unit. The fungus or mycobiont is the main symbiotic partner and can usually form reproductive organs. The mycobiont therefore determines the botanical name and the systematic classification of the lichen. The alga or phycobiont is a unicellular representative of the green algae or the blue algae.
The symbiotes are sometimes so closely connected that they cannot survive without each other. The phycobiont, the green algae or blue algae, uses photosynthesis to extract energy from sunlight and use it to form sugar alcohols, which the fungus feeds on. In turn, the mycobiont, the fungus, ensures that the algae can absorb water and minerals. The fungus also offers protection, for example against damage or excessive sun.
A lichen is a mutualistic symbiosis of a fungus and an alga, with both organisms benefiting from the interaction. The fungus, which determines the general shape of the lichen, surrounds the algae. The fungus can only make this general shape in symbiosis. The algae embedded in the fungal tissue take care of photosynthesis: they produce carbohydrates from inorganic compounds under the influence of sunlight. Instead of the sugar normally produced by plants, the algae produce special sugar alcohols (polyols) used by the fungus.
The algae also benefit from the fungus.
The fungus retains water, which comes from the air (rain and fog), so that the algae can use it for photosynthesis.
The fungus secretes acids, which help the algae absorb minerals from a mineral substrate.
The fungus lies over the algae and therefore offers protection against intensive sunlight.
Some fungi are poisonous and thus protect the algae against damage.
Lichens are fungi with a specific nutritional physiology: symbiosis. They are the textbook example of (mutualistic) symbiosis: they are the result of the close coexistence of two different types of organisms: a fungus (the "mycobiont") and a green algae and/or a blue-green algae (the "phycobiont"). These are often so closely linked that they do not have the ability to survive outside the partnership, but for many green algae it is plausible that they can also survive independently.
Many lichens grow very slowly (sometimes no more than 0.1 mm per year), and as a pioneer therefore grow mainly where they cannot be displaced by seed plants. For example, they are often found on bark or on bare rock such as tombstones, roof tiles and walls, on which they can live, and sometimes even penetrate, unlike plants. Lichens can manage with few nutrients and often get them from the rain and dust in the air. In the event of dehydration, they can also remain in a resting phase for a long time, sometimes years, and become physiologically active again after adding water. That is why lichens are an important component of life in the polar regions and the high mountains, where water is only present for much of the time in a frozen (and therefore unusable) state. Lichens are an important part of the tundra and Antarctic flora. An example in the Arctic is the reindeer moss, which is the main food of the reindeer for much of the year in Lapland. Also, lichens are among the few "organisms" that can withstand a two-week stay in the vacuum and the very strong UV radiation of the universe.
Many lichens mainly reproduce asexually by powdered