Question

why are lichens called mutualists?

who are the partners in the mutualisim?-what is exchange?

Answer 1

Lichen fungi require carbon as a food source. It forms a mutualistic relationship between a fungus and a photosynthetic organism, usually symbiotic algae and/or cyanobacteria, that are photosynthetic. The lichen symbiosis is thought to be a mutualism, since both the fungi and the photosynthetic partners, called photobionts, benefit. Tha's why, the Lichens are called mutualists.

fungus (usually a member of the Ascomycota or Basidiomycota phyla) and  photosynthetic organism (a eukaryotic alga or a prokaryotic cyanobacterium) are the usual partners in the mutualism.

The photosynthetic organism provides carbon and energy in the form of carbohydrates. Some cyanobacteria fix nitrogen from the atmosphere, contributing nitrogenous compounds to the association. In return, the fungus supplies minerals and protection from dryness and excessive light by encasing the algae in its mycelium.