Lichens are symbiotic association of Algae and Fungi. What is symbiosis?

Explanation

Symbiosis is a close physical associa-tion between the individuals of a pair of species.
b)1.They are pioneers of rock,vegetation.
2.They are source of food and fodder.
3.Medicinal uses: Peltigera canina is used as acure for hydrophobia, hobaria pulmonaria is used in the treatment of lung disease.
4.Lichens can be used as indicator of air pollution.