Question

lichen is a combination of an alga and a fungus. how do they help each other?

Answer 1

Fungus is non green but it  provides raw  material to algae which is green and algae provides food to fungus.

Answer 2

Lichens are the symbiotic association between an algae and a fungus. The algae is generally a green  alga or blue green alga. The fungus is an ascomycete or basidiomycete. It is believed that the alga contributes organic food from photosynthesis and the fungus is able to absorb water and mineral salts. The fungus can also conserve water and this enables some lichens to grow in extreme dry conditions where no other plants can