Question

What are lichen? how are they useful?

Answer 1

lichen are the symbiotic association of algae and fungi they live sybiotically ( depends on each other for survival)
lichen are useful in indicating the air pollution level as they grow in non polluted areas so one can find out by seeing lichen that the area is polluted or not
hope it helps

Answer 2

Lichen are symbiotic association between algae and fungi ..they are pollution indicator.