Lichens are not one, but two organisms, a partnership of two kingdoms. Lichen is made of a fungus (usually ascomycete) and an alga (gray or green).
There are 20,000 kinds of lichen, each with a different variety of fungus. There are fewer algae in many lichens; so fewer algae are involved.
A lichen body is made of sturdy fungus hyphae, and the alga is sheltered within. The fungus provides the alga with water and mineral nutrients; the alga in turn leaks out nutrients, which is absorbed by the fungus.
This partnership is so tough that lichen can live where no other creature can live. If the weather turns too hot, cold, or dry, the lichen goes into a state of suspended animation until the environment is desirable.
Because lichens are only 5% algae, and because the alga grows very slowly, lichen grow only about 5 mm a year. Therefore lichen live for centuries, or in a few cases, millennia.
Lichens only weakness is that in getting nutrients from rain in polluted areas where sulphur dioxide is dissolved into the rain, lichen absorbs the poison and often dies as a result.
This is a perfect example of symbiotic relationship.