Why lichens are called dual organism?

Lichens have symbiotic relationship between algae and fungus hence they are called as dual organisms.

Lichens grow from the substrate of fungi in an algae environment.

The algae and fungi are in mutual relationship where algae help to produce food and fungus help to absorb the nutrients. Lichens are of various size and forms, divided into thallus and filament body parts.

Lichens are of variety of colors due to the presence of usnic acid.