Why were lichens identified as a separate systematic group of organisms?

1) Lichens are an unusual symbiosis of fungus and algae, that is, it is not a mushroom or a plant.

2) According to the method of obtaining food, they are autotrophs, synthesized by photosynthesis and form organic compounds, and heterotrophs, with the help of mycelium filaments, are able to consume nutrients.

3) They absorb water with their entire surface: dew, fog, rains, therefore they can settle in bare, unsuitable places for living, and when dying, they form a nutritious fertilizer, thereby ensuring the continuation of the life of other plants.