Lichens are not a single organism — they are a stable symbiosis between fungi and photosynthetic partners (algae or cyanobacteria). The fungus provides structure and water retention; the photosynthetic partner provides carbon.

They colonize bare rock, producing acids that break it down into proto-soil, enabling plant colonization — a keystone role in succession. A lichen can persist on rock that would kill most other organisms.

The classification problem: lichens were classified as plants for centuries because they look like plants. They are neither fungi nor algae alone; the symbiosis is the organism. Taxonomy had no category for them.

Related: [[mycorrhizal-networks]], [[succession-theory]]