Here's why:
* Lichen is a symbiotic relationship: It's a combination of a fungus and an alga living together. So, an antonym would need to represent something that's not a symbiotic organism.
* Lichen is a specific type of organism: It's not like a plant, animal, or bacteria.
However, we can think of words that represent opposites in terms of function or appearance:
* Individual organisms: "Fungus" or "alga" would be opposite in the sense that lichen is a combination of both.
* Free-living: An antonym might be something that's not dependent on another organism for survival, like a tree or a flower.
* Complex structure: An antonym could be something simple and single-celled, like a bacterium.
Ultimately, the best antonym depends on the context of your question. Can you provide more information about how you're using the word "lichen"?