lichen

Merriam-Webster's Vocabulary Builderの"symbiousis" 項にあった例文を引用します。
The lichen that grows on rocks is produced by the symbiosis of a fungus and an alga, two very different organisms.
The prefix sym-, "with," in symbiosis produces the notion of cooperation. The symbiotic relationship between an alga and a fungus results in a creation--lichen--that neither organism could achieve on its own. Symbiotic associations are not limited to the botanical world.
Entertainment celebrities exist in symbiosis with the press that swarms about them. Neither group could exist, let alone flourish, without the other.
"lichen" は写真で見た感じでは苔の一種だと思っていましたが、"symbiosis of a fungus and an alga, two very different organisms" とはビックリです。
英英辞書での "lichen" の説明も見てみます。

・Oxford English Dictionary: A simple slow-growing plant that typically forms a low crusty, leaflike, or branching growth on rocks, walls, and trees.
Lichens are composite plants consisting of a fungus that contains photosynthetic algal cells. Their classification is based upon that of the fungal partner, which in most cases belongs to the subdivision Ascomycotina, and the algal partners are either green algae or cyanobacteria. Lichens obtain their water and nutrients from the atmosphere and can be sensitive indicators of atmospheric pollution