microphyte
/ˈmʌɪkrə(ʊ)fʌɪt/Biology
noun
Any microscopic or very small organism considered (or at one time considered) to belong to the plant kingdom; especially a microscopic alga.
Origin
Mid 19th century; earliest use found in David Page (1814–1879), geologist and author. From micro- + -phyte. Compare French microphyte.