protistan vs microorganism

protistan and microorganism both are nouns.

Word	Noun	Adjective	Verb	Adverb
protistan	Yes	No	No	No
microorganism	Yes	No	No	No

As nouns, microorganism is a hypernym of protistan; that is, microorganism is a word with a broader meaning than protistan:

protistan: free-living or colonial organisms with diverse nutritional and reproductive modes
microorganism: any organism of microscopic size

Other hypernyms of protistan include micro-organism.

protistan (noun)	microorganism (noun)
free-living or colonial organisms with diverse nutritional and reproductive modes	any organism of microscopic size