diatom vs phytoplankton

diatom and phytoplankton both are nouns.

Word	Noun	Adjective	Verb	Adverb
diatom	Yes	No	No	No
phytoplankton	Yes	No	No	No

As nouns, phytoplankton is a hypernym of diatom; that is, phytoplankton is a word with a broader meaning than diatom:

diatom: microscopic unicellular marine or freshwater colonial alga having cell walls impregnated with silica
phytoplankton: photosynthetic or plant constituent of plankton; mainly unicellular algae

Other hypernyms of diatom include alga, algae.

diatom (noun)	phytoplankton (noun)
microscopic unicellular marine or freshwater colonial alga having cell walls impregnated with silica	photosynthetic or plant constituent of plankton; mainly unicellular algae