phytoplankton vs diatom

phytoplankton and diatom both are nouns.

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

As nouns, diatom is a hyponym of phytoplankton; that is, diatom is a word with a more specific, narrower meaning than phytoplankton:

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

Other hyponyms of phytoplankton include planktonic algae.

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