diatom vs alga

diatom and alga both are nouns.

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

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

diatom: microscopic unicellular marine or freshwater colonial alga having cell walls impregnated with silica
alga: primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves

Other hypernyms of diatom include phytoplankton, algae.

diatom (noun)	alga (noun)
microscopic unicellular marine or freshwater colonial alga having cell walls impregnated with silica	primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves