Linked from
The 24 pages that link to Diatom, each with the reason it gives.
ProtistBroader topic: Diatoms are abundant aquatic protists with distinctive silica shells.
Secondary endosymbiosisBroader topic: Their plastids descend from a red-algal endosymbiont.
FucoxanthinBroader topic: Diatoms are major marine producers of fucoxanthin beyond brown algae.
ChromistaBroader topic: Diatoms are a major photosynthetic group commonly assigned to Chromista.