Microscopic photosynthetic organisms with
siliceous secondary plant walls termed "frustules." Mostly 20 - 100 µm,
and generally of two forms: pennate or centric. Marine and freshwater
organisms of various ecological preferences -- planktonic, benthic, etc.
Diatoms are destroyed by hydrofluoric acid so they are never present
in routine palynological preparations, and diatom processing involves
treatment with strong oxidizing agents, which destroys pollen.