Diatom

Electron microscope photo of a 10-micron-long diatom, Achnanthidium minutissimum (Kützing) Czarnecki.

Diatoms are single-celled algae living in water, encased in tiny glass shells. They are the bottom of the food chain. Since groundbreaking work by Ruth Patrick in the twentieth century, it is recognized that the mix of diatom populations in fresh water serves as a good indicator of the health of a body of water. Algae, including diatoms, account for nearly a quarter of the world's oxygen.

The intricacy and symmetry of diatom fossils are renowned for their beauty.

Any algae not considered diatoms are instead considered to be soft algae. Phycologists tend to specialize in one or the other, but not both. Likewise, phycologists tend to specialized in either freshwater or salt water algae, but not both.