he neighborhood: Where do diatoms live?
Almost all living diatoms require sunlight to survive and photosynthesize, limiting them to the uppermost 200 meters of the water column. This sunlit region of the water column is referred to as the photic zone. The waters below the photic zone are a world of darkness. Since the entire surface of the ocean is exposed to sunlight at least part of the year, diatoms live practically everywhere at the sea surface.

Diatoms are classified into two categories according to their lifestyle: planktonic or benthic. Since all diatoms are photosynthetic, both planktonic and benthic diatoms are restricted to living within the photic zone. Centric diatoms are mostly planktonic, floating near the sea surface, while pennate diatoms are mostly benthic, living on the seafloor, or attached to floating objects. Some diatoms even change their lifestyle preference between juvenile and adult forms, abandoning their floating positions and attaching themselves to the seafloor later in life. Although most benthic diatoms do not move, some pennate diatoms can travel a short distance across the seafloor. Even this weak ability to move would be unusual for a terrrestrial plant. Can you imagine if grass could get up and move around!

All diatoms must live in the photic zone.