The group listed here that contains silica in its surface covering is the:

ciliates

red algae

diatoms

water molds

The group that contains silica the diatoms. Diatoms are unicellular algae. They have a silica based cell wall known as a frustule, which gives each diatom a particular shape.

Diatoms exist as single cells though some maybe found in colonies that create beautiful shapes which can be leaf - shaped, oval, round and other shapes.

The outer shells of diatoms are used in industry in the manufacture of many products such as toothpaste. The gritty texture in toothpaste is due to addition of diatoms.