Abundant in both freshwater and marine habitats, diatoms may be responsible for up to a quarter of the Earth’s primary productivity. Their abundance is attested by the mining of “diatomaceous earth”, fossil deposits of diatom frustules which are used for various purposes including horticulture and the manufacture of dynamite.
Diatom frustules are made of glass (silica). The biomineralization of silica is being studied by nanotechnologists who are interested in the ability of these cells to produce such finely-patterned and intricate patterns.
There are two types of diatoms: the pennate (with bilateral symmetry) and the centric (with radial symmetry). This difference is solely morphological and does not appear to correspond with separate evolutionary lineages.
The chloroplasts contain chlorophylls a, c1 and c2, and appear brown because of the accessory pigment fucoxanthin. Food stored in the form of oils are often visible as oil droplets in the cell.
For more information and references, see the Teacher’s Guide to the documentary film Diatoms, Life in Glass Houses by J Pickett-Heaps. The Diatoms of the United States website has an illustrated online identification key.
The above are pennate diatoms.
A centric diatom which forms long filaments. Its features such as the pattern of dots on the frustule are radially symmetrical, but the cylinders of each cell are stacked end-to-end to form the strands.
Some diatom species attach themselves to substrates by secreting mucilage or building up long stalks. The ones pictured here may be Gomphonema.
Diatoms are capable of gliding motility across a substrate. One can observe them moving in a slow and stately manner across a glass slide or cover-slip under observation. Motility in diatoms is limited to species that possess an organelle called the raphe.
Many testate amoebae use frustules of diatoms from their diet to build up their tests.