Illustrated here are two different kinds of diatom, class Diatomeae, phylum Chrysophyta. These organisms belong to the plant Kingdom. Both of these are planktonic, marine specimens.

The discoid forms (above left), Actinocyclus sp., belong to a group of diatoms that are renowned for their birefringent colours. These specimens were mounted upon a victorian microscope slide.

The living chain diatom (above right) was shot in 3-D and in doing so, it became obvious that the cells are triangular in cross-section and that the yellow nuclear mass within is suspended in the centre of the cell by cytoplasmic strands.

Diatoms are unicellular organisms that lack a flagellum. They range from 0.0015 to 2 mm in size and their siliciceous tests may be cylindrical, elliptical, polygonal or lancet shape, some forming long chain-like colonies. Diatoms inhabit both freshwater and marine environments and belong to both the planktonic and benthic communities. Some planktonic forms can regulate their buoyancy, and many benthic species are able to crawl slowly over the substrate. It is thought that some diatoms secrete a mucus-like substance that aids their locomotion. Though the majority of diatoms are free-living and autotrophic, some are symbionts or parasites upon, for example, foraminifera, corals and algae.

At present about 70,000 species, both fossil and recent, have been described. Diatoms are both abundant and of vital importance. They are thought to comprise approximately a quarter of plant life by weight and produce at least a quarter of the oxygen we breathe. In life they provide high quality nutrition for a wide range of animals, from small protozoans to large baleen whales. In death they fall to the ocean floor and create an oil-rich plasma layer that is, over thousands of years, transformed into petroleum. Their silicieous tests are commonly mined and used as filters and abrasives. In addition to their economic value diatoms are important environmental indicators. Extant diatoms enable biologists to accurately assess the pollution levels in certain ecosystems, whereas extinct diatoms aid palaeontologists to reconstruct palaeoclimates and interpret the depositional environment of certain strata.

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