Diatoms occur in almost all aquatic habitats, marine or freshwater, where they can be freely suspended in water (planktonic), moving through sediments, or attached to rock or other surfaces. A few grow on land, on soil or damp rock faces, and some live as endosymbionts within foraminifera. They are all small. The largest is Ethmodiscus rex, which is a small disc 2 mm in diameter; it occurs in the equatorial Pacific Ocean. The smallest diatoms are only a few micrometres long and cannot be identified with a light microscope.
One of the heterokonts
Diatoms are a class or division of algae (Bacillariophyceae or Bacillariophyta), which are related fairly closely to the brown seaweeds. Both groups belong to a larger cluster of organisms called the ‘heterokonts', because their motile flagellate cells have two dissimilar flagella, one bearing stiff hairs, the other being smooth. The cells of heterokont algae are usually golden or brown in colour, because the green chlorophylls are masked by orange and yellow carotenoid pigments.
Cell walls of silica
The special feature of diatoms is that each cell is enclosed in a complex, highly ornamented cell wall made not of organic compounds, as in most plants and algae, but of silica. Diatoms are able to absorb dissolved silicate from the environment and transform it into elaborate solid structures. Materials chemists are now studying this process to gain insights into how new ceramics and composite materials might be produced.
The shape, pattern, size, and structure of the cell wall is species-specific, but in every case, the fundamental plan of the cell wall is the same: it consists of two halves, which overlap each other as in a Petri dish. Diatoms have an extraordinary life cycle, in which average cell size gets smaller as the cells multiply. The process of diminution continues for months or years, before being reversed by a phase of comparatively rapid expansion (taking days), which is almost always associated with sexual reproduction.
The glassy cell walls of diatom cells are particularly resistant to decay and it is this that makes diatoms very important for detecting changes in climate and water quality. Fossil diatoms ('diatomite') are used as inert fillers in many industrial products and processes, and in toothpaste.
Evolution of diatoms
Diatoms evolved at around the same time as the mammals and the flowering plants - they are relative newcomers to the planet. During the Tertiary (the last 65 million years) they have been the predominant group of algae, in terms of both photosynthesis and numbers of species.
For more information on RBGE diatom research please visit our dedicated site: Algae World.