Bacillariophyta: Diatoms
Examples: Tabellaria, Amphipleura.
Characteristics: Golden-brown colour from fucoxanthin masking chlorophylls a and c; beta-carotene; various xanthophylls. Each cell is enclosed in a unique type of siliceous cell wall which takes the form of a box with an overlapping lid.
Diatoms are unicellular or colonial coccoid algae. Have membrane-bound chloroplasts and nuclei. Most are aquatic, occurring in the sea or in freshwater, but some occur on damp soil or rocks. Marine phytoplankton largely consists of diatoms and in temperate oceans where there is upwelling bringing nutrients to the surface they are responsible for the very high primary productivity in these areas.
Dead diatoms accumulating under such high-productivity areas form diatom oozes. Geological deposits derived from such oozes, mainly Tertiary in origin, are now mined as diatomite or diatomaceous earth used for water filtration, in toothpastes (as an abrasive) and in deodorants and decolouring agents.
It is estimated that the total primary production on earth is about 1.4 x 1014 kg of which 20-25% is contributed by marine planktonic diatoms and a further 15-20% by other plantktonic algae. These algae are thus critical for the ecosphere integregity of "spaceship Earth".
Because the siliceous cell wall is so well preserved the diatoms have an extensive fossil record, more extesive than any other group of algae, and can be used to determine whether deposits have a marine or freshwate origin. The oldest fossil diatoms date from the early Cretaceous (120 million years ago) and were marine.
There are two types of diatom: centric and pennate. The centric diatoms are radially symmetrical (above) and the pennate diatoms are bilaterally symmetrical (below). Centric diatoms appear to have evolved first.
There are about 250 genera of living diatoms and around 12,000 described species, by far the most numerous group of algae. There are varying opinions as to the numbers of diatom species, some authors speaking of 10,000, some of 100,000, and some of in excess of 1,000,000 species. At least 75,000 names have been applied to diatoms.
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