Heterokontophyta: Diatoms

Examples: Tabellaria, Amphipleura.

Characteristics: Golden-brown colour from fucoxanthin masking chlorophylls a and c; beta-carotene; various xanthophylls and oils. Each cell is enclosed in a unique type of siliceous cell wall which takes the form of a box with an overlapping lid like a pill-box

Diatoms are unicellular or colonial coccoid algae belonging to the Kingdom Chromista, Phylum Heterokontophyta. Most are aquatic, occurring in the sea or in freshwater, but some occur on damp soil, mosses, trees and rocks.

A considerable portion of marine and freshwater phytoplankton 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. Some planktonic diatoms are now known to have symbiotic Rhizobium-like bacteria similar to those in legume roots on land that fix atmospheric nitrogen.  The only other algae known to have such symbiotic bacteria are some marine haptophyte algae. 

Diatoms have silica cell walls, and these fit together like a Petri-dish or pill box and when diatoms reproduce the "lid" and the "dish" separate and each forms a new "dish" and thus a diatom population will tend to get smaller with repeated cell divisions. The "normal" size is eventually restored by sexual reproduction.

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 a mild abrasive) and in deodorants and decolouring agents. Diatoms in lungs are often used by forensic scientists as an indication of drowning and the types of diatoms found can reliably show where drowning took place.

Because the siliceous wall is so well preserved the diatoms have an extensive fossil record, more extensive than any other group of algae, and can be used to determine whether deposits have a marine or freshwater origin. The oldest confirmed fossil diatoms date from the Lower Cretaceous (125 million years ago) and were marine. Diatoms have recently been shown to have a "urea cycle" previously thought to be unique to animals, but this seems to have evolved separately.

There are two types of diatom: centric and pennate. Centric diatoms are radially symmetrical and pennate diatoms are bilaterally symmetrical.

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