Classification
The classification of heterokonts is still unsettled, and they may be treated as a division (or phylum), kingdom, or something in-between. Accordingly, groups like the diatoms may be ranked anywhere from class (usually called Diatomophyceae) to division (usually called Bacillariophyta), with corresponding changes in the ranks of their subgroups.
Diatoms are traditionally divided into two orders:
- centric diatoms (Centrales), which are radially symmetrical
- pennate diatoms (Pennales), which are bilaterally symmetrical. The former are paraphyletic to the latter.
A more recent classification divides the diatoms into three classes or subclasses and a number of orders:
- centric diatoms
- Cl ass Coscinodiscophyceae or Subclass Coscinodiscophycidae Round & R.M.Crawford
- Anaulales Round & R.M.Crawford
- Arachnoidiscales Round
- Asterolamprales Round
- Aulacoseirales R.M.Crawford
- Biddulphiales
- Chaetocerotales Round & R.M.Crawfor
- Chrysanthemodiscales Round
- Corethrales Round & R.M.Crawford
- Coscinodiscales Round
- Cymatosirales Round & R.M.Crawford
- Desmomastigales
- Ethmodiscales Round
- Hemiaulales Round & R.M.Crawford
- Leptocylindrales Round & R.M.Crawford
- Lithodesmiales
- Melosirales R.M.Crawford
- Orthoseirales R.M.Crawford
- Paraliales R.M.Crawford
- Rhizosoleniales
- Stictocyclales Round
- Stictodiscales Round & R.M.Crawford
- Thalassiosirales
- Triceratiales Round & R.M.Crawford
- pennate diatoms without a raphe
- Class Fragilariophyceae) or Subclass Fragilariophycidae F.E.Round
- Ardissoneales F.E.Round
- Climacospheniales Round
- Cyclophorales Round & R.M.Crawford
- Fragilariales P.C.Silva
- Licmophorales Round
- Protoraphidales Round
- Rhabdonematales Round & R.M.Crawford
- Rhaphoneidales Round
- Striatellales F.E.Round
- Tabellariales Round
- Thalassionematales Round
- Toxariales Round
- pennate diatoms with a raphe
- Class (Bacillariophyceae) or Subclass Bacillariophycidae D.G.Mann
- Achnanthales P.C.Silva
- Bacillariales Hendey
- Cymbellales D.G.Mann
- Dictyoneidales D.G.Mann
- Eunotiales P.C.Silva
- Lyrellales D.G.Mann
- Mastogloiales D.G.Mann
- Naviculales Bessey
- Rhopalodiales D.G.Mann
- Surirellales D.G.Mann
- Thalassiophysales D.G.Mann
It is probable there will be further revisions as understanding of their relationships increases.
Diatoms generally range in size from ca. 2-200μm, and are composed of a cell wall composed primarily of silica. This siliceous wall can be highly patterned with a variety of pores, ribs, minute spines, marginal ridges and elevations; all of which can be utilised to delineate genera and species. The cell itself consists of two halves, each containing an essentially flat plate, or valve and marginal connecting, or girdle band. One half, the hypotheca, is slightly smaller than the other half, the epitheca. Diatom morphology varies. Although the shape of the cell is typically circular, some cells may be triangular, square, or elliptical.
Cells are solitary or united into colonies of various kinds, which may be linked by siliceous structures; mucilage pads, or stalks; mucilage tubes; amorphous masses of mucilage and threads of polysaccharide (chitin), which are secreted through strutted processes. Major pigments of diatoms are chlorophylls a and c, beta-carotene, fucoxanthin, diatoxanthin and diadinoxanthin. Diatoms are primarily photosynthetic. A few, however, are obligate heterotrophs, while others can live heterotrophically in the absence of light, provided an appropriate organic carbon source is available. Storage products are chrysolaminarin and lipids.
Round & Crawford (1990) and Hoek et al. (1995) provide more comprehensive coverage of diatom taxonomy.
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