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Diatoms Bacillariophyta

For this CD-ROM, we use the overall classification of the diatoms proposed by Round et al. (1990), which recognized three classes: the Coscinodiscophyceae, the Fragilariophyceae, and the Bacillariophyceae. During the last 10 years, molecular genetic studies have shown that this classification does not accurately reflect phylogeny. For example, the centric diatoms, which were all classified in the Coscinodiscophyceae by Round et al., have proved to be ‘paraphyletic’ with respect to the pennate diatoms, which Round et al. (2004) put in two classes, the Fragilariophyceae (araphid pennate diatoms) and the Bacillariophyceae (raphid pennate diatoms). In other words, the ancestor of the two pennate diatom groups was not also the ancestor of the centric diatoms; instead, it was a centric diatom belonging to just one of many centric diatom lineages. However, despite much research, diatomists have yet to agree on a satisfactory replacement for the Round et al. scheme. A new classification was proposed by Medlin & Kaczmarska (2004), but this has not been fully evaluated and it has the practical disadvantage that two of the three major groups they recognize cannot be distinguished using light microscopical data. We have therefore retained the 1990 scheme here, though we realize its inadequacies.

Classes within Bacillariophyta


Fragilariophyceae Round in Round et al. (1990)
The Diatoms: Biology and Morphology of the Genera. Cambridge.
Cells elongate and usually symmetrical about at least two planes, one of which is the pervalvar plane. Stria pattern bilateral, orientated about a longitudinal rib (the sternum), as in a feather, but raphe always absent. Costae and septa may be present. Rimoportules usually present. Many genera within the class have many small, discoid plastids lying under the valve face, but some have one, two or a few plastids. Many genera have rows of small spines at the junction of the valve face and mantle which allow sibling cells to form filamentous colonies. However, taxa also form zig-zag or stellate colonies, where cells are joined at the base by mucus secreted from apical pore fields, or clusters radiating from a common pad or stalk.


Bacillariophyceae Haeckel 1878 emend. D.G. Mann in Round et al. (1990)
The Diatoms: Biology and Morphology of the Genera. Cambridge.
Cells elongate and often symmetrical about at least two planes, one of which is the pervalvar plane; heterovalvar cells also occur. Stria pattern bilateral, orientated about a longitudinal sternum, as in Fragilariophyceae; raphe present (sometimes subtended internally by siliceous bridges), either within the longitudinal rib or to one side of it (Eunotia). Costae and septa may be present. Rimoportules absent (except Eunotia and related genera). Usually one, two or four plastids per cell, occupying characteristic positions in relation to the raphes; rarely many small, discoid plastids. Chain-like, zig-zag or stellate colonies rarely formed, but epiphytic species often form stalks for attachment.


Coscinodiscophyceae Round et R.M. Crawford in Round et al. (1990)
The Diatoms: Biology and Morphology of the Genera. Cambridge.
Cells circular, triangular or quadrate, or elongate, usually symmetrical about at least two planes, one of which is the pervalvar plane. Stria pattern radial, orientated about a small boss or ring. Raphe always absent and costae and septa very rare. Rimoportules usually present. Mostly with many small discoid plastids. Sometimes connected into filamentous or zig-zag colonies by spines, pads of mucus or chitin threads.

Comments to: Diatom Key Development Team