Diatoms: cleft confervas, or carved at the joints

Robert K. Edgar
February 2013

1 - Introduction

Generations of novice diatomists have been introduced to diatoms with a box-model representing these unicellular individuals as each enclosed in a glass cell wall -- a two-part box, with a bottom and an overlapping cover, like a petri dish or a pill box (Figures 1 & 2). They have also usually encountered the etymology of the group's name -- a combination of two Greek words: δια- (dia-) and τεμνειν (temnein) = diatemnein, meaning "cut in half". Recently, as information about diatoms has exploded on the web and as new audiences interested in climate change, aquatic toxins, nanotechnology, biofuels and microbial evolution meet diatoms for the first time, this morphological précis and the name's roots continue to be commonly encountered -- and often causally linked. From the viral entry in Wikipedia (2008-2013),

diatoms (Greek (dia) = "through" + temeneion (temnein) = "to cut", i.e. "cut in half" ...characteristic feature ... cells encased within a unique cell wall made of silica ... [consisting] of two asymmetrical sides with a split between them, hence the group name." [2009, my bold, Note 1],

to the sterling review in 2009 of the life of a diatom in the journal Nature by Virginia Armbrust,

... diatoms. ... Their name is derived from the Greek diatomos, meaning 'cut in half', a reference to their distinctive two-part cell wall made of silica. [again, my bold]

One is much struck by the descriptive aptness of the name's roots to the basic cell wall structure, indeed, to the idiosyncratic feature that captures the group's morphological uniqueness. However, while this causal linkage of roots and structure may infuse much current usage, it does not accurately portray the historical genesis of the name. As applied to this group the word diatoms is a little over 200 years old, but our knowledge of the bipartite structure and siliceous nature of the diatom cell wall is not -- the name precedes the knowledge by decades. How is this possible? Was the namer prescient? Or merely lucky? Or was something else intended in choosing the name despite this striking goodness-of-fit?

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Figure 1 (left). A scanning electron microscope view of the cell wall of the diatom Stephanodiscus, showing its two overlapping saucer-shaped halves (v = valves) held together by a set of belt-like girdle bands (gb). Image: S. M. Edgar & E. C. Theriot, University of Texas at Austin. [Blue text indicates the figure or text is interactive.]

Figure 2 (right). A brightfield light microscope view of a section of the cell wall of a Pinnularia, showing its two overlapping halves. The "scissors line" in each image shows where the cut is implied by the usage cited above.


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