Having struggled to find words to describe the movement of Bacillaria paxillifer in last week’s post (“In the shadow of the Venerable Bede”), I have now uploaded a video, taken by Chris Carter, to YouTube showing a Bacillaria colony in action. In my photograph in last week’s post you will see the colony fully extended. Think of each cell as if it were one section of an extendable ladder. Chris’ video starts with the “ladder” fully retracted.
Imagine a single cell of Bacillaria in isolation. This will, in time, divide but the structure of Bacillaria’s silica cell wall (“frustule”) is such that the two cells remain intact via a “tongue and groove” structure associated with the raphe. The raphe is the part of the diatom cell responsible for movement but the tongue-and-groove structure means that the two cells can only move in relation to one another: sliding along the “track” along the centre of the valve (see lower photograph). Now imagine each of these cells dividing again, to give four cells joined in this way. Another division will produce eight cells, and so on.
A cleaned valve of Bacillaria paxillifer, an image from the ADIAC database [http://rbg-web2.rbge.org.uk/ADIAC/db/adiacdb.htm]. Note the central “ladder” (a “fibulate raphe system”) which forms the “tracks” along which adjacent cells move. The scale bar is 10 micrometres (= 1/100th of a millimetre). Photo: Micha Bayer.
Bacillaria paxillifer was one of the first diatoms to be described, being relatively large and distinctive. It was originally classified as Vibrio paxillifer in 1786, which is intriguing as Vibrio is now understood as a genus of bacteria (including V. cholerae, the organism responsible for cholera). In 1788, however, a German naturalist, Johann Friedrich Gmelin decided that Bacillaria was sufficiently distinctive that it deserved its own genus. And so it was that Bacillaria was the first genus of diatoms to be formally described. In the process, it lent it’s name to the class into which all other diatoms were eventually placed. And that, Best Beloved, is the reason why, in the formal taxonomic literature, diatoms are referred to as Bacillariophyceae or Bacillariophyta.
Schmid, A.-M. M. (2007). The “paradox” diatom Bacillaria paxillifer(Bacillariophyta) revisited. Journal of Phycology 43: 139-155.