The paradox that is Bacillaria

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.

Bacillaria_paxillifer

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.

Reference

Schmid, A.-M. M. (2007). The “paradox” diatom Bacillaria paxillifer(Bacillariophyta) revisited. Journal of Phycology 43: 139-155.

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All things bright and beautiful?

Jarrow_Bridge_140419

Jarrow Bridge, looking downstream from near St Paul’s church, 18 April 2014.

Walking up from the site where I collected the diatoms I wrote about in the previous post, I passed St Paul’s church, dating back to Saxon times. However, as it was Good Friday, the church was busy with worshippers, so I could not go in to look at the ancient stained glass. Just beyond the church is a bridge over the muddy creek that is the mouth of the River Don. The stones in the bed of the creek just upstream of the bridge had a whitish coating of sewage fungus which is, in my experience, never a healthy sign in a stream, tidal or not. Sewage fungus is much less common now in Britain than it was in the past, which is an indication of the gradually improving state of our rivers. The term “sewage fungus” is, in fact, a misnomer as the organisms that are lumped under this term include both fungi and bacteria. The important point is that they are not photosynthetic and so rely upon complex organic compounds in the water in order to obtain the energy that they need to grow. “Complex organic compounds”? Go figure.

Sewage_fungus_Jarrow_140418

Sewage fungus (probably Beggiatoa) smothering stones in the tidal creek just upstream of Jarrow Bridge.

Mixed in amidst the sewage fungus and green algae (Ulva sp.) on the rock there were lots of diatoms. The one that caught my eye was a large sigmoid Nitzschia gliding amidst the Beggiatoa filaments. There were several of these moving around the slide but what was most interesting to me was that each had at least one, and in some cases half a dozen, much smaller diatoms sitting on them as they moved around. Being a relative novice to brackish and marine habitats, I do not know what species these epiphytes were, though I suspect that they were Amphora, possibly A. exigua. The constant motion of the Nitzschia made it impossible to capture this crisply with my stacking software and I lack the hard heart necessary to kill diatoms purely to obtain a better photograph.

sigmoid_Nitzschia_and_Ampho

A sigmoid Nitzschia with a payload of Amphora spp. collected from the tidal creek near Jarrow Bridge, 18 April 2014.

I wrote “possibly Amphora exigua” based on the flimsiest of evidence. The latest monograph on Amphora, a massive tome, describes the details of the silica frustule in minute of detail but the ecological comments are vague. I should, it tells me, now call this diatom “Halamphora exigua” but, under “Distribution and ecology” all it can tell me is “not precisely known”. It is a sad reflection on the strange world of diatoms where, it seems, we know the shape of everything yet the meaning of nothing.

Reference

Levkov, Z. (2009). Amphora sensu lato. In: Diatoms of Europe, Volume 5. (H. Lange-Bertalot, ed). A.R.G. Gantner Verlag K.G. 916pp.