First record of Platessa bahlsii in the UK?

I have spent the past two or three weeks looking at diatoms from an almost unique perspective.   Rather than trying to name them by looking down a microscope and observing their physical properties, I have been looking at their barcodes (see “When a picture is worth a thousand base pairs …”).   We have over 100 different known diatoms in a “library” of barcodes, each of which has been grown in the laboratory, named from photographs and then had its DNA extracted and sequenced.  However, we also, now, have results from some mixed samples collected from rivers and analysed by a new approach called “Next Generation Sequencing”.   The challenge is trying to match the sequences found in the field samples with the diatom species that they match.  Sometimes there is clear agreement, but in some cases I had to go back to my microscope slides to check the names of things that should, in theory, have matched something in our library.

The diatom in the illustration below is one such example of a “near miss”, which made me scratch my head and have another look.   As it did not appear to match any of the illustrations in my books, I also sent photographs off to a colleague, Luc Ector, in Luxembourg.  He pointed me towards some pictures in a recent paper from the USA, leading to a provisional identification of Platessa bahlsii Potapova 2012.



Platessa cf bahlsii Potapova 2012 from the River Teise at Caddingford, Kent (TQ 691 488), 29 September 2011.

Though I call this a “new record” I am fairly sure that I have seen it before and suspect that it is probably easy to misidentify.  These diatoms are less than 10 micrometres (1/100th of a millimetre) long and, mostly, occur in relatively small numbers only sporadically, so this is the taxonomic equivalent of a “perfect storm”.   It is only in the relatively rare instances when a larger population is encountered along with good facilities and enough time that it is possible to track down the true identity.  

Indeed, is Platessa bahlsii the “true identity” of this organism?  Here we stray into deep water.   P. bahlsii was first described from the USA and there is a vigorous debate on the extent to which diatoms are cosmopolitan or have restricted geographical distributions.   Another problem is that the genus Platessa is, itself, only 10 years old and not everyone would agree with the decision to create this genus.  A colleague once pointed out that whilst a species is a rigorous and objective biological concept, all higher levels of taxonomy are, to some extent, negotiable.   Twenty years ago, this species would have been placed in the genus Achnanthes.   Since then, freshwater species of Achnanthes have been split into ten separate genera, two of which have quietly fallen out of favour already.  Looking at the results from our barcode study, I suspect that another shake-up may be necessary before we reach a classification of this group of diatoms that is biologically realistic.   That will mean more names changing and more confusion in the meantime. Hey ho …


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