My name is Legion …

I promised to write a little more about Gomphonema subclavatum, one of the diatoms we encountered in the previous post.   I picked this one out for more attention because it is one of many diatoms that have changed names in recent years and it is sometimes interesting to scratch around to understand why this has happened.

Had I seen this particular species fifteen years ago I would have called it Gomphonema clavatum without hesitation.  Although G. subclavatum was recognised as a distinct species back in the nineteenth century, for most of the twentieth century it was treated as a variety of G. longiceps, which Krammer and Lange-Bertalot then subsumed into G. clavatum.  If you look at their plate of G. clavatum, you will see a huge range of sizes and shapes so it is perhaps no surprise that people subsequently realised that there was more than one species lurking under this name.

Gomphonema subclavatum from Cregduff spring, Co. Mayo, Ireland, September 2017.  Photographs: Bryan Kennedy.  Scale bar: 10 micrometres ( = 100th of a millimetre).

When this happens, taxonomists ask which of the various contenders was the Gomphonema clavatum seen by the person who originally described the species.  This involves going back to the museum collection where that person deposited the material that they examined and taking another look.  This process of “typification” helps determine which of the forms is the rightful inheritor of the name.   Erwin Reichardt decided to have a go at this process for G. clavatum and went to examine the samples, now in the Museum für Naturkunde in Berlin, on which Christian Gottfreid Ehrenberg had based his original description.  However, he could find nothing that resembled G. clavatum, with the closest match being G. olivaceum.

I’m reading a biography at the moment that contains the warning that “history is always a matter of trying to think into the minds of people who think differently from ourselves”.  That serves as a useful reminder that Ehrenberg knew far less about the biology of diatoms than we do today, but was also limited by the technology available.  Not only were his microscopes far less sophisticated than ours but also capturing the essence of the organisms he saw in print was far from straightforward (see “Picture this?”).  The idea of Gomphonema clavatum that we had until Reichardt re-examined the type material was the result of a 180-year game of “Chinese whispers”: each generation matching their specimens to inadequate images and descriptions, then making their own images which, in turn, became the basis for their successor’s identifications.  By the time Krammer and Lange-Bertalot wrote their Flora, it was finally possible to reproduce high quality micrographs, rather than line drawings but over a century of taxonomic drift meant that their images are no longer connected to the right name.  Their plate actually shows two species: the larger forms with undulate margins belong to G. longiceps Ehrenberg 1854) whilst the smaller specimens are G. subclavatum.   That assumes, of course, that there are no further twists to come.  As I alluded in my previous post, morphology might not be telling us the whole story for this genus.

The unfortunate twist, also mentioned in my previous post, is that the taxonomic confusion in the past means that we don’t actually get any sharper ecological insights in the present as a result of unravelling these names.   Anyone looking at ecological data associated with “Gomphonema clavatum” from twenty years ago needs to know that this could represent either G. longiceps or G. subclavatum or one of a number of other species that have been split away in recent years.  There is always a hope that this better understanding of taxonomy will yield fruits as we go forward but I’m always suspicious that someone else might come along and rearrange things yet again…

Reference

Krammer, K. & Lange-Bertalot, H. (1986).  Süsswasserflora von Mitteleuropa. 2/1 Bacillariophyceae 1: Naviculaceae. Spektrum Akademischer Verlag, Heidelberg.

Reichardt, E. (2015). The identity of Gomphonema clavatum Ehrenberg (Bacillariophyceae) and typification of five species of the genus Gomphonema described by C.G. Ehrenberg.  Diatom Research 30: 141-149.

The biography to which I refer is Tom Wright’s new book on Paul (SPCK, 2018).

 

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Return to Pangong Tso

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Pangong Tso, from the Indian shore, looking towards China, July 2015 (photograph: Heathe Kelly).

You may remember that a year ago I wrote some posts about a high altitude lake on the India-China border (see “Subaquatic landscapes in Pangong Tso” and references therein).   This year, Heather made a second trip to Pangong Tso (described here) under the auspices of Indus Experiences and brought me back another sample from the littoral zone.   There was a beautiful thick biofilm here, an unusual bright yellow-brown colour and a jelly-like consistency, but bubbling away as the algae photosynthesised busily.   Once again, local vodka was pressed into service as a preservative and, once again, peering through my microscope a few days later, I could see that the sample was dominated by the same long-stalked Gomphonema species that I recorded a year ago (see “Diatoms from Pangong Tso”). The jelly-like consistency did worry me, as this is not what I would expect of a pure growth of diatoms and I did wonder if there were cyanobacteria growing amongst the diatoms that had not survived the journey home in their marinade of cheap vodka.

Pangong_littoral_diatoms_Ju

Growths of diatoms (predominately a long-stalked Gomphonema sp) on a boulder in the littoral zone of Pangong Tso, India, July 2015. The right hand image is a close up showing oxygen bubbles being produced by the the jelly-like masses. Photographs: Heather Kelly.

Intriguingly, the Gomphonema seems to occur in two forms: a fatter form, with a width around eight micrometres, and a narrower form, about six micrometres wide. I’ve written before about how diatoms tend to get shorter over time (see “Diminishing with age”). What I did not make clear in this post is that cell breadth tends to stay relatively constant during this process.   This does not happen with every species but it is interesting to see that the fat and narrow forms have overlapping sizes, so it is not a simple matter of the narrow ones being the far end of the size reduction sequence. More work is definitely needed here although, alas, I don’t think Pangong Tso is on the itinerary for next year’s visit to India.

Gomphonema_Pangong_July15

Gomphonema sp from the littoral zone of Pangong Tso, north India, July 2015.   a. – d. represent the “fat” form; e. – h. are the narrower form(s). Scale bar: 10 micrometres (= 1/100th of a millimetre).

Diatoms and dinosaurs

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Ennerdale Water, looking north from the outflow to the River Ehen, April 2015

My monthly visit to the River Ehen coincided with a week of warm weather, clear skies and low flows.   There were some glorious views of the Lake District fells shrouded in early morning mist when I first arrived and I spent a few minutes trying to capture them with my camera before turning my attention to work.

We are now into our third year of regular sampling and are beginning to get a feel for the seasonal dynamics of the algae in the stream. By this time of year, we expect to see the biomass falling as the invertebrates start to become more active and browse away the luxuriant winter growths (see “A very hungry chironomid”). Many of the stones do appear to have less algae growing on them but, at one site, there were still some very conspicuous growths of diatoms. I did wonder if these were growths of Didymosphenia geminata (see “A journey to the headwaters of the River Coquet”); I have never seen this species in the Ehen but several aspects of the habitat here are such that it is possible that it could thrive.   However, when I got a sample under my microscope, the growths turned out to be long-stalked Gomphonema species, with a variety of other diatoms, including Tabellaria flocculosa, Fragilaria spp., Hannaea arcus, Brachysira neoexilis and at least a couple of members of the Achnnathidium minutissimum complex.

Gomphonema_clumps_Ehen_Apri

Clumps of diatoms – mostly Gomphonema – on a stone from the River Ehen, April 2015.

The Gomphonema species in the River Ehen have perplexed us before. Our best bet is that the larger ones, in particular, are G. gracile.   However, we are straying into areas where names abound although the limits between species are often described in vague terms in the literature. Dawn Rose and Eileen Cox described an experiment recently in which they grew two cultures of a different diatom, Gomphonema parvulum, in culture.   Diatoms, you may remember, get smaller with successive cell divisions (see “Diminishing with age…”) until a point is reached when sexual reproduction is initiated.   When this happened with one of their cultures, they were surprised to see cells that our Floras would have called Gomphonema gracile emerge from the auxospores. They continued to grow these and watched as the G. gracile cells diminished in size until they resembled G. parvulum again. The problem is, as they point out in their paper, most diatomists base their opinions on the structure of the cleaned frustule rather than the behaviour of the living cell.

Gomphonema_Ehen_April15

Gomphonema cells from the River Ehen, Cumbria, April 2015.  The five images on the left show a size reduction series in valve view; the right hand image shows two cells in girdle view with the stalk just visible.   Scale bar: 10 micrometres (1/100th of a millimetre).

Why, you may ask, don’t more people do experiments of this type in order to define the limits between species of diatoms?   It is a good question. The day before my visit to the Ehen, I had read an article in my newspaper about a similar study that had confirmed that the dinosaur Brontosaurus was probably a distinct genus rather than being part of the genus Apatosaurus, as had been assumed for most of the 20th century.   The scientists based their conclusion on careful measurements of different properties of dinosaur skeletons, just as most diatomists measure and describe the diatom frustule. What else can a palaeontologist do?   Their material is, after all, long dead.   Diatomists work with the living world. It is, admittedly, fiddly and time-consuming work to prepare cultures of single species from field communities, and you need special incubators to grow these cultures once they have been isolated. But it is not impossible.

Most of us who study diatoms are over-anxious to get rid of the organic constituents of the cell in order to focus on the silica remains. We are, in effect, contemporary palaeontologists. Most palaeontologists would love the opportunity to get some definitive evidence of species limits (mating experiments on Brontosaurus: that would be quite a sight!).   They must think that us diatomists are a bizarre bunch indeed.

Reference

Rose, D.T. & Cox, E.J. (2014). What constitutes Gomphonema parvulum? Long-term culture studies show that some varieties of G. parvulum belong with other Gomphonema species. Plant Ecology and Evolution 147: 366-373.

Tschopp, E., Mateus, O. & Benson, R.B.J. (2015). A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda).   PeerJ 3: e857 [https://dx.doi.org/10.7717/peerj.857]

 

Food for thought in the River Ehen

Last week’s trip to the River Ehen involved close scrutiny of the hydrographs and weather forecast in the days preceding the trip, followed by some crude extrapolations that predicted that the river would be low enough for us to enter. Nevertheless, there were some anxious moments when we first arrived and gazed into the fast-flowing water.   It was an occasion when I was glad of my veterinary gloves which afford some protection as I have to plunge my arms into river almost to my shoulder in order to pluck stones from the bottom.

The top surface of many of the stones had conspicuous chocolaty-brown patches which are a sign that the diatoms are growing vigorously.   We had been surprised that the amount of algae in the river had been quite low compared to our previous measurements in the autumn and had wondered whether the mild weather had allowed the natural grazers in the river to persist for longer than was the case last year.   Now, with a distinctly autumnal chill in the air, the diatoms no longer seem to be held in check which lends credence to this theory.   If true, it is a good demonstration of just how important “top down” processes are in controlling the quantities of algae that we see in our streams and rivers.

Ehen_Mill_biofilms_141106

Chocolate-brown patches of diatoms on the upper surfaces of cobbles on the bed of the River Ehen (Mill) in November 2014.

Under the microscope, these brown films resolved into a mass of diatoms, along with a few filaments of Bulbochaete (see “The River Ehen in August”). There were two species of Gomphonema species which live at the end of long stalks: G. acuminatum (see “The River Ehen in February”) and G. truncatum, plus other, smaller Gomphonema species.   There were also a lot of long, needle-like cells of Fragilaria tenera, many of which were joined at their bases to form hedgehog-like bundles of cells. My guess is that these start life attached to either the rocks or other algae but that they easily detach, especially when manhandled onto a microscope slide, and appear to be free-living.

Mill_diatoms.141106

Common diatoms from the River Ehen (Mill), November 2014: a. Gomphonema truncatum valve view; b. G. truncatum girdle view; c. Gomphonema acuminatum valve view; d. G. acuminatum girdle view; e. Fragilaria tenera valve view; f. F. cf. tenera girdle view.   Scale bar: 10 micrometres (1/100th of a millimetre).

Ten or fifteen years ago I would not have referred to this diatom as “Fragilaria” but, instead, would probably have called it Synedra tenera.   Older identification guides (such as West and Fritsch’s British Freshwater Algae, from 1927) differentiate between Synedra and Fragilaria based on whether or not the cells form ribbon-like colonies (Fragilaria) or are solitary or form radiating clusters (Synedra). From the 1980s onwards this simple dichotomy was unpicked just enough to demonstrate that it was too simplistic, but not enough to enable our esteemed taxonomists to put all the pieces back into a coherent whole (a story for another day …).   However, the evidence at present leans to the narrow needle-like species that once belonged to Synedra actually sharing more characteristics with true Fragilaria.   I call this species Fragilaria tenera with mild trepidation, knowing that there is at least one other needle-like Fragilaria present in the upper Ehen and, moreover, that a paper has recently been published (though I have not yet read it) in a rather obscure German taxonomic journal that dissects this complex into yet more fragments. Research on Fragilaria has, alas, generated rather more heat than light over the past couple of decades.

Frag_tenera_stack_Ehen_#1

A colony of Fragilaria tenera from the River Ehen, image produced using Helicon Focus stacking software. Individual cells are approximately 50-60 micrometres long.

Reference

Lange-Bertalot, H. & Ulrich, S. (2014). Contributions to the taxonomy of needle-shaped Fragilaria and Ulnaria species 1. Lauterbornia 78: