More about Gomphonema vibrio

Gomphonema vibrio is part of a complex of species that has only begun to be unravelled in the past few years.   In the first edition of the Süsswasserflora von Mitteleuropa in 1930, Hustedt included it as one of three varieties of G. intricatum, along with G. pumilum and G. dichotum.  By the time of the second edition (1986), however, Krammer and Lange-Bertalot had subsumed G. intricatum into G. angustum, creating a single species that spanned an enormous range of size (see their Plate 164 if you don’t believe me).   A few years later they revised this opinion, and unpicked the G. angustum complex, reinstating several of the taxa that they had originally subsumed and also recognising some more recently described species (many by Erin Reichardt).   There may well be more changes to come as this group has not yet been subjected to critical study by molecular geneticists.

One of the other species in this melange is Gomphonema pumilum, a much smaller diatom that is common in both running and standing waters (Hustedt’s comment on the species complex only referred to a preference for “stagnant waters”).   We have met it a few times previously (see, for example, “Pleasures in my own backyard”) and I also found it in a 1999 sample from Croft Kettle whilst searching for G. vibrio.   However, I then turned to an older slide, based on a sample collected in 1872 and given to me by John Carter (see “Remembering John Carter”).   This had some cells of G. pumilum but also some that exceeded the quoted dimensions for G. pumilum (length: 12 – 36 mm; width: 3.5 – 5.5 mm) and which fell within the size range for G. vibrio.   I suspect that we are, in fact, dealing with a mixture of the two species and if this is a common situation then it may explain why Hustedt had difficulties unpicking the two species.   When I arranged the images of G. vibrio and G. pumilum that I found in this sample in order of diminishing size, there is a continuum between the two forms.  We now know that width is a better discriminator than length and, armed with this, we can see a difference between the two species. But that is one of the benefits of hindsight.

Gomphonema pumilum from Croft Kettle, May 1999.  a. – e.: valve views; f., g.: girdle views.   Scale bar: 10 micrometres (= 100th of a millimetre).

Gomphonema vibrio (h. – k.) and G. pumilum (l. – m. [and n.?]) from “Hell Kettles”, 1872.  Scale bar: 10 micrometres (= 100th of a millimetre).

This raises a question about the reliability of the size ranges quoted in the literature   A couple of the smaller valves of G. vibrio were less than 7 mm wide.  Yet, in other respects, they were more similar to the “true” G. vibrio valves than to those of G. pumilum.  The answer will vary from species to species but, as a general rule, we should not be too bothered if the extremes of a population stray a little beyond the values quoted in the literature.   These are usually based on the largest and smallest cells found in a thorough scan of one or more populations, but not necessarily on observations of an initial cell (the largest in a population) or of cells at the point immediately before sexual reproduction is initiated (the smallest).  We simply don’t have that information for most species so, as a result, should be prepared to accept larger and smaller valves into a species if they are qualitatively similar to, and quantitatively part of a continuum with, the rest of the population.  My post “Diatoms and the Space-Time Continuum”, also on Gomphonema, offers some further insights into this story.

Reference

Hustedt, F. (1930).  Susswasserflora von Mitteleuropa 10: Bacillariophyceae.  Gustav Fischer, Jena.

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

Krammer, K. & Lange-Bertalot, H. (1991). Susswasserflora von Mitteleuropa 2: Bacillariophyceae. 4 Teil: Achnanthaceae. Kritische Ergänzungen zu Achnanthes s.l., Navicula s.str., Gomphonema. Spektrum Akademischer Verlag, Heidelberg.

Reichardt, E. (1997).  Taxonomische revision des Artencomplexes um Gomphonema pumilum (Bacillariophyceae).  Nova Hedwigia 65: 99-129.

Reichardt, E. & Lange-Bertalot, H. (1991).  Taxonomische revision des Artencomplexes um Gomphonema angustum – G. intricatum – G. vibrio und ähnliche taxa (Bacillariophyceae).  Nova Hedwigia 53: 519-544.

Note

In my post on Gomphonema rhombicum, I mentioned that the location on the type slide is given as “Appleby”, which was not very precise.   My 1872 slide is labelled “Hell Kettles, Durham”.  “Hell Kettles” is the name for the pair of ponds, of which Croft Kettle, which I described in my earlier post, is the larger.   However, the location “Durham” is not very illuminating.   The closest town to Croft Kettle is Darlington, whilst Durham City is 40 km to the north.   “Durham”, in this context, could refer to the county, which covers 2721 square kilometres and habitats from calcareous ponds such as these to moorland pools.   A slide label offers very little space to give precise details of location but, in both these cases, a little more information would be useful.   The likelihood is that Firth had more detailed notes elsewhere but these have been lost over time, so we are left with these scant words.   There is a lesson here for all of us in how we record the meta-data that accompanies our samples.

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Croft Kettle through the magnifying glass …

Croft_Kettle_150529_#3

Cymbella-dominated community of epiphytic algae around Chara hispida stems in Croft Kettle, May 2015.

The final stage of my journey of discovery through Croft Kettle is a three-dimensional diorama in which the various components that I have described in posts since my visit on 29 May are reassembled into something approaching their natural state.   In this image, I have tried to show how much of the yellow-brown gunk which you can see in my post of 1st June is, in fact, the stalks of Cymbella cymbiformis, which form a dense matrix around the Chara stems. This, in turn, creates a habitat within which other diatoms can move around. In my illustration, I have included cells of Navicula radiosa and Amphipleura pellucida as well as Rhopalodia gibba and several crystals of calcite. The Rhopalodia intrigues me: Chris Carter’s photographs in my post from 7 June show it attached to Chara stems but I did also see it moving around in samples dominated by the Cymbella stalks.   Adhering strictly to a sessile lifestyle when Cymbella stalks are growing all around you and creating a light-capturing canopy is probably not a great survival strategy and the capacity to move amidst this forest of stalks must give the Rhopalodia more opportunities.   On the other hand, Rhopalodia is not really optimised for motility, as both of it’s raphe slits are on the same side, in contrast to Navicula and relatives where they are on opposite sides. Diatoms exude mucilage through the raphe which attaches to the surface and then gives them something to push against. In this Cymbella forest, these stalks will, presumably, provide that point of contact.   As the diatom pushes against the stalk, it will need to connect with another stalk before it can progress. Having two raphe slits on opposite sides of the valve increases the chances of this happening (in much the same way as a climber using both arms and both legs to work his or her way up a narrow chimney). Having both on the same side, presumably, reduces the chance of successfully adhering to another stalk.   It is, I suspect, a case of “needs must”: limited motility is still better than no motility at all.   If you look carefully, you’ll see that I’ve also included some cyanelles in the Rhopalodia cells.

It is slightly disingenuous for me to suggest that this image is purely the result of my own observations.   They are, for sure, the starting point but I find myself referring to several books as I constructed the image. The view down the microscope has a very flattened perspective, which means that it can be difficult to get an impression of the three-dimensional appearance of a diatom such as Rhopalodia.   For this, I referred to scanning electron micrographs in The Diatoms: Biology and Morphology of the Genera. However, these show the diatom frustules as opaque so I then need to refer back to my own images in order to build up a view of the cell interior.   I can start from my own observations but, after a few days, the chloroplasts of some species start to degrade, so I also turned to Eileen Cox’s book on identification of live diatoms. This is good for details of the plastids, but not so good for stalks so, for these, I am back to peering down my microscope at fresh material.   Finally, I found an excellent new book on charophytes that had some great illustrations that helped me understand the structure of the stem of Chara.

This interplay between direct observation and existing knowledge is necessary and, indeed, there are noble precedents (see “I am only trying to teach you to see …”). However, it also carries the possibility that we promulgate the errors of the past; we look at the natural world through eyes conditioned by the opinions and interpretations of others.  But, then, my picture is no more than an accumulation of my own opinions and interpretations.   In science, as in history, we always walk backwards into the future …

References

Cox, E.J. (1996). Identification of Freshwater Diatoms from Live Material. Chapman & Hall, London.

Round, F.E., Crawford, D.M. & Mann, D.G. (1990). The Diatoms: Biology and Morphology of the Genera.   Cambridge University Press, Cambridge.

Urbaniak, J. & Gąbka, M. (2014). Polish Charophytes: An Illustrated Guide To Identification. Wroclaw University of Environmental and Life Sciences. Wroclaw.

Alice’s adventures in Croft Kettle …

I cannot leave the subject of Croft Kettle without mentioning one tangential association of this small pond with the world of literature.   Croft Kettle is just over a kilometre from the village of Croft-on-Tees, just south of Darlington and, in the middle of the 19th century, the rector of Croft was the Reverend Charles Dodgson.   His eldest son was also Charles Dodgson, better known by his pen-name Lewis Carroll.   Several writers have explored the ways in which north-east England fuelled his imagination and provided raw material for Alice in Wonderland and Alice’s Adventures Through The Looking Glass (most notably Bryan Talbot’s excellent Alice in Sunderland, 2007, Jonathan Cape, London). One of these, a geologist called Tony Cooper from the British Geological Survey, even brought Croft Kettle into the story.   Croft Kettle is a sinkhole, formed by the dissolution of gypsum (calcium sulphate) and there was a local legend that the pond was bottomless, leading Cooper to wonder whether this deep, deep hole in the ground so close to where he lived was an inspiration for the rabbit hole into which Alice fell and tumbled “down, down, down”. When Carroll was older, his father moved from Croft to become the Dean of Ripon, also in north-east England and another region where there were many sinkholes.   Some of these appeared quite suddenly, with catastrophic consequences for houses built in the vicinity and, intriguingly, Cooper notes that the original model for Tenniel’s illustration of Alice, lived in a house affected by such subsidence in Ripon.   Curiouser and curiouser, as Alice might have said.

Return to Croft Kettle

It has been over three weeks since I last wrote about Croft Kettle. However, with the diversions to Milan and Trento behind me, I can now settle down and continue to sort out the images I had taken from the samples I collected back in May.   I’ve also dug out some other slides from previous visits in order to show the full range of diversity that we’ve encountered over the years.

One of the most abundant diatoms gliding through the mass of Cymbella stalks surrounding the Chara stems was Navicula radiosa, illustrated below. The name derives from this diatom’s strongly radiate striae, which are not visible in the live specimens that I photographed.   What you can see clearly are the two long, narrow plastids (chloroplasts), one on either side of the valve.   Another Navicula species which I have seen in this habitat at Croft Kettle, but which was not obvious in the samples I collected in May is illustrated in the next illustration: Navicula oblonga.   This is enormous by diatom standards: the largest individual I found in a sample I collected in May 1999 was just over a fifth of a millimetre long. It is hard to fit an entire valve of N. oblonga into a field of view to photograph at 1000x magnification, so the images below were all taken at 400x. N. oblonga is a relatively rare diatom in the UK in my experience, with an apparent preference for hard waters, extending into slightly brackish conditions.

Croft_Kettle_150529_#2

More diatoms associated with Chara hispida stems in Croft Kettle, May 2015: a, b., c.: Navicula radiosa; d. Amphipleura pellucida.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

My return to Croft Kettle was prompted, you may remember, by a talk about a fossil lake in the Sahara which had a similar diatom flora (see “The desert shall rejoice and bloom …”). Both Cymbella cymbiformis and Navicula oblonga featured in the assemblages that Nassouma Yahiaoui found there, along with representatives of Epithemia and Mastogloia, both of which I found in my May 2015 samples, though neither presented themselves in particularly photogenic poses.   I did include some photographs of another species of Epithemia in my description of Cassop Pond, which is also associated with the Permian limestone.

Navicula_oblonga_Croft_May1

Navicula oblonga associated with Chara stems in Croft Kettle, May 1999. Scale bar: 25 micrometres (= 1/40th of a millimetre).

One other interesting species that I found at Croft Kettle, though it was not present in Nassouma’s Guern Toil profile, was Amphipleura pellucida. This is a long, delicately-featured diatom with a small, H-shaped plastid in the centre.   A silica rib runs along the centre of the valve; this splits as it approaches the poles to contain a short raphe slit. The combination of silica rib and raphe resembles the end of a sewing needle. Microscopists have long been interested in Amphipleura pellucida because the striae are extremely-closely spaced (37-40 in 10 micrometres).   This means that they can only be resolved by the very best optics and, consequently, slides containing A. pellucida are used as test objects. The slide I used to photograph the specimens below was given to me by John Carter (see “Remembering John Carter”) and was made from material collected in 1872 by “Firth”. Some hunting on the internet suggests that this was William Allott Firth, who was a Quaker from Yorkshire. Croft Kettle is less than a kilometre from the Yorkshire – Durham county boundary and Darlington, the nearest town, has strong Quaker connections too.

It is a good idea to have a test slide with Amphipleura pellucida to hand when you are buying a microscope.   The sales reps whose job it was to demonstrate new microscopes used to breeze into our lab in shiny suits reeking of cheap aftershave and talk the talk about how wonderful their product was.   We would then hand them our test slide and say “resolve that”.   After twenty minutes fiddling with the microscope set-up they would usually make some excuse and retreat with their tails between their legs.   I got a certain sadistic pleasure from watching these fast-talking laboratory sales representatives being defeated by a handful of gunk collected by a Victorian amateur natural historian.

Amphipleura_pellucida_Croft

Ampipleura pellucida from a sample collected from Hell Kettles in 1872 by Robert Issac Frith. Scale bar: 10 micrometres (= 1/100th of a millimetre). Bottom right: the slide from which the specimens were photographed.

More about Croft Kettle

In my post on Croft Kettle, I commented on the long stalks possessed by Cymbella cymbiformis. These were difficult to capture with my camera, partly because the Cymbella cells readily detach themselves from their stalks and partly because the tangle of stalks exceeds the depth of field available to the microscopist. Instead, I have tried to capture the view through the microscope eyepiece in a drawing.

Croft_Kettle_150529_#1

Croft Kettle, epiphytic algae associated with Chara hispida stems, May 2015. Drawn at x400 magnification.   The narrow stalks on the left hand side are about five micrometres in diameter.

There is a tangle of stalks on the left hand side, along with two cells each of Cymbella cymbiformis, Navicula radiosa and Rhopalodia gibba. Note, too, the narrow filament of Oedogonium, complete with oogonia (see “Love and sex in a tufa-forming stream …”).   The Rhopalodia cells have glided free from the tangle of stalks.

As I looked at these rich communities of algae I started to wonder if would make a good subject for a painting, so I have been continuing to examine the material I collected in order to build up a sense of what the three-dimensional community around the Chara stems would have looked like. One interesting observation came when I had a look at some of the narrow branchlets of Chara. I wanted to see which algae were directly attached to the Chara surface (more about this in a moment) but the feature that was most noticeable was the quantity of calcite crystals deposited around the stems.   These give Chara its characteristic stiff stems that are rough to the touch.   The calcite is deposited as a by-product of photosynthesis; intriguingly, Chara shares this property with many tufa-forming algae and bryophytes but not with its close relative Nitella, which is much softer to the touch (see “Finding the missing link in plant evolution…”).

Chara_hispida_and_calcite

Calcite crystals deposited around the tip of a branchlet of Chara hispida from Croft Kettle, May 2015.   Image composed using Helicon Focus stacking software. Scale bar: 10 micrometres (= 1/100th of a millimetre).

One of the questions that was puzzling me was the habit of the diatom Rhopalodia gibba within the community of algae on and around the Chara stems.   In many of my specimens, Rhopalodia seemed not to be attached to the Chara but, instead, glided amidst the tangle of Cymbella stalks growing around the Chara stems; however, I also saw a few cells directly epiphytic on the Chara stems, and this also seems to be the habit that Chris Carter has captured in some of his images of Rhopalodia. I suspect that Rhopalodia, and many other diatoms are opportunistic and can adopt slightly different habits depending upon the prevailing conditions. There is no point, for example, in doggedly sticking to an epiphytic habit if this m

eans sitting in the deep shade cast by a Cymbella forest. Whatever the textbooks say.

Rhopalodia_gibba_Croft_Kett

Rhopalodia gibba associated with Chara hispida stems in Croft Kettle, May 2015. b. is a valve view; e. is a girdle view and a.,c. and d. are intermediate between the two positions. Scale bar: 25 micrometres (= 1/40th of a millimetre).

Rhopalodia_on_Chara_CCarter

Rhopalodia growing on Chara. Photographs by Chris Carter.

Rhopalodia is a genus with an unusual morphology. The raphe follows the dorsal margin (i.e. the left hand side of b. in the figure above) but this means that, in girdle view (i.e. looking from above), both raphes are on the same side of the valve.   I have often assumed that having raphe slits on opposite sides assists motility, by giving the cell two planes by which it may attach (much like a climber working his way up a narrow chimney).   It is possible that being attached to the surface is the preferred habit; motility would become an advantage only when the energy that this process consumes is offset by that supplied by the extra photosynthesis that can take place when it moves away from the shaded areas and into the canopy.   I have never seen any work done to address this topic, but it would make an interesting study.

Another interesting feature of Rhopalodia is the presence of cyanelles, organelles derived from cyanobacteria or similar prokaryotic algae. We also encountered these in Epithemia (see “A return to Cassop”) where I also mentioned that they may be involved in nitrogen fixation.   Cyanelles deserve a post all of their own at some point in the future, so I will just leave you for now with another of Chris Carter’s excellent photographs, in which the cyanelles of a Rhopalodia sp. are highlighted.   They are near-transparent, with thin membranes and are easy to confuse with the vacuoles that contain the polysaccharide chrysolaminarin (these tend to be more refractive). Very easy to overlook.

As I was putting this post together, I noticed that West and Fritsch noted that Rhopalodia gibba was “common in all kinds of localities.”   This surprised me, as I have only ever seen it at a handful of sites in the UK. It did make me wonder if West and Fritsch, writing in 1927, were right, and that it has declined significantly subsequently. A species that has a competitive advantage at low nitrogen concentrations will not have had an easy life in the period after West and Fritsch wrote, as agricultural intensification and widespread use of fertilisers led to increases in the concentration of nitrogen in surface water.

Rhopalodia_with_cyanelles_C

A girdle view of Rhopalodia sp. with cyanelles indicated by arrows.   Note, too, the characteristic lobed chloroplast. Photograph by Chris Carter.

Reference

West, G.S. & Fritsch, F.E. (1927). A Treatise on the British Freshwater Algae.   Cambridge University Press, Cambridge.

The desert shall rejoice and blossom …

Ratnieki_NDM_May15

The Nordic Diatom Meeting, Ratnieki, Latvia, May 2015.  

The motivation for my recent trip to Latvia (see “Following in Arthur Ransome’s footsteps …”) was to attend the Nordic Diatom Meeting, which took place in the University of Riga’s conference centre, Ratnieki, set in the Latvian countryside. It was a small, informal and very friendly meeting, fuelled by enormous quantities of food and enlivened by an excursion that managed to compress an overview of Latvian history from the Iron Age to the present into one afternoon of sightseeing.

It is a little unfair to pick out one of the presentations, but there was a moment on the first afternoon when I sat up with a start as the story of a fossil lake in the Saharan desert unfolded.   There is, in the middle of the Western Great Erg (a huge sand-covered area of the Sahara in southern Algeria), within which there are several depressions. Some of these contain deposits that suggest that they were once lakes.   An Algerian PhD student, Nassima Yahiaoul, told us about her study of an outcrop in one of these depressions, Guern Touil, which was composed largely of diatomite, a rock consisting largely of the remains of dead diatoms.   This is good evidence that, in a moister period perhaps 7000 years ago (the precise date is not yet known), this area was not a bleak, unforgiving desert, but a freshwater or brackish lake.

What made me take particular notice, however, was the diatoms that she found when she analysed these deposits.   These included Cymbella cymbiformis, Epithemia argus, Denticula tenuis, three species of Mastogloia and Navicula oblonga, a very large and distinctive species.   None of these are particularly common in the streams and lakes that I study in temperate Europe but, curiously, several of these occur together in a small pond about 30 kilometres away from where I lived. This pond is, itself, botanically quite distinctive, and it was a strange sensation to sit in the Latvian countryside and hear about another with such an uncanny resemblance but which is so far distant in both space and time.

Guern_Touil_outcrop_Yahiaou

The outcrop of diatomite in the Guern Touil depression, Western Great Erg, Algeria, studied by Nassima Yahiaoul.

The place that Nassima’s description evoked for me is Croft Kettle, a small pond is fed by subterranean springs emanating from the Permian limestone.   It is fringed by the saw sedge, Cladium mariscus but the edges of pond then shelve very steeply and the submerged vegetation is dominated by Chara hispida and C. vulgaris.   Whether Nassima’s pond ever looked like the illustration below is debatable (there are fossil forests near Guern Touil so the idea of a tree-fringed oasis at some point in the Holocene is not wholly fanciful). The bare evidence that palaeoecologists produces often needs to be catalysed by the imagination, and the imagination, in turn, feeds off analogies. So long as we treat these speculations with a healthy dose of caution, all is good.

Croft_Kettle_May15

Croft Kettle, a Site of Special Scientific Interest in County Durham, just south of Darlington, photographed in May 2015.

As the pond is fed by subterranean springs, the water in Croft Kettle is very clear, allowing the dense Chara beds to extend into the depths.   I could only reach the very edge of these beds when I visited a few days ago, but I was struck by the large quantity of yellow-brown diatom growths that smothered the Chara.   Under the microscope, these proved to be composed of a dense tangle of a stalked diatom, probably Cymbella cymbiformis, within which other diatoms such as Rhopalodia gibba and Navicula radiosa were moving.   The Cymbella is the same one that Nassima found in Guern Touil and I could also see representatives of three of the other genera that she described. I have recorded some of the other species that Nassima recorded from here, but they were not showing themselves today.

Croft_Kettle_Chara_May15

An underwater view of the margins of Croft Kettle, showing the dense beds of Chara, smothered by growths of diatoms, May 2015.

The quantity of diatoms that I saw in Croft Kettle was surprising, especially as I normally expect grazers to be very active at this time of year.   The yellow-brown growths resembled those that I reported from the River Ehen in April (see “Diatoms and dinosaurs”). Those were of a Gomphonema species which, like Cymbella cymbiformis, grows on the end of long stalks. These, in turn, create a tangled matrix within which other species of diatom can live.

The Cymbella cells become detached from their stalks very easily, which means that it is easier to photograph isolated cells than the complete stem plus stalk complex.   The tangle of stalks is also difficult to capture in a photograph due to the very shallow depth of field available when you are using medium-and high-magnifications.   That brings me back to the topic of imagination: the microscopist needs this just as much as the palaeoecologist, if s/he is to gain an insight into the nature of communities that have been wrenched out of their natural habitat and squashed under a cover slip.   More so, indeed, for the diatomist, who habitually marinades samples in a sauce of oxidising agents to leave just the silica frustule behind.   But here I go again … droning on about the need to understand diatoms in their living state.   Forgive me …

Enough for today: Croft Kettle is a pond with many fascinating – and one or two very unexpected – stories to tell.   Plenty to keep me going for a few more posts …

Cymbella_cymbiformis_Croft_

Cymbella spp. growing on Chara in Croft Kettle, May 2015; a. – c.: Cymbella cf. cymbiformis; d. – e.: two as yet unknown Cymbella sp. Scale bar: 10 micrometres (= 1/100th of a millimetre).

References

Wheeler, B.D. & Whitton, B.A.(1971). Terrestrial and Sub-aquatic vegetation. The Vasculum 56: 25-37.

Hudson, J.W., Crompton, K.F. & Whitton, B.A. (1971). Ecology of Hell Kettles; 2. The Ponds. The Vasculum 56: 38-45.