Diminishing with age …

Peering down my microscope following my latest trip to the River Ehen, I saw the characteristic curved outlines of cells the diatom Hannaea arcus. It is a species that is most abundant in the spring time and, then, only in relatively unpolluted streams. What surprised me was that it was far less abundant in my sample this year than from samples collected at the same time last year. Of course, as I only visit once a month and this species only thrives for a few weeks, I may have missed the peak of its growth. Or some as-yet unknown combination of the organism’s life-cycle and local environmental fluctuations may have conspired to keep numbers lower than last year.


Live cells of Hannaea arcus from the River Ehen, near Ennerdale Village, May 2014. Scale bar: 10 micrometres (100th of a millimetre).

One other feature that struck me is that the diatoms I was looking at this year seemed to be shorter than those I saw last year. I dug out an old slide to check this and the difference is quite striking. Last year, I saw cells that were 100 micrometres or even longer in a few cases. This year, the longest I saw was 70 micrometres. Such fluctuations in size are common in diatoms and relate to the way the cells divide. The silica cell wall, the frustule, is in two parts, which overlap in the manner of the two halves of a Petri dish or old-fashioned date box. When the cell divides, each of these halves becomes the larger half of one of the two daughters so the average size of the population drops. This is repeated many times until, eventually, cell size diminishes to a threshold whereupon sexual reproduction is initiated.


Cleaned cells of Hannaea arcus collected from the same site on the River Ehen as the live cells photographed above, but a year earlier, in spring 2013. Scale bar: 10 micrometres (100th of a millimetre).

We do not know much about the specifics of the life-cycle of Hannaea arcus but a colleague, David Jewson, suspects that many freshwater benthic diatoms have a two-year life cycle. The reduction in size between 2013 and 2014 would support this assertion. The big question, then, is what size will the cells be when we return to the river in spring 2015?

One aside on Hannaea arcus: a couple of months ago, I wrote about the late John Carter (“remembering John Carter”) and I recalled seeing a short paper that he wrote in 1946. In those days, Hannaea arcus was known as Ceratoneis arcus and he wrote about a sample he had collected in 1927 which was an almost pure growth of this species. That, in itself, is uncommon but it was the location that surprised me: he had found it in a water-filled hole, nearly twenty feet (6.5 metres) up an oak tree. Having only known him as a staid old man, the image of the younger John Carter clambering through the branches of an oak tree in search of algae brought a wry smile to my face.


Carter, J.R. (1946). Diatom notes: the importance of records. The Microscope 6: 70-73.


More about John Carter

John Carter, who I wrote about in my last-but-one post (“Remembering John Carter”) had a great interest in Achnanthes, the old genus to which Platessa bahlsii (see post of 12 March 2014) would have once belonged.   In the early 1960s he wrote a series of papers for the journal The Microscope entitled “The genus Achnanthes as it occurs in British fresh waters”, and which included a key and many of John’s highly detailed drawings (see figure).  “I feel”, he wrote, “that many forms are overlooked simply because of their minute size and the extreme fineness of sculpture on the valve.”   He spent many hours carefully drawing what he could see down his microscope and, indeed, described several new species.  I have long believed that drawing is an important discipline for focussing attention on the properties of organisms and John was an exemplar of this.  Within a few years of his death, however, the advent of high-resolution digital imaging made it much easier to put several individuals from each of several populations alongside one another in order to study this variation more objectively (see the images of Platessa bahlsii as an example).


An illustration from John Carter’s 1961 paper on the genus Achnanthes.  These drawings show several species most of which would now be placed in the genus Planothidium.

He attempted, along with two other amateur microscopists, to draw all the diatoms then known from Britain, and their Atlas of British Diatoms was published three years after his death in 1996.  It is still sits on the bookshelf beside my microscope, as much for the aesthetic pleasure that I gain from his meticulous drawings of the larger diatoms such as Pinnularia.  The book is now, alas, out of print.

My favourite memory of John Carter? I mentioned to him how I found the German in Hustedt’s 1930 edition of the Susswässerflora von Mitteleuropa much easier to understand than that in Krammer and Lange-Bertalot’s revised version.   He smiled as he recalled a conversation with Lange-Bertalot: “Horst, I said, you speak wonderful English but you write terrible German.”   I recounted this anecdote to a German colleague last year who looked horrified and told me that Lange-Bertalot is admired in Germany for his very meticulous and precise technical German.  This story sums up for me John Carter’s archetypal no-nonsense Yorkshire attitude that treated everyone from students to eminent professors in exactly the same manner.


Carter, J.R. (1961).  The genus Achnanthes as it occurs in British fresh waters.  The Microscope 12: 320-325.

Hartley, B., Barber, H.G. & Carter, J.R. (1996).  An Atlas of British Diatoms (edited by P.A. Sims).  Biopress, Bristol.

Remembering John Carter

I wondered, as I re-read the previous post, what my late mentor and friend John Carter would have called the diatom I was writing about.   When I first started looking seriously at diatoms in the early 1990s, there was no-one in my laboratory in Durham with any experience from whom I could learn, and Brian Whitton suggested I went up to visit John at his home near Hawick, in the Scottish Borders.   I rung to arrange a date and, a couple of weeks later, made the two hour drive up through Northumberland and across the border to his house in the small village of Denholm.   Stepping into John’s study was like stepping back in time fifty years: it was dark and dusty, with piles of books and file boxes lining the walls and stacked on tables, along with boxes of microscope slides.  In the fireplace there was some of the equipment that he used to digest his diatom samples – apparatus that really belonged in a laboratory fume cupboard.   And, on a narrow desk against one wall, an old brass microscope equipped with a tilting mirror rather than its own light source.


John Carter in his study at Denholm, in Scotland.  An uncredited photograph from his obituary in Diatom Research

We spent the day in this study, John peering down his microscope and calling out the names of the diatoms he saw along with a commentary on the diatomists he had known (Hustedt, I remember him telling me, was a member of the Nazi party, which made it difficult for him to re-integrate with the scientific community after the war).  I perched on a chair beside him taking notes and occasionally squinting down the microscope to see for myself what he was describing.  I often, too, got a pithy assessment of the state of the slides that I had brought with me.   After a couple of hours of this, we would be summoned by his wife into the dining room for a hearty casserole, and while we ate they would quiz me about my children and talk about their years in the Borders.

Later, back in Durham, I would go back through the slides and try to reconcile my notes with what I could see under the laboratory’s much more modern microscope, ever marvelling at just how much detail John had been able to see with his old equipment.   It was a steep learning curve but, after half a dozen visits to John, it all began to make sense, and I gradually gained the confidence I needed to identify diatoms on my own.   Not long after that we had a telephone call in the laboratory to say that John Carter had died.  I felt that a door onto an older, more civilised, way of doing science had closed.