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.