The weather for my second visit to Druridge Bay this year was rather better than I enjoyed when I was here back in May. This time around, the sun was shining and the huge expanse of sand spread out invitingly before me. This was Northumberland at its very best … glorious scenery and just a handful of other tourists with whom to share it.
Druridge Bay at low tide in July 2013.
There were a series of conspicuous ripple marks on the firm sand of the lower part of the beach and, looking closely at the “valleys” within these ripple marks, I saw a few patches, each just a couple of centimetres across, that were noticeably darker than the damp sand surrounding them. These, like the chocolate-brown patches I scraped from the mud at Whitburn, were composed largely of diatoms, though a different set of species, as the sand presents a very different habitat to the mud I had sampled there. The most abundant species here was a thin bobbin-shaped diatom, with two chloroplasts mounted end-to-end in the short “body” from which projected two spines, each longer than the body itself.
This diatom looks very similar to the Nitzschia acicularis that we saw in the River Wear in a recent post. The arrangement of the chloroplasts is identical, and the cells were also busily moving around the slide, in a similar manner to cells of Nitzschia. However, when viewed under an electron microscope, there are a number of differences that have led many to classify this into a separate genus, Cylindrotheca.
Patches of Cylindrotheca closterium from the “valleys” formed by ripple marks on the beach at Druridge Bay, close to the low tide mark. The lowermost patch is about four centimetres across. A single cell of C. closterium is illustrated on the right-hand side. Scale bar: 25 micrometres (1/40th of a millimetre).
Scanning around the slide, I found some other species of diatoms including the Pleurosigma that we met at Whitburn, and some cells of a planktonic genus, Chaetoceros. Another that caught my attention consisted of a chain of cells each with a pyramidal base from which emerged a long spine. Until 1990 this would have been regarded as a marine species in the genus Asterionella, which we met a few weeks ago. It shares similar-shaped cells which form similar-shaped colonies although, on closer examination, both cell and colony shape are quite different. Here, the “spine” is much narrower and the chloroplasts are squeezed into the basal part whilst the colonies are loose helices rather than closed star-shaped colonies. As a result, this is now regarded as part of a separate genus, Asterionellopsis.
A colony of Asterionellopsis glacialis from the same sample as the Cylindrotheca closterium (above). The inset shows a single cell. Scale bar: 25 micrometres (1/40th of a millimetre).
The chain of Asterionellopsis cells is quite hard to make out in the illustration because all that I found in the sample were surrounded by what appeared to be organic detritus. Perhaps I had stored the sample too long before examining it? I’ll need to revisit the area to see if I can find some cleaner specimens before too long.
Both Cylindrotheca and Asterionellopsis illustrate an important trend in recent studies of the algae, as our ideas about the affinities between groups of organisms change as we learn more about them. In both these cases, classifications that made sense to earlier generations peering through light microscopes have been revised as more recent phycologists use scanning electron microscopy and, increasingly, molecular biological methods, to examine them. Indeed, there is still no consensus about Cylindrotheca closterium, with one very recent identification guide still regarding it as part of Nitzschia. Faced with a pile of algal identification guides, some old, some new, we are little better off than Alice when confronting Humpty Dumpty:
“When I use a word,” Humpty Dumpty said, in rather a scornful tone, “it means just what I choose it to mean — neither more nor less.”