I drove from Bogle Bridge, my sample site on Stockerley Burn, up a low hill and down the other side into the next valley where another tributary stream of the River Browney flowed. I scrambled down to the stream and hunted amongst the stones for any algae. There were fewer green filaments here but I did find a small area of a stone covered with a dark brown mat that intrigued me. My initial suspicion was that it was a mat of the filamentous blue-green alga Phormidium, but it turned out to be composed almost entirely of Nitzschia sigmoidea, a diatom that we last encountered in the brackish waters around Jarrow (see “All things bright and beautiful?”).
Nitzschia sigmoidea from Smallhope Burn, Knitsley Bridge, August 2014. The upper image shows a single cell, the lower image two cells shortly after division. Scale bar: 20 micrometres (= 1/50th of a millimetre).
If you look back at the post on Nitzschia sigmoidea at Jarrow, you’ll see that it was smothered in epiphytes. Some of the cells here at Smallhope Burn also had attached epiphytes, though this time it belonged to a different genus.
I have called this epiphyte “Synedrella subconstricta”, though it has had several different names during the course of my career. When I started, it was Synedra parasitica var. subconstricta but it has also been moved to the genus Fragilaria and, from there, to Pseudostaurosira. Some have regarded it as a variety, others as a distinct species. What is generally agreed is that it is usually found attached to Nitzschia sigmoidea, and occasionally to other large motile diatoms. Frank Round regarded this as a major reason for creating a new genus, Synedrella, for this and a related taxon, S. parasitica. Don’t let this species name misguide you: there is no evidence that it is a parasite; it is more likely to be a commensal relationship of some kind. I am less convinced by more recent efforts to move it to the genus Pseudostaurosira, which generally forms long chains rather than sharing the habit of single cells with a short but flexible stalk. This move reflects the tendency of my fellow diatomists to spend too long staring at the empty cells of diatoms and not enough time watching them in their live state. I have hunted around to see if there is any molecular evidence to support any of these genus names but, so far, have not had any luck. How, I wonder, would we get the pure culture that we need as our source of DNA when the diatom seems to be an obligate epiphyte?
And, one more question that puzzles me: what advantage does a sigmoid outline confer onto a diatom? You’ll see from the second of my two images that there is also a cell of another sigmoid genus, Gyrosigma, in the same habit. This is a relative of Pleurosigma, which we at Whitburn last year (“Microscopic monsters in mud”) and there are also sigmoid species in the genus Stenopterobia. The sigmoid habit must have evolved several times though it is interesting that all of these are motile genera and two (Nitzschia and Stenopterobia) also include straight species. Any ideas?
Nitzschia sigmoidea with an epiphytic cell of Synedrella subconstricta and, just above them, a cell of Gyrosigma (probably G. acuminatum). Scale bar: 10 micrometres (1/100th of a millimetre).
Medlin, L., Jung, I., Bahulikar, R., Mendgen, K., Kroth, P., Kooistra, W.H.C.F. (2008). Evolution of the diatoms. VI. Assessment of the new genera in the araphids using molecular data. Nova Hedwigia Beiheft 133: 81-100.
Medlin, L., Yang, I. & Sato, S. (2012). Evolution of the diatoms. VII. Four gene phylogeny assesses the validity of selected araphid genera. Nova Hedwigia Beiheft 141: 505-514.
Round, F.E. & Maidana, N.I. (2001). Two problematic freshwater araphid taxa re-classified in new genera. Diatom Research 17: 21-28.