Plectonema [Thuret] Gomont   

Kingdom: Eubacteria
Phylum: Cyanobacteria – blue-green algae
Order: Oscillatoriales

British distribution (genus): Evidently widespread, terrestrial, freshwater and marine.
World distribution (genus): Cosmopolitan?

Plectonema on wall
Gelatinous colony of presumed Plectonema gracillimum on wall of old railway viaduct, Paisley, Renfrewshire, March 2002.

For some time, a feature of the old railway viaduct by the station in Paisley, Renfrewshire, has been that water oozes intermittently from the stonework, and that this causes rather unpleasant looking yellow-brown patches of "gunk". With some trepidation, and a silent prayer that it would not turn out to be a mass of faecal bacteria of an origin one would rather not think about, a sample was collected in March, 2002.

The "gunk" proved to be a mucilaginous mat of very fine trichomes of a blue-green alga (cyanobacterium) that belonged to the Oscillatoriales. Originally, on the simple basis that it was an "Oscillatoria in a sheath", it was identified as a Lyngbya. However, the publication of Whitton's account of the British cyanobacteria (as Cyanophyta, in John, et al., 2002) has shown that previously available generic keys were too simplistic, and that a reassessment of this identification was needed. The narrow trichomes (about 2.5 µm in diameter), in sheaths that were colourless to distinctly yellow under the microscope, do not easily match any of the Lyngbya species described in that work, while the difficulties of separating Lyngbya from Plectonema or Phormidium are emphasised (see Whitton, op. cit., pg. 60). It was noted that other recent workers have moved many of these taxa into a new genus, Leptolyngbya Anagnostidis & Komárek, characterised by narrow trichomes and firm, narrow sheaths.

Undoubtedly, the species illustrated here is a Leptolyngbya, but this genus has not yet been generally adopted in British literature. Following Whitton's account, the alga in question seems most likely to be Plectonema gracillimum Zopf ex Hansgirg (= Leptolyngbya gracillima (Hansgirg) Anagnostidis & Komárek), though no information on cell length is given.

It should be added that the mucilaginous mat contained a few diatoms and minute cells that seemed most likely to be other, minute cyanobacteria (presumably Chroococcales). While other bacteria were present (and probably multiplied after collection of the sample) they formed no appreciable growths, nor was any "sewage fungus" community present. Though unsightly, these algal mats evidently pose no health risk to users of the adjacent car-park.

Presumed Plectonema gracillimum from viaduct wall, originally photographed at ×500.
Presumed Plectonema gracillimum from viaduct wall, originally photographed at ×1250 (oil immersion).

The genus Plectonema resembles Lyngbya in that single trichomes are enclosed in gelatinous sheaths, which tend to be thin, firm and often coloured. It differs from Lyngbya primarily by the occurrence of 'false branching', where a break in a trichome results in one or both adjacent ends continuing to grow out in a new sheath at right angles, forming a 'T-junction'. This was not noted when the material was examined, but in fact it was not searched for, though the absence of true branches was confirmed. (Possibly, there is a false branch shown in the bottom left of the upper photomicrograph, but it is not clear.) As with other members of the Oscillatoriales, heterocysts and akinetes are absent. Nine British species are recognised by Whitton, of which four are primarily marine and others are freshwater or terrestrial.

P. gracillimum is a species that forms blue to yellow-brown mucilaginous mats on soil, on damp walls (notably in greenhouses) and also on metalliferous spoil heaps and in areas where there is seepage of zinc-rich waste. Whitton uses the name for declared, rather arbitrary identifications based on a simple morphological definition, with recognition that other evident Plectonema taxa, with narrower or wider filaments, also occur on old spoil heaps. At present, P. gracillimum appears to be the best name for the alga illustrated on this web-page.

•   John, D.M., Whitton, B.A., & Brook, A.J., ed. (2002). The freshwater algal flora of the British Isles, Cambridge University Press, Cambridge.

© A.J. Silverside
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