File:Cyanobacteria - Nostoc commune - geograph.org.uk - 1470398.jpg

English: Cyanobacteria - Nostoc commune. The area in which these colonies were found is the foreground that is shown covered with snow in the following photo: 1158049.

The seaweed-like mass shown here measured 15 cm across. It is a colony of cyanobacteria; specifically, it is Nostoc commune. (For more on cyanobacteria, see 1470400, taken on the same day, in the same place.)

[See http://www.algaebase.org/search/species/detail/?species_id=24479 (at Algaebase) for some pictures, and for detailed taxonomic information.]

Within its outer membrane, the colony contains a brownish gel in which chains of green cells are suspended. The colonies vary greatly in appearance according to how wet they are, but are generally olive-green to brown.

Although I've shown a single specimen here, the area was covered in seaweed-like patches, up to several square metres in size. Two factors account for the abundance: (1) for the west of Scotland, the summer of 2009 was the second wettest since records began in 1914, and (2) there is great deal of concrete here, from large chunks to tiny fragments (the Nostoc pruniforme link, above, explains why this favours their growth).

The abundance of N. commune was not confined to this spot; later on the same day, I noticed that a little concrete footbridge (NS 3984 7708) across the Murroch Burn in Kilmalid was also densely covered. Incidentally, when they dry out, Nostoc colonies become extremely inconspicuous.

[Note: Cyanobacteria are bacteria, not algae, but the existing image category of "Algae" was chosen for convenience.]