One of the ironies of teaching a course on algal identification in the Lake District is that we actually take the students out of the Lake District on the first field trip in order to introduce them to the enormous variety of Cyanobacteria (blue-green algae). This is because the southern part of the Lake District, where the FBA is located, is situated on the Silurian Slates, which means that the streams, lakes and tarns have fairly soft water. Cyanobacteria, on the other hand, tend to be most abundant and diverse in hard water, so we drive about 40 minutes south and east from Windermere to a limestone escarpment called Whitbarrow, where there are a number of calcareous flushes and springs that are ideal for our purposes.
We always visit the floor of an abandoned quarry in this area which has several such flushes. The quarry owners had systematically removed the limestone until they had reached the Silurian Slate underneath. This, in turn, formed an impermeable layer that intercepted any water that had percolated through the limestone. The quarry floor was, typically, slippery with calcium-rich water that had seeped out from the surrounding limestone, as well as the mucilage that the algae produced. There were also, dotted around, several unprepossessing brown objects that, to the untrained eye, could easily be mistaken for the droppings of a small animal or bird (a Peregrine falcon was circling overhead during our visit). Once the students have the courage to pick these up, they see that they are composed of a firm jelly-like substance that is, we persuade them, actually an alga and should, therefore, be dropped into one of their specimen tubes to take back for closer investigation.
Animal, vegetable or mineral? Colonies of Nostoc commune on the floor of Whitbarrow Quarry, May 2014.
Once back in the FBA’s laboratory (complete with panoramic views of Windermere), we can dissect out small pieces of the jelly-like material and squash it onto a microscope slide. What they see when they peer down their microscopes is a plethora of chains of bead-like cells of a Cyanobacterium called Nostoc commune. Most of the cells have contents that have a granular appearance, with a background of bluish-green photosynthetic pigments. A few of the cells, however, are rounder and clearer: these are the “heterocysts”, cells that are especially adapted to “fix” atmospheric nitrogen and so help the organism survive in nutrient-poor habitats. The jelly-like matrix slows the rate at which water evaporates from the colonies, with the outer layers drying to form a tough, leathery skin around the colony.
Nostoc commune from Whitbarrow Quarry under the microscope. The cells are approximately 5 micrometres (1/200th of a millimetre) across.<br
There is a fascinating short paper by Malcolm Potts on the origin of the name “Nostoc”. Because Nostoc colonies often appeared very quickly following heavy rain (because the dried colonies absorb water quickly and swell in size), there was a belief in Medieval times that Nostoc fell from the sky. A German mystic and alchemist, Parselus, was the first to use the name “Nostoc”, claiming that it was “…excrement blown from the nostrils of some rheumatick planet. The name, indeed, is strongly suggestive of both the Old English word Nosthryl and the German term nasenloch both, as Potts politely explains, “…that part of the human anatomy intimately associated with extracellular polysaccharide.”
Potts, M. (1997). “Etymology of the Genus Name Nostoc (Cyanobacteria)” (pdf). International Journal of Systematic Bacteriology 47 (2): 584. doi:10.1099/00207713-47-2-584