A couple of kilometres away from the stream featured in the previous post is an old quarry that we visit each year as part of this course (see “Nosing around for blue-green algae …”). In a damp flush at the edge of the quarry floor, we found some patches of what looked, from a distance, like sheep droppings. A useful strategy, shared by most of the human race, is to assume that anything that looks like a sheep’s dropping probably is a sheep’s dropping, and not to go prodding at this with a finger. However, a curious soul in the distant past with a sense of adventure ignored this precept and discovered that a few of these were, in fact, growths of Cyanobacteria. Most natural historians, wisely, focus their attention on more spectacular aspects of life on earth; however, a few of us have retained this childish instinct to poke at anything that looks like sheep’s droppings.
Scytonema sp. from a flush at Whitbarrow Quarry, Cumbria, May 2015
A small part of one of these growths, teased out and mounted on a cover slip, reveals itself to have the characteristics of the genus Scytonema although, today and despite a long hunt, I could not find any filaments that presented themselves in a suitable position to photograph. The illustration below, therefore, is of a growth of Scytonema from another calcareous site in Cumbria. The Cyanobacteria is, you may remember, the modern name for the “blue-green algae” which is often confusing as many Cyanobacteria are not blue-green in colour. What we can see here is a chain of cells (a “trichome”) which are surrounded by a thick sheath (“trichome” plus “sheath” equals “filament”, in Cyanobacteriological lore). The sheath is a yellow-brown colour, due to a pigment called “scytonemin” which acts as a sunscreen, absorbing ultra-violet radiation and, in the process, obscuring the blue-green colour of the trichome within.
Scytonema sp. from a calcareous flush at Sunbiggin Tarn, May 2005. Scale bar: 10 micrometres (= 1/100th of a millimetre).
Two other characteristics of Scytonema are visible in the photograph. Both the left and right hand pictures shows “false branches”: if the trichome breaks for any reason, either or both of the broken ends keep growing and break out of the filament. The left hand illustration is a single false branch and, just to the left of the branch you can see the distinct “heterocyst”, a cell where nitrogen fixation takes place. The wall of the heterocyst is thicker than that of other cells, as nitrogen fixation can only occur in the absence of oxygen.
Walking back from the quarry towards the minibus, Allan pointed up at dark patches on the cliffs looming over us. It was Gloeocapsa alpina, the same species that we met a short while ago in a cave on Malta (see “The mysteries of Clapham Junction …”). The cliffs at Whitbarrow are, in effect, a vertical “desert” from the point of view of any organism that aspires to live there. These Cyanobacteria, with their ability to “re-boot” on those occasions when conditions are favourable for growth, have an advantage here. One guesses that the damp climate of north-west England is slightly more forgiving than that of a Mediterranean hillside but it is still a tough habitat in which to survive.
Growths of Gloeocapsa alpina (arrowed) on the cliffs at Whitbarrow Quarry, May 2015.