Mougeotia was not the only alga that intrigued me in Ennerdale Water during my recent visit (see “Fifty shades of green …”). Alongside the green tufts, and also just at water level, there were dark spots and patches on the rock that yielded to a gentle scrape with my finger nail. The colour suggested Cyanobacteria, so I popped a little into a sample bottle to examine later.
Patches of Stigonema mamillosum and Scytonema cf crustaceum growing at water level on granite boulders on the southern shore of Ennerdale Water, October 2016. The scale bar is approximately one centimetre.
The surprise, when I looked down my microscope, was not that it was cyanobacteria, but that there were at least three genera mixed together. The first of these was Scytonema cf crustaceum, characterised by a thick brown sheath and the presence of double “false branches”, formed when both ends of a broken filament continue to grow and, eventually, burst out of the sheath (see “Poking around amongst sheep droppings”). In the image below you can see the narrow blue-green filament of cells within the much broader sheath.
Also present was Stigonema mamillosum, a representative of a genus with a more advanced morphology than other Cyanobacteria, with branched filaments that can be several cells thick (see “More from the River Atma”), and Calothrix sp., which has tapering filaments in a much thinner sheath. All three genera have the capability to fix atmospheric nitrogen, so thrive in nutrient-poor habitats such as Ennerdale (see also “Both sides now …”). Calothrix, in addition, is able to scavenge phosphorus from the water, releasing enzymes from the long colourless hairs (just about visible to the right of my photograph).
Scytonema cf crustaceum from the littoral zone of Ennerdale Water, October 2016. Scale bar: 20 micrometres (= 1/50th of a millimetre).
Stigonema mamillosum and Calothrix sp from the littoral zone of Ennerdale Water, October 2016. Scale bar: 20 micrometres (= 1/50th of a millimetre).
I found superficially-similar growths on rocks on the north east side of the lake, but it was clear, even from the appearance in my sample bottle, that this was something different. The tangles of filaments from the southern shore of the lake, where I had started, had no other form when suspended in water, than an amorphous blob. However, the material from the north-east side formed distinct “tufts”. The superficial similarities continued when I peered down the microscope: once again the chains of blue-green cells were enclosed within a thick brown sheath and, once again, there were false branches. This time, however, the false branches were single, not double, and formed acute angles with the “parent” filament, rather than the near perpendicular double false-branches that we saw in Scytonema. These features are characteristic of Tolypothrix (Brian Whitton suggests T. distorta) and it is these acute branches that impart the “bushy” appearance to the colony. Like the cyanobacteria that I found on the southern shore, Tolypothrix is capable of nitrogen fixation so, its presence here is confirmation of the nutrient poor status of the lake.
Tolypothrix distorta (var. penicillata?) from the littoral zone of Ennerdale Water, October 2016. a: low power view of a tuft of filaments (approximately 5 mm in length); b: filaments showing single false branching (x100 magnification); c: medium power (x400) view of false branch. Scale bar: 20 micrometres (= 1/50th of a millimetre).
Nitrogen-fixation involves busting apart the strong bonds of atmospheric nitrogen in order that the cell can use the nitrogen to build the proteins that it needs to function. This requires a lot of energy and, as a result, the investment is only worthwhile if other sources of nitrogen are very scarce. That energy could, otherwise, be diverted to more useful purposes. The presence of so many different types of nitrogen-fixing organism around Ennerdale is sending out a clear sign that this is a nitrogen-poor habitat. Algae such as Mougeotia cannot fix nitrogen, and they presumably have to make other sacrifices (a slower growth rate, perhaps?) in order live alongside these Cyanobacteria. As far as I know, the energy costs of scavenging phosphorus from organic compounds in the water has not been calculated but the same principle must apply: the cell has to create more of the phosphatase enzymes than normal, in order to produce a surplus that can leak through the cell membrane and react with organic molecules in the vicinity. Again, that all requires energy that can be used for other purposes. In contrast to nitrogen fixation, this is an ability that Cyanobacteria share with some other algae including, possibly, Mougeotia.
Finding these algae in a one of the most remote lakes in the country, where the impact of humans is very low, I start to wonder how many of our other lakes would have had such an assemblage of organisms before agricultural intensification and the rise in population numbers. Nature is, naturally, parsimonious in the way it distributes the inorganic nutrients plants need. Necessity, we are told, is the mother of invention and the diversity we see in near-pristine habitats such as Ennerdale Water is as much the result of plants and algae finding their own individual solutions to grabbing their share of the scant resources available. There’s enough here for a BBC natural history documentary … apart from an anthropomorphic mammal or bird. Which is another way of saying … no chance …