Last week took me back at Ennerdale Water in the Lake District to see how the algae in the littoral zone had changed since my previous visit (see “Both sides now …”). Back in July, we had found very few algae visible with the naked eye at most of the sites around the margin that we visited; three months on, the situation is very different, with obvious growths at many locations. As Ennerdale is a remote lake with few human influences, any changes we see are likely to be the result of natural processes rather than “pollution”, so that makes the rapid increase in quantity of algae very intriguing.
One location was particularly intriguing: it was on the south west shore, where the steep scree-laden slope of Crag Fell enters the lake. The littoral zone has some large stable boulders washed by waves blown down the lake from the high fells to the east. The boulders had a covering of mosses on their upper surface and this moss, in turn, had been colonised by green algae.
Under the microscope, these growths were revealed to be the filamentous green alga Mougeotia, a relative of Spirogyra, which I have written about in a number of previous posts (it is often common in the River Ehen, for example, which flows out of Ennerdale: see “The River Ehen in February”). The curious aspect of this particular population was that there were signs of sexual reproduction. Mougeotia, along with Spirogyra and many other filamentous green algae, is usually observed in the vegetative state (see “The River Ehen in March” and “The perplexing case of the celibate alga”).
Boulders in the splash zone of Ennerdale with growths of Mougeotia over mosses (left) and growing directly on the rock surface (right). The top photograph shows a view from Kirkland across Ennerdale with Great Gable in the background.
Filaments of the green alga Mougeotia in an early stage of conjugation, with papilla growing from the lower filament towards the upper one. Scale bar: 20 micrometres (= 1/50th of a millimetre).
Conjugation involves cells in two adjacent filaments developing outgrowths (“papilla”) that meet and fuse, creating a copulation canal between the two cells. The cell contents (“protoplasts”) of both cells contract and then they both moves, amoeba-like, into the canal where they fuse to form a zygote.
The image above suggests that the upper filament may be playing hard to get, rebuffing the amorous advances of the lower filament. I don’t know enough about conjugation of these algae to know whether the enthusiasm for sex differs between filaments, but it is also possible that what I photographed is an artefact of filaments that may well have been establishing cosy relationships with neighbours before being dragged first from the lake and then onto a slide for my voyeuristic pleasure. What may have been, in Ennerdale, a patchwork of stable relationships between filaments becomes, amidst the chaos of sampling and slide preparation, a picture of phycological bacchanal.
The lower picture shows a later stage of conjugation, with a zygote forming in the copulation canal. The process takes place in three dimensions and it was difficult to obtain a crisp image, even using Helicon Focus stacking software but it gives an idea of what is taking place. The zygote will, eventually, form a tough exterior wall and sink to the bottom of the lake where they will survive until conditions become favourable again.
Filaments of Mougeotia at a later stage of conjugation: the cell contents are in the process of fusing to form a zygote. Scale bar: 20 micrometres (= 1/50th of a millimetre).
The question I have been asking myself is why this particular population has chosen to conjugate at this particular time and place. I have visited the River Ehen regularly since 2012 and have found Mougeotia or relatives on almost every visit, yet this is the first time that I have seen conjugation. There are various theories: low nitrogen concentrations have been suggested as something that promotes conjugation in Mougeotia’s relative Spirogyra, but this is unlikely to be a factor in a nutrient-poor lake such as Ennerdale. A more likely explanation may be found in the graph below, which shows lake levels in Ennerdale over the past year.
Lake levels in Ennerdale Water (from www.riverlevels.uk, measured at NY 088 153, near the outflow to the River Ehen) for the year preceding our visit in October 2016.
The alga had been growing, remember, in the splash zone. If you look at the graph, you will see that the lake had recently been almost 30 centimetres higher than it was now and, indeed, had fluctuated quite a lot over the past month or two. My suspicion is that falling lake levels, and the accompanying risk of drying out, may also have been a factor for initiating conjugation. Another possibility is that this is a seasonal occurrence that I was fortunate enough to stumble upon, and there is some evidence that dormancy is related to temperature, possibly allowing the zygotes to overwinter in the bottom muds before the increased solar radiation in the spring initiates germination, followed by meiosis (reduction division) to produce the germlings from which next season’s filaments will grow.