The reason behind my trip to the Lake District a couple of weeks ago was to teach a short course on identification of freshwater macroalgae with Allan Pentecost (see “Heatwave? What heatwave” and subsequent posts for more about last year’s course). One of the sites we visit with the students is a small stream flowing off Whitbarrow, a Carboniferous limestone outcrop in southern Cumbria. The bed of the stream is covered with tufa, formed from calcium carbonate precipitated from the water. We bring the students here because there is usually a good variety of cyanobacteria for them to learn to recognise in the field and to sample for later investigation in the laboratory. Amongst these cyanobacterial growths, however, we also saw a few patches of green filaments on the stream bed, which we also took back with us.
Sampling Whitbarrow tufa stream in May 2015.
These filaments turned out to be growths of the green alga Oedogonium. You may remember that I wrote a post last year with the title “The perplexing case of the celibate alga …” in which I commented that Oedogonium, though a common genus in freshwaters, is difficult to identify to species because this requires the reproductive organs which are rarely seen in the wild.
Our population of Oedogonium, however, was fertile, and this enabled us (Allan, to be strictly honest, as he knows the algae of tufa-forming streams extremely well) to name it. The images below show the distinctive swollen oogonia within filaments of narrow cells (compare these with the much broader cells observed in “A case of mistaken identity?”). These oogonia look as if they have already fused with the male antheridia to form zygotes, which will eventually be released. These zygotes can lie dormant for a long time, which makes sexual reproduction a useful technique for overcoming adverse conditions (see also: “The River Ehen in March”). Not very romantic, I know, but that’s the reality of life at the unprepossessing end of biodiversity.
Oedogonium calcareum from Whitbarrow tufa stream, May 2015, showing oogonium. Arrows indicate position of “caps” (scar tissue from intercalary cell division) a. scale bar: 20 micrometres (= 1/50th of a millimetre); b. & c.: scale bar: 10 micrometres (= 1/100th of a millimetre).