In my post on Croft Kettle, I commented on the long stalks possessed by Cymbella cymbiformis. These were difficult to capture with my camera, partly because the Cymbella cells readily detach themselves from their stalks and partly because the tangle of stalks exceeds the depth of field available to the microscopist. Instead, I have tried to capture the view through the microscope eyepiece in a drawing.
Croft Kettle, epiphytic algae associated with Chara hispida stems, May 2015. Drawn at x400 magnification. The narrow stalks on the left hand side are about five micrometres in diameter.
There is a tangle of stalks on the left hand side, along with two cells each of Cymbella cymbiformis, Navicula radiosa and Rhopalodia gibba. Note, too, the narrow filament of Oedogonium, complete with oogonia (see “Love and sex in a tufa-forming stream …”). The Rhopalodia cells have glided free from the tangle of stalks.
As I looked at these rich communities of algae I started to wonder if would make a good subject for a painting, so I have been continuing to examine the material I collected in order to build up a sense of what the three-dimensional community around the Chara stems would have looked like. One interesting observation came when I had a look at some of the narrow branchlets of Chara. I wanted to see which algae were directly attached to the Chara surface (more about this in a moment) but the feature that was most noticeable was the quantity of calcite crystals deposited around the stems. These give Chara its characteristic stiff stems that are rough to the touch. The calcite is deposited as a by-product of photosynthesis; intriguingly, Chara shares this property with many tufa-forming algae and bryophytes but not with its close relative Nitella, which is much softer to the touch (see “Finding the missing link in plant evolution…”).
Calcite crystals deposited around the tip of a branchlet of Chara hispida from Croft Kettle, May 2015. Image composed using Helicon Focus stacking software. Scale bar: 10 micrometres (= 1/100th of a millimetre).
One of the questions that was puzzling me was the habit of the diatom Rhopalodia gibba within the community of algae on and around the Chara stems. In many of my specimens, Rhopalodia seemed not to be attached to the Chara but, instead, glided amidst the tangle of Cymbella stalks growing around the Chara stems; however, I also saw a few cells directly epiphytic on the Chara stems, and this also seems to be the habit that Chris Carter has captured in some of his images of Rhopalodia. I suspect that Rhopalodia, and many other diatoms are opportunistic and can adopt slightly different habits depending upon the prevailing conditions. There is no point, for example, in doggedly sticking to an epiphytic habit if this m
eans sitting in the deep shade cast by a Cymbella forest. Whatever the textbooks say.
Rhopalodia gibba associated with Chara hispida stems in Croft Kettle, May 2015. b. is a valve view; e. is a girdle view and a.,c. and d. are intermediate between the two positions. Scale bar: 25 micrometres (= 1/40th of a millimetre).
Rhopalodia growing on Chara. Photographs by Chris Carter.
Rhopalodia is a genus with an unusual morphology. The raphe follows the dorsal margin (i.e. the left hand side of b. in the figure above) but this means that, in girdle view (i.e. looking from above), both raphes are on the same side of the valve. I have often assumed that having raphe slits on opposite sides assists motility, by giving the cell two planes by which it may attach (much like a climber working his way up a narrow chimney). It is possible that being attached to the surface is the preferred habit; motility would become an advantage only when the energy that this process consumes is offset by that supplied by the extra photosynthesis that can take place when it moves away from the shaded areas and into the canopy. I have never seen any work done to address this topic, but it would make an interesting study.
Another interesting feature of Rhopalodia is the presence of cyanelles, organelles derived from cyanobacteria or similar prokaryotic algae. We also encountered these in Epithemia (see “A return to Cassop”) where I also mentioned that they may be involved in nitrogen fixation. Cyanelles deserve a post all of their own at some point in the future, so I will just leave you for now with another of Chris Carter’s excellent photographs, in which the cyanelles of a Rhopalodia sp. are highlighted. They are near-transparent, with thin membranes and are easy to confuse with the vacuoles that contain the polysaccharide chrysolaminarin (these tend to be more refractive). Very easy to overlook.
As I was putting this post together, I noticed that West and Fritsch noted that Rhopalodia gibba was “common in all kinds of localities.” This surprised me, as I have only ever seen it at a handful of sites in the UK. It did make me wonder if West and Fritsch, writing in 1927, were right, and that it has declined significantly subsequently. A species that has a competitive advantage at low nitrogen concentrations will not have had an easy life in the period after West and Fritsch wrote, as agricultural intensification and widespread use of fertilisers led to increases in the concentration of nitrogen in surface water.
A girdle view of Rhopalodia sp. with cyanelles indicated by arrows. Note, too, the characteristic lobed chloroplast. Photograph by Chris Carter.
West, G.S. & Fritsch, F.E. (1927). A Treatise on the British Freshwater Algae. Cambridge University Press, Cambridge.