Cymbella-dominated community of epiphytic algae around Chara hispida stems in Croft Kettle, May 2015.
The final stage of my journey of discovery through Croft Kettle is a three-dimensional diorama in which the various components that I have described in posts since my visit on 29 May are reassembled into something approaching their natural state. In this image, I have tried to show how much of the yellow-brown gunk which you can see in my post of 1st June is, in fact, the stalks of Cymbella cymbiformis, which form a dense matrix around the Chara stems. This, in turn, creates a habitat within which other diatoms can move around. In my illustration, I have included cells of Navicula radiosa and Amphipleura pellucida as well as Rhopalodia gibba and several crystals of calcite. The Rhopalodia intrigues me: Chris Carter’s photographs in my post from 7 June show it attached to Chara stems but I did also see it moving around in samples dominated by the Cymbella stalks. Adhering strictly to a sessile lifestyle when Cymbella stalks are growing all around you and creating a light-capturing canopy is probably not a great survival strategy and the capacity to move amidst this forest of stalks must give the Rhopalodia more opportunities. On the other hand, Rhopalodia is not really optimised for motility, as both of it’s raphe slits are on the same side, in contrast to Navicula and relatives where they are on opposite sides. Diatoms exude mucilage through the raphe which attaches to the surface and then gives them something to push against. In this Cymbella forest, these stalks will, presumably, provide that point of contact. As the diatom pushes against the stalk, it will need to connect with another stalk before it can progress. Having two raphe slits on opposite sides of the valve increases the chances of this happening (in much the same way as a climber using both arms and both legs to work his or her way up a narrow chimney). Having both on the same side, presumably, reduces the chance of successfully adhering to another stalk. It is, I suspect, a case of “needs must”: limited motility is still better than no motility at all. If you look carefully, you’ll see that I’ve also included some cyanelles in the Rhopalodia cells.
It is slightly disingenuous for me to suggest that this image is purely the result of my own observations. They are, for sure, the starting point but I find myself referring to several books as I constructed the image. The view down the microscope has a very flattened perspective, which means that it can be difficult to get an impression of the three-dimensional appearance of a diatom such as Rhopalodia. For this, I referred to scanning electron micrographs in The Diatoms: Biology and Morphology of the Genera. However, these show the diatom frustules as opaque so I then need to refer back to my own images in order to build up a view of the cell interior. I can start from my own observations but, after a few days, the chloroplasts of some species start to degrade, so I also turned to Eileen Cox’s book on identification of live diatoms. This is good for details of the plastids, but not so good for stalks so, for these, I am back to peering down my microscope at fresh material. Finally, I found an excellent new book on charophytes that had some great illustrations that helped me understand the structure of the stem of Chara.
This interplay between direct observation and existing knowledge is necessary and, indeed, there are noble precedents (see “I am only trying to teach you to see …”). However, it also carries the possibility that we promulgate the errors of the past; we look at the natural world through eyes conditioned by the opinions and interpretations of others. But, then, my picture is no more than an accumulation of my own opinions and interpretations. In science, as in history, we always walk backwards into the future …
Cox, E.J. (1996). Identification of Freshwater Diatoms from Live Material. Chapman & Hall, London.
Round, F.E., Crawford, D.M. & Mann, D.G. (1990). The Diatoms: Biology and Morphology of the Genera. Cambridge University Press, Cambridge.
Urbaniak, J. & Gąbka, M. (2014). Polish Charophytes: An Illustrated Guide To Identification. Wroclaw University of Environmental and Life Sciences. Wroclaw.