Oedogonium presents a real challenge to an ecologist. As I mentioned in my previous post, there are many species and these are found in a wide variety of conditions. In order to identify the species we need the reproductive organs but, as is the case for several filamentous freshwater algae, these are rarely seen in the wild. I did consult two colleagues on whether it was possible to induce Oedogonium filaments to grow these in the laboratory, but both told me that this was difficult. The theory is that you are more likely to find reproductive organs in situations where the alga has been allowed to dry slowly. This is a useful survival strategy as the spores are usually very resistant to desiccation and can survive long periods out of water. However, converting theory to practice is not straightforward.
But how, I wondered, was the section on Oedogonium in the Freshwater Algal Flora produced? ‘From secondary sources’, came back the reply. In other words, the author of this part of the Flora had relied descriptions and illustrations in earlier publications. As the most thorough work on Oedogonium in the UK was performed by the Wests, father and son, in the late nineteenth and early 20th centuries, this means that there has been no thorough overview of Oedogonium here for over 100 years. I searched the database Web of Science and found just 14 papers that reported studies on the taxonomy of Oedogonium in the intervening years. Just two of these were from European laboratories: one in 1991 in Czechoslovakia and a Polish study from 1979. That’s not very much, considering the large number of species and their very broad distribution.
Just as we can identify some flowering plants from their vegetative characteristics alone, so some people have tried to identify Oedogonium using just the properties of the filaments. However, there is not very much to go on, apart from the length and width of the cells. The best attempt is that by my colleague Susi Schneider in Norway (see “A brief excursion to Norway”). She differentiated eight types of Oedogonium in Norwegian rivers based on cell dimensions and noted a significant relationship between these types and phosphorus concentrations in the rivers where they grew. Interestingly, the narrow forms were associated with low nutrients whilst the broader ones were found in more nutrient rich conditions. The population I found in Stockerley Burn was relatively broad which suggests, using Susi’s criteria from Norway, that this is a nutrient-rich stream. I am, however, reluctant to import Susi’s categories directly to the UK because our rivers are very different from those in Norway. However, I think it would be interesting to see whether the broad principles could be used here, even if we needed a slightly different calibration.
These struggles with Oedogonium also suggest that this is a genus that would benefit from a molecular genetic study, which would be a much more powerful means of differentiating between forms of Oedogonium although, unless we cracked the secret of either finding or culturing fertile Oedogonium it will be difficult to reconcile the DNA results with classical taxonomy. Until then, I fear, Oedogonium, represents yet another case of the “trailing edge” of science, where we may be in danger of forgetting faster than we learn.
Schneider, S.C. & Lindstrøm, E.-A. (2011). The periphyton index of trophic status PIT: a new eutrophication metric based on non-diatomaceous benthic algae in Nordic rivers. Hydrobiologia 665: 143-155.