Few of the participants in the UK / Ireland diatom ring-test that I described in my previous post felt any need to thank me for my choice of slide for our 50th test. The slide came from a spring in County Mayo, Ireland, which is part of the Agricultural Catchments Programme, a large study into the effect of farming on water quality. The sample itself came from the stems and leaves of the submerged water cress (Nasturtium officinale*) plants which fill the entire channel. It was a real stinker, with a mess of Gomphonema forms, several of which did not neatly fit any species description that we could find. A conservative reckoning is that there were at least eight different Gomphonema “species” and that raises a further question about what it was about this habitat that led to so much diversity within a single genus within a single sample.
First, a quick tour around some of the Gomphonema forms that we found. There was general agreement that the most common type was close to G. micropus Kützing 1844 but not a perfect match to published descriptions (the stria density, in particular, was too low). The situation was further complicated because the status of G. micropus was questioned at times, with it being treated as a variety of G. parvulum and placed in the G. angustatum complex by different authorities during the 20th century. Then there were a number of valves with more rounded ends and a higher striae density than G. micropus but which, if you look closely, are not symmetrical around the long axis. We thought that these were close to G. cymbelliclinum Reichardt & Lange-Bertalot 1999. Unfortunately, there were also quite a lot of valves that had intermediate properties, making it hard, in many cases, to say whether it was one species or the other.
Gomphonema cf micropus from Cregduff spring, Co. Mayo, Ireland, September 2017. Photographs: Bryan Kennedy. Scale bar: 10 micrometres ( = 100th of a millimetre). The image at the top of the post shows Cregduff spring (photo by Lauren Williams)
Gomphonema cf cymbelliclinum from Cregduff spring, Co. Mayo, Ireland, September 2017. Photographs: Bryan Kennedy. Scale bar: 10 micrometres ( = 100th of a millimetre).
We also found some valves that were close to descriptions of Gomphonema utae Lange-Bertalot & Reichardt 1999 and some that were close to G. parallelistriatum Lange-Bertalot & Reichardt 1991. We also found representatives of the G. parvulum complex, G. tergestinum and G. subclavatum (more about this one in the next post).
Gomphonema cf utae from Cregduff spring, Co. Mayo, Ireland, September 2017. Photographs: Bryan Kennedy. Scale bar: 10 micrometres ( = 100th of a millimetre).
Gomphonema cf parallelistriatum from Cregduff spring, Co. Mayo, Ireland, September 2017. Photographs: Bryan Kennedy. Scale bar: 10 micrometres ( = 100th of a millimetre).
So what is going on here? There are, I suspect, two key elements to the story that we need to explain. The first is the limits of species within Gomphonema. I’ve touched on this before (see “Diatoms and dinosaurs”) and some recent studies that combine morphological and molecular biological evidence also cast doubt on our ability to differentiate within this genus using classical approaches. Whilst I was struggling to disentangle the species in this sample, I had a conversation with an eminent taxonomist and she hinted darkly that Gomphonema was “over-described”. There is a readiness to “split” established taxa and describe new species that, in her opinion, runs ahead of the evidence.
The limitations of taxonomy cannot explain all of the variation that we observed in this sample, so the second question to ask is what it is about the conditions here that allow so many representatives of one genus to thrive. I’ve touched on this subject before (see “Baffled by the benthos (1)” and “Baffled by the benthos(2)”). In these posts I introduced G. Evelyn Hutchinson’s “paradox of the plankton” in which he suggested that environments that look uniform, to mortals six orders of magnitude larger than algae are, in fact, considerably more heterogeneous and, so offer more opportunities for “variations on a theme” to thrive. In the second post I went on to suggest that this type of diversity imparts resilience to an ecosystem and so should be looked upon as a positive feature of the ecosystem when doing ecological status assessments.
There is, however, one final possibility that, to my knowledge, has not yet been explored. The presence of transitional forms in the diatom assemblage at Cregduff may be an artefact of our inability to differentiate biological species based on a limited range of morphological criteria on offer. However, it is also possible that we are looking at a situation where the Linnaean species are not reproductively isolated from one another, allowing hybridisation. The concept of a “hybrid swarm” is well known in some other groups (e.g. orchids) but has never been formally demonstrated in diatoms. However, the wide morphological diversity within a single genus in one sample alongo with, in some cases, apparent continua of variation, does raise questions about speciation within thi genus.
The final twist to this story is that, from the point of view of current ecological status assessments, all this diversity has little effect. Though everyone grumbled about the difficulties in naming the Gomphonema species, the results, as the box-and-whisker plot in the previous post show – were less variable than in many of our other ring tests. What I suspect happened is that the underlying taxonomic confusion means that many of these taxa have “mid-range” scores for the TDI (and other indices), so the final calculation cancels out the identification issues. Bear in mind that this may not always be the case!
* I understand that this is the correct name now, rather than Rorippa nasturtium-aquaticum. See Al-Shehbaz, A. & Price, R.A. (1998). Delimitation of the genus Nasturtium (Brassicaceae). Novon 8: 124-126.
The two papers that deal with variation within Gomphonema to which I refer are:
Abarca, N., Jahn, R., Zimmermann, J. & Enke, N. (2014). Does the cosmopolitan diatom Gomphonema parvulum (Kützing) Kützing have a biogeography? PLOS One 9: 1-18.
Kermarrec, L., Bouchez, A., Rimet, F. & Humbert, J.-F. (2013). First evidence of the existence of semi-cryptic species and of a phylogeographic structure in the Gomphonema parvulum (Kützing) Kützing complex (Bacillariophyta). Protist 164: 686-705.