Our latest ring test* slide took us on a vicarious journey to the beautiful River Don in Aberdeenshire. Maybe because I have been doing this job for so long, but the quality of the landscape was clear to me as I peered through my microscope 500 kilometres away: the range of diatoms that I could see would not have thrived anywhere with more than the lightest touch from humankind.
One of the clues for me lay in some of the smallest diatoms on the slide. It took some discussion amongst my fellow experts, but we eventually came up with a list of five different species of Achnanthidium (all illustrated below) which, together, constituted about a third of all the diatoms on the slide (admittedly, because they are small, they constitute rather less than a third of the total volume of diatoms, but that is another story ….). The mere presence of several Achnanthidium species is, in my experience, usually a sign of high habitat quality (see “Baffled by the benthos (2)”) but unravelling the identities of the different species with a light microscope is challenging.
Achnanthidium minutissimum from Medwin Water, Scotland. Photographs from the Diatom Flora of Britain and Ireland by Ingrid Jüttner. Scale bar: 10 micrometres (= 1/100thof a millimetre).
Achnanthidium pyrenaicum from the River Don, Towie, Aberdeenshire. Photographs by Lydia King. Scale bar: 10 micrometres (= 1/100thof a millimetre).
The genus Achnanthidium is a good example of the delicate co-existence between “identification” and “taxonomy” in the world of diatoms. Individuals from this genus are usually small so anyone using a light microscope for routine analyses will be working right at the optical limits of their equipment whilst anyone with a serious interest in taxonomy will depend upon a scanning electron microscope for the insights needed for critical differentiation between species.
This divergence between the working methods of “identifiers” and “taxonomists” means that it is rarely possible to name every individual of Achnanthidium with complete confidence. The ones that present clearly in valve view (i.e. face-up) can mostly be assigned to a species based on features we can see with a light microscope, but it is not always straightforward for those seen in girdle view (i.e. on their side) or which are partly obscured by other diatoms or extraneous matter on the slide. In this example from the River Don, we also noticed that smaller individuals of A. gracillimum lost their characteristic rostrate/sub-capitate ends and were, as a result, not easy to differentiate from A. pyrenaicum.
Achnanthidium gracillimum from the River Don, Towie, Aberdeenshire. Photographs by Lydia King. Scale bar: 10 micrometres (= 1/100thof a millimetre).
What continues to mystify me is why so many closely-related species can live in such close proximity. It is Achnanthidium that prompt this question here, but other genera display similar tendencies (see “When is a diatom like a London bus?”). And this immediately generates another question: why are more people not asking this question of diatoms and, indeed, microscopic algae in general?
The answer to that question falls into two parts. The first is that understanding the precise ecological requirements of microscopic algae is not a trivial task, and assumes that you are able to get several closely-related species to live in culture (which, itself, assumes you know the precise ecological requirements of each … you see the problem?). There is, as a result, a tendency to avoid experimental approaches and, instead, look for how species associate with likely environmental variables in datasets collected from sites exhibiting strong gradients of conditions. However, this assumes that the forces that drive the differentiation between species work at the same scale at which we sample (see “Our patchwork heritage …” for more on this).
Underlying this, however, is a deeply-held belief, dating back at least forty years, that the niches of freshwater diatoms are determined primarily by the chemistry of the overlying water. This is a dogma that has served us well when using diatoms for understanding the effects of environmental pollution but which is, ultimately, a limitation when trying to explain why we found five separate Achnanthidium species in a single sample, all exposed to the same stream water.
Achnanthidium lineare (first three images from the left) and A. affine (two images on the right) from River Don, Towie, Aberdeenshire. Photographs by Lydia King. Scale bar: 10 micrometres (= 1/100thof a millimetre).
I will go one step further: this dogma is so deeply held that referees rarely challenge the weak evidence that is produced to demonstrate different responses to environmental conditions between closely-related species. There are certainly variations in environmental preferences between Achnanthidium species, but these are best expressed as trends rather than unambiguous differences and I have never seen such trends subject to rigorous statistical testing.
I blame better microscopes: greater magnification and resolution has revealed such a baffling amount of diversity that all the energy of bright diatomists is absorbed unravelling this rather than trying to explain what it all means (see “The meaning of … nothing”). If we were bumbling along with the quality of equipment that Hustedt depended upon, then maybe we would be cheerfully lumping all these forms together and focussing on functional ecology instead. Maybe.
* see “Reaching a half century” for more about the ring test scheme