Theme and variations

Following our visit to the cities of the Silk Road (see “Daniel and his den of diatoms …”) in April we turned our eyes in the opposite direction and, within an hour of leaving Tashkent, we had left the flat plains behind and climbing into the foothills of the Tien’shan mountains.   The intensive agriculture of the lowlands gave way to pine forests and, as the road started to twist and turn up the slopes, we started to get tantalising glimpses of the snow-capped mountains which straddle Uzbekistan’s eastern border with Krygyzstan.

As ever, I looked for opportunities to combine business and pleasure, collecting one sample from a small calcareous seepage in the hills near the village of So’qoq and another from a stream running through mixed geology near the village of Kumyshkang, where we were staying in a Soviet-era dacha.   Sampling the seepage drew some curious looks from two women who were collecting water mint from further downstream, and yielded an almost pure growth of a diatom that is either Achnanthidium pyrenaicum or a close relative.   This would have been, by the way, the diatom that I would have expected to find were I to sample a remote, unpolluted calcareous stream in the UK.

Achnanthidium cf pyrenaicum from a calcareous stream near So’qoq in eastern Uzbekistan (41°18’45.6” N 69 ° 51’40” E).  a. – d.: rapheless valves; e. – g.: raphe valves; h.: girdle view.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Later in the day, we explored a side valley of an unnamed river that flows through the village of Kumyshkang.  The steep landscape on the south side of the valley had a thin cover of scrubby vegetation (in contrast to the wooded slopes on the other side) and the stream tumbled off the hillside towards the river below.  The biofilm, partly as a result of this harsh environment and partly, I suspect, due to grazing by invertebrates, was very thin but, nonetheless, quite diverse, with Achnanthidium minutissimum predominating.  There were a lot of outcrops of pink granite in the hillsides around the stream, but there were other rocks too, including shales and slates.   The flora here, as a So’qoq, would not look out of place in samples I find in the UK although the mix of taxa is not what I would expect if granite was the predominant rock in the catchment.   I travel light, without meters to check the chemical composition of the water, so there is no way to confirm this.  Except by going back one day better prepared …

Diatoms from a stream near Kumyshkang, Uzbekistan (41°18’45.6” N 69 ° 51’40” E, approx. 1400 m above sea level).  .   i.: Ulnaria ulna; j. – l.: Achnanthidium minutissimum; m.: A. cf. pyrenaicum; n., o.: A. cf caledonicum; p.: Achnanthidium girdle view; q.: Navicula tripunctata; r. Navicula sp.; s. Gomphonema gracile; t. Gomphonema sp.; u. Surirella brebissonii var. kutzingii; v. Diatoma moniliformis; w. Nitzschia sp.; x. Planothidium lanceolatum; y. Reimeria sinuata; z.: Encyonema ventricosum; aa.: Encyonopsis sp.   Scale bar 10 micrometres (= 100th of a millimetre).

I should add a caution about names applied to Asian diatoms using identification literature written for European freshwaters, especially after my comments in “Back to the Himalayas …”.   Until the 1980s there was a widespread belief that diatom species were cosmopolitan and could be found all around the world.  This belief became self-fulfilling as, armed with this assumption, biologists set out with books written by and for Europeans and blithely applied the names to the diatoms that they found.  From the 1980s, however, papers started to appear in which people took a closer look at variation in some of these apparently cosmopolitan species and argued that there were, in fact, substantial differences between forms from different locations, and that there were, in fact, much greater numbers of diatoms than previously thought, and that many of these were restricted to particular geographic regions.   But then, in 2002 Bland Finlay and colleagues challenged this emerging view by arguing that it was not diatoms that were restricted in their distributions, it was the locations where these detailed studies had been performed that were rare.   In other words, given enough time and effort on the part of diatomists, we should expect to see these so-called endemic species cropping up in samples from all over the world.

This created a brouhaha within the diatom world which resulted in some further papers that questioned Finlay’s assertions and argued from theoretical grounds that there was no reason why diatoms should not be restricted to a limited geographical area.  As the new century progressed, diatomists added molecular barcoding to their armouries and this offered partial support for both positions: some diatoms – or at least some strains of some diatoms, Nitzschia palea and Gomphonema parvulum, for example – do appear to be genuinely cosmopolitan whilst others do not.  Of course, Finlay and colleagues always hold the trump card in this respect: it is not possible to disprove the existence of any so-called endemic species elsewhere in the biosphere until every conceivable habitat has been examined. But a truce, of sorts, does seem to be emerging.

Sampling the calcareous seepage near So’qoq, April 2017.  The picture at the top of the post shows the valley upstream of Kumyshkang.

The truth may well lie between the two extreme positions.  Maybe many diatoms really are widely distributed because random dispersal mechanisms for microscopic organisms are highly effective, as Finlay and colleagues argue.  But every time a few viable cells of a diatom species land on a suitable habitat, their small pool of genetic variability will either thrive or disappear.   When they thrive, the story of Darwin’s finches will be replayed and a combination of genetic drift and selective pressures will create variations on the original theme, just waiting for an observant biologist to come along and discover the new species.

The question that intrigues me is whether or not the bugs that crawl across the submerged stones in search of food ever notice the difference.   One of my perennial bugbears is that the careful taxonomic work that has resulted in the discovery of all this diversity within diatoms is rarely accompanied by ecological analyses of similar rigour.   In particular, do these different forms of what we once regarded as “cosmopolitan” species actually have any effect on how energy flows through the ecosystem?  Do they, in other words, taste different to the invertebrates that crawl across the stones in search of food?  Or, as Bland Finlay hinted in a subsequent review article, are these different genotypes, in effect, variations on the same basic “ecotype”?   In which case, a casual observer crouching beside a foreign stream may not know the precise name of every species he encounters but still may have a pretty good idea of how these fit into the bigger picture of aquatic diversity.

References

Finlay, B.J. (2002). Global Dispersal of Free-Living Microbial Eukaryote Species.  Science (New York) 296: 1061-1063.

Finlay, B.J. (2004). Protist taxonomy: an ecological perspective.  Philosophical Transactions of the Royal Society Series B 359: 599-610.

Finlay, B.J., Monaghan, E.B. & Maberly, S.C. (2002). Hypothesis: the rate and scale of dispersal of freshwater diatom species is a function of their global abundance. Protist 153: 261-273.

Kemmarec, 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.

Mann, D.G. & Droop, S.J.M. (1996).  Biodiversity, biogeography and conservation of diatoms.  Hydrobiologia 336: 19-32.

Telford, R.J., Vandvik, V. & Birks, H.J.B. (2006). Dispersal limitations matter for microbial morphospecies. Science (New York) 312: 1015.

Trobajo, R., Clavero, E., Chepurnov, V.A., Sabbe, K., Mann, D.G., Ishihara, S. & Cox, E.J. (2009). Morphological, genetic and mating diversity within the widespread bioindicator Nitzschia palea (Bacillariophyceae). Phycologia 48: 443-459

Vyverman, W., Verleyen, E., Sabbe, K., Vanhoutte, K., Sterken, M., Hodgson, D.A., Mann, D.G., Juggins, S., van de Vijver, B., Jones, V., Flower, R., Roberts, D., Chepurnov, V., Kilroy, C., Vanormelingen, P. & de Wever, A. (2002). Historical processes constrain patterns in global diatom diversity. Ecology 88: 1924-1931.

A view of the Tien’shan mountains from near So’qoq, Uzbekistan.

Advertisements