A stream flowed through Gubeikou, the village beside the Great Wall where we stayed. It ran in an artificially-straightened channel, crossed by several small bridges, fords and stepping stones and, when I first saw it, a high percentage of its surface area was covered by floating mats of algae. This, of course, piqued my interest. However, when I returned the following morning equipped with rudimentary sampling equipment, the locals were busy clearing these flocs out of the stagnant areas. The prospect of trying to communicate my interest in algae despite an almost total lack of Mandarin was too much and I skulked off, returning later to find a few of these flocs, along with some submerged leaves smothered in algae in a couple of sheltered backwaters. Even so, the sight of a wuharin (foreigner) peering intently at the less-savoury aspects of their local stream attracted plenty of curious stares from passing locals.
A floc composed of leaves from an aquatic monocotyledon and associated algae in the stream at Gubeikou, Beijing Province, China, April 2016.
Local liquor is the travelling diatomist’s best friend, as it can be pressed into use as a preservative. However, I did not add any baijiu to this sample, as this would have damaged the green algae that dominated the flocs. Instead, I stuck the samples into a corner of my suitcase and hoped for the best. This was rather optimistic on my part as, ten days later, when I finally had a chance to get the sample under my microscope, the green alga had disappeared completely and the sample was dominated by diatoms, particularly chain-forming araphid species, of which I could make out at least two species, even in this raw state.
A chain of diatoms, possibly Staurosira binodis associated with a green algal floc collected from Gubeikou stream, April 2016, along with (at the bottom of the picture), an out-of-focus chain of a smaller diatom. Scale bar: 20 micrometres (= 1/50th of a millimetre).
What I saw when I peered down my microscope at the samples, once they had been properly prepared and mounted, encapsulated many of the challenges faced by the modern diatomist. Had I collected this sample thirty years ago, I would have confidently named most of what I could see. The prevailing assumption was that diatom species were mostly cosmopolitan and I would have picked up a copy of Hustedt’s 1930 Flora from my bookshelf and matched the shapes that I saw in my Chinese sample with the illustrations. However, looking at the sample through a mind conditioned by the developments over the past thirty years, I see subtle deviations from the outlines with which I am familiar, and I start to wonder …
It doesn’t help that the most abundant group in the sample from the algal floc were chain-forming araphids of the genera Staurosira and Pseudostaurosira, a group where there is much uncertainty over species and generic limits even within the geographical areas that I do know quite well. The sample does, nonetheless, illustrate a principle that I discussed last year as, once again, we see several of these closely-related forms occurring together in the same habitat(see “When is a diatom like a London bus?”). It suggests to me that, whatever the subtleties exist in species composition, the same general factors are ordering the community, whether in western Europe or China. The sample from the submerged leaves had a quite different composition, dominated by Nitzschia species. Some of the species looked familiar but at least two of those are known to be complexes that have still only been partially unravelled.
Chain-forming araphid diatoms associated with a green algal floc in the stream at Gubeikou, Beijing Province, China, April 2016. a., b.: Staurosira cf. binodis; c.,d.: Pseudostaurosira cf. elliptica; e.,f.: Fragilaria capucina. Scale bar: 10 micrometres (= 1/100th of a millimetre).
That there are patterns in the distribution of diatoms seems to be beyond dispute. There has been an enormous amount of research on this topic in recent years, much of it in reaction to a paper by Bland Findlay and colleagues who argued that biogeographic concepts were of limited applicability to microscopic organisms. Yet we also know that some species are cosmopolitan, and the situation is further complicated because most diatomists base deductions about distribution on morphological criteria (what the [dead cell] looks like) and assume that this aligns with the biological species concept (ability of interbreeding pairs to produce fertile offspring) without further testing (the papers listed below are amongst the exceptions). Finally, the limited geographical scope most studies, coupled with the prevailing belief that biogeographical variation exists, means that it is too easy to assume that a species has a restricted distribution. It raises interesting questions about what we mean by a term such as “species” when considering diatoms, but that question will have to wait for another day.
My bigger concern is that the diatomist sees ecology in terms of nouns, whereas the dynamic systems that we study (and whose condition we are expected to advise upon) are perhaps better envisioned as a series of verbs. Seen like this, the taxonomic complexity that diatomists love to unravel distils down into little more than a source of energy for the next trophic level. Biogeographic differences only become important when they affect this flow of energy and, as we are often dealing with subtle variations in shape and size of cells, I doubt that all this taxonomic work will lead to radically different conclusions about the state of the environment. But I may be wrong. The problem is that this leads into a vicious circle: to answer questions about the extra information contained in all this diversity, we first have to unravel this diversity. But this, in turn, takes up time that could be spent asking equally valid questions about ecosystem functioning. Yet the unstudied diversity may, itself, be a confounding variable in studies on ecosystem functioning. I’d like to think that diatomists get wiser as they get older; however, I am not fully convinced that this is always the case …
Nitzschia species associated with submerged monocotyledon leaves in Gubeikou stream, Beijing Province, China, April 2016. a., b.: Nitzschia cf fonticola; c., d.: Nitzschia amphibia (girdle and valve views respectively); e.: Nitzschia palea sensu lato. Scale bar: 10 micrometres (= 1/100th of a millimetre).
Findlay, B.J., Monaghan, E.B. & Maberley, 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.
Rimet, F., Trobajo, R., Mann, D.G., Kermarrec, L., Franc, A., Domaizon, I. & Bouchez, A. (2014). When is sampling complete? The effects of geographical range and marker choice on perceived diversity in Nitzschia palea (Bacillariophyta). Protist 165: 245-59.
Trobajo, R., Mann, D.G., Chepurnov, V.A., Clavero, E. & Cox, E.J. (2006). Taxonomy, life cycle and auxosporulation of Nitzschia fonticola (Bacillariophyta). 2: 1353-1372.
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.