Diatoms from the Troodos mountains

Back in April, I wrote two posts about the algae from a stream draining a chromite mine in the Troodos mountains in Cyprus (see “Survival of the fittest (1)” and “Survival of the fittest (2)”).  I also planned to write a post about the diatoms growing in the stream but the slide I prepared has been sitting on my desk over the summer whilst I was distracted by other things.  However, I have just started looking at some samples from metal-enriched streams in the northern Pennines and, curious to see whether a Cypriot chromite mine had similar effects, I blew the dust off the slide and slipped it under my microscope.

The principal effect of toxic pollution is to reduce the number of species found and, in this respect, my sample from the outflow of the Hadjipavlou mine outflow was true to form, containing just eight species.  The most abundant of these was Meridion circulare, accounting for one in four of all the cells.  What is more, many of the cells were visibly distorted (see images a., c. and d., in particular, in the plate below).  This is quite a common phenomenon in metal-polluted streams (see “A twist in the tale”) though I have not seen it quite so obviously in Meridion circulare before. My own pet theory is that one of the enzymes involved in laying down the silica cell wall has a metal co-factor that is displaced by heavy metals.

Meridion circulare from thepebbles from the stream draining Hadjipavlou chromite mine in the Troodos mountains, Cyprus, March 2019.  Scale bar: 10 micrometres ( = 1/100th of a millimetre).   The photograph at the top of the post shows snow on the Troodos mountains near the mine.

The only other diatom that was at all common in the sample was Hantzschia amphioxys, which also occurred alongside a smaller population of Hantzschia abundans.  I’ve not come across Hantzschia in metal-enriched streams before: it is a species that is most often associated with habitats that are not permanently submerged.  That may be the case at Hadjipavlou but the water that flows from mines comes from groundwater rather than rainfall so would not be subject to the strong seasonal variations that we associate with Mediterranean streams.  It is hard to draw a firm conclusion from a single visit.   Unlike Meridion circulare, however, neither population of Hantzschia showed any obvious distortion, perhaps due to the Hantzschia cells being more heavily silicified than those of Meridion circulare.

The extent to which cellular distortions are obvious does vary between species, as can be seen in “A twist in the tale …”  which compared three different representatives of the same genus in a metal-polluted stream.  I chose the word “obvious” with care as I do think that these phenomena are more easily seen in long thin cells than in shorter ones.  In the same Pennine streams where distorted Fragilaria are common, for example, I can also see distorted cells of smaller diatoms such as Achnanthidium minutissimum.  But you need a keen eye to spot these reliably.   Some other people have used fluorescent stains to look at other cellular irregularities, such as the position of the nucleus and damage to the nuclear membrane, but these require specialist approaches whereas distortions to cell outlines can be spotted from a standard analysis.

Hantzschia abundans (k., l.) and Hantzschia amphioxys (m. – p.) in the from the stream draining Hadjipavlou chromite mine in the Troodos mountains, Cyprus, March 2019.  Scale bar: 10 micrometres ( = 1/100th of a millimetre). 

A few years ago I was involved in a study of diatoms from streams in Cyprus and I dug out some of these data in order to put the Hadjipavlou sample into context.  One immediate surprise was that many of the “reference” (i.e. pristine or near-pristine) samples in that survey also had relatively low diversity.   The 45 samples in this subset had, on average, nine species, and a mean Shannon diversity index of 1.7, compared to eight species and a Shannon diversity index of 1.42 for the Hadjipavlou sample.   I’ve never been a fan of diversity indices as measures of ecological quality (see “Baffled by the benthos (2) and links therein”) although I suspect that average diversity at Hadjipavlou measured over a period of time will always be low whereas average diversity at unimpacted sites is more likely to fluctuate. Equally, low diversity coupled with a second strand of evidence, such as distorted valves, is a useful sign to an ecologist that something untoward is happening.

Number of taxa (left) and Shannon diversity (right) recorded in 45 samples from “reference” sites (i.e. minimal evidence of anthropogenic alteration) in Cyprus.  The arrows indicate the location of the Hadjipavlou stream within this dataset. 

The irony of writing about a heavily-polluted stream in the Troodos mountains is that the geological conditions which created the metal-rich veins hereabouts also create conditions for many plants endemic to Cyprus.   The serpentine and other ultramafic rocks create metal-rich soils within which few plants can survive (more about these here. I suspect that few of the plant enthusiasts drawn to Cyprus will ever cast more than a cursory glance at the green flocs adorning the abandoned mines of the Troodos mountains.

References

Licursi, M., & Gómez, N. (2013). Short-term toxicity of hexavalent-chromium to epipsammic diatoms of a microtidal estuary (Río de la Plata): Responses from the individual cell to the community structure. Aquatic Toxicology 134-135: 82-91.  https://doi.org/10.1016/j.aquatox.2013.03.007

Messy bedrooms …

When I was tramping around the Shetland Islands earlier this year (see “Hyperepiphytes in the Shetland Islands“), looking at the algae that live in the freshwater lochs, I noticed some meandering hieroglyphs made from fine sediment on the tops of some of the stones in the littoral zone.   I see these occasionally at other places too, and know that they are the “galleries” of caseless caddis flies.  Caddis flies are close relatives of the butterflies and are best known because many of their larvae use “found materials” (in contemporary art jargon) to construct cases to protect themselves.  Some species use fine gravel, silt and sand, some use fragments of plants, some have cases that are very neat, some have a more haphazard approach to construction.  However, a few families of caddis flies eschew cases and, instead, build these galleries.

Many caddis fly larvae, whether cased or not, are grazers, scraping the algae off the rocks on the bed of the stream or lake.   There is evidence that the cases offer some protection against predators such as trout which, by increasing survival rate, means that it is worthwhile for the caddis larvae to divert some of their hard-earned energy into building these.   Presumably, their caseless cousins gain the same advantage to building their galleries but recent research has suggested that these galleries offer a further benefit.

Think of caddis larvae as adolescent caddis flies.  Now imagine that the caddis gallery is the equivalent of an adolescent’s bedroom.   Horribly messy, in other words.   Let’s leave that image of a teenager behind (as most human teenagers know their way to the bathroom) and consider what happens to all that waste material that emerges from the far end of a caddis larva’s digestive system.   This nutrient-rich “ manure” encourages algae, meaning that our caseless caddis flies are, in fact, gardeners and are able to tap into this extra energy resource within their galleries in order to grow.   That brings us back to the analogy with teenagers, as these also frequently graze in their bedrooms (the diatom Campylodiscus is even the same shape as a Pringle, whose empty containers litter the bedroom floor of my own progeny).   I guess it is a good thing that caddis larvae don’t wear socks as, with six legs and two prolegs, the mess inside the gallery would be indescribable.

Galleries of caseless caddis flies (possibly Psychomiidae) on the top surface of a cobble from Sand Loch, Shetland Islands with (right) a close-up of a single gallery. The photograph at the top of the post shows Sand Loch in May 2019.

A recent study in the Lake District has shown that this “gardening” means that the algae which grow in the fine sediment from which the galleries are constructed are different to those found elsewhere on the rock surface, with a greater proportion of diatoms, which are considered to be more palatable to invertebrates than other types of algae.  Some caddis flies are thought to go even further, and can selectively remove and discard the algae that are least palatable (some Cyanobacteira, for example).

It is possible that up to 40% of the larva’s energy needs are met from the gallery itself.   The tube is, in fact, not a static construction: the larva pokes its head out in order to graze the algae immediately in front of the gallery, and extends the gallery as the food supply within easy (and safe) reach is exhausted.   At the same time, it is consuming the alga-rich rear part of the gallery (reminiscent of Hansel and Gretel eating the gingerbread house?).   A gallery only has a life-span of 10 days in the laboratory; whether this is the same under field conditions is not clear but that gives us some idea of the transience of these structures.   This rapid turnover means that the caddis larva is always feeding on succulent early-succession species, rather than the tougher and less digestible algae that might appear in more mature biofilms.

I also see similar galleries on the bed of the River Ehen from time to time but have been told that these are formed by non-biting midge (chironomid) larvae, rather than by caddis.  I presume that the same processes are happening in these although I have not been able to find much written in the literature.

Organisms that can significantly alter the habitat in which they live, and affect the conditions experienced by other species in the habitat are termed “ecosystem engineers”.  Beavers are good examples, as their dams can have significant effects on organisms extending for hectares.  Yet, in their own small way, caseless caddis larvae are also ecosystem engineers.  As are adolescent boys.   Which makes me wonder, having only talked until now about the algae in their galleries, whether caseless caddis larvae also have patches of mould extending up their walls.

Galleries made by chironomid larvae on a boulder in the River Ehen, March 2019.

References

Hart, D. D. (1985). Grazing insects mediate algal interactions in a stream benthic community. Oikos 44: 40-46. https://doi.org/10.2307/3544041

Johansson, A. (1991). Caddis larvae cases (Trichoptera, Limnephilidae) as anti-predatory devices against brown trout and sculpin. Hydrobiologia 211: 185-194. https://doi.org/10.1007/BF00008534

Ings, N. L., Hildrew, A. G., & Grey, J. (2010). Gardening by the psychomyiid caddisfly Tinodes waeneri: Evidence from stable isotopes. Oecologia 163: 127-139. https://doi.org/10.1007/s00442-009-1558-8

Ings, N. L., Grey, J., King, L., McGowan, S., & Hildrew, A. G. (2017). Modification of littoral algal assemblages by gardening caddisfly larvae. Freshwater Biology 62: 507-518. https://doi.org/10.1111/fwb.12881

Otto, C., & Johansson, A. (1995). Why do some caddis larvae in running waters construct heavy, bulky cases? Animal Behaviour 49: 473-478. https://doi.org/10.1006/anbe.1995.0061

The devil lies in the detail …

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 (= 1/100^thof a millimetre). 

Achnanthidium pyrenaicum from the River Don, Towie, Aberdeenshire.  Photographs by Lydia King.  Scale bar: 10 micrometres (= 1/100^thof 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 (= 1/100^thof 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 (= 1/100^thof 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

Spheres of influence

Back to Moss Dub for this post because Chris Carter has sent me some stunning images of the filamentous desmid Desmidium grevillei that I talked about in my earlier post.   I mentioned that it is surrounded by a mucilaginous sheath, which was just apparent in my brightfield image.   Chris has added Indian ink to the wet mount.  The ink forms a dense suspension in the water but is repelled by the mucilage around the desmid cells, resulting in a much better impression of the extent of the sheath around the cell than is otherwise possible.

Desmidium grevillei from Moss Dub, photographed by Chris Carter using Indian ink to highlight the mucilage sheath around the cells. 

Indian ink is a negative stain, which means that it is the background, rather than the specimen itself, which takes up the colour.   This, in turn, alters the passage of light through the sample and appears to improve the contrast of the final image.   Chris’ images of the apical view show this well, and also illustrates the complicated three-dimensional arrangement of the chloroplasts within each semi-cell.   His photographs also show the pores through which the mucilage is secreted.

The curious thing about this negative stain is that, whilst it appears to emphasis a halo of nothingness around the Desmidium filament, it is actually drawing our attention to something important.   In his presidential address to the British Phycological Society in 1981 A.D. Boney referred to mucilage as “the ubiquitous algal attribute” and goes on to list the many functions that the slimes produced by a wide range of algal groups may perform.  Not all will apply to our Desmidium but Boney does use desmids as examples of some of the roles slime may play: it can be, for example, a buoyancy aid, keeping the desmids in the well-lit regions of a lake or pond and it can protect cells against desiccation if a pond or lake dries out.  It may also play a role in helping desmids adhere to their substrates and there is also evidence that mucilage layers may help to protect algae from toxins.

Apical view (at four different focal planes) of Desmidium grevillei from Moss Dub, photographed by Chris Carter, September 2019.

But that’s only part of the story.   There is two-way traffic across the membranes of algal cells, with essential nutrients moving into the cell but, in some cases, enzymes moving in the opposite direction.  If nutrients are in short supply then these enzymes can help the cell by breaking down organic molecules in order to release nutrients that can then be absorbed. Those enzymes take energy to manufacture, and the sheath of gunk around the filament means that there is a lower chance of them diffusing away before doing their job (see “Life in the colonies …”).   The same principle applies to sexual reproduction too, with mucilage serving, in some cases, as “sperm traps” or simply as the phycological equivalent of KY Jelly.

It is not just the algae that benefit from this mucilage: the outer layers, especially, can be colonised by bacteria which will also be hoovering up any spare organic molecules for their own benefit with, no doubt, some collateral benefits for the organisms around them.  The connection is probably too tenuous to count as a symbiosis with the desmids but we could think in terms of mutual benefits.

So that “nothing” really is a “something”, and that is before we consider the role of these extracellular compounds in the wider ecosystem.  I mentioned the role of similar compounds in consolidating the fine sediments on coastal mudflats in “In the shadow of the Venerable Bede” to give a flavour of this.   The least prepossessing aspect of the least prepossessing plants can, given time, change landscapes.  That should give us all pause for thought.

Close-up of Desmidium grevillei filament with focus on the left-hand cell adjusted to show the apical pores.   Photographed by Chris Carter from material from Moss Dub collected in September 2019.

Reference

Boney, A.D. (1981). Mucilage: the ubiquitous algal attribute.  British Phycological Journal 16: 115-132.

Domozych, D. S., & Domozych, C. R. (2008). Desmids and biofilms of freshwater wetlands: Development and microarchitecture. Microbial Ecology https://doi.org/10.1007/s00248-007-9253-y

Sorentino, C. (1985). Copper resistance in Hormidium fluitans (Gay) Heering (Ulotrichaceae, Chlorophyceae). Phycologia 24: 366-388. https://doi.org/10.2216/i0031-8884-24-3-366.1

 

The little tarn of horrors …

In addition to desmids, we found several other algae in the samples collected from Cogra Moss.  One of these consisted of colonies of cells in mucilaginous masses attached to floating mats of vegetation (which looked like terrestrial grasses).  We decided that these were probably Chrysocapsa epiphytica, the second representative of the Chrysophyta I’ve described in this blog this year (see also “Fade to grey …”).  As is the case for Chromulina, much of what we know about Chrysocapsa epiphytica is down to the patient work of John Lund who first described this species back in 1949.

Colonies of Chrysocapsa epiphytica growing on submerged vegetation at Cogra Moss, Cumbria, September 2019.  Cells are 7.5 – 15 micrometres long and 7.5 – 12 micrometres wide. 

He described the various mucilaginous lobes as “reminiscent of the …. human brain”.  The spherical, oval or ovoid cells form a layer, two to four cells deep, at the surface of the colony.   The cells have the typical yellow-brown colour of chrysophytes and, though it is hard to see the chloroplasts in this photograph, John Lund says that there are usually two, sometimes four, in mature cells.

Its presence in a soft-water lake probably means that it is a species that relies on dissolved carbon dioxide rather than bicarbonate as its raw material for phytosynthesis (see “Concentrating on carbon …” for some background on this).   We know, from laboratory studies, that most chrysophytes rely exclusively on carbon dioxide, and lack the capacity to use bicarbonate.  This confines them to water where the pH is low enough to ensure a supply of carbon dioxide (the chemistry behind this is explained in “Buffers for duffers”. It may also explain why Chromulina lives in surface films rather than submerged in the pond (the locations where we’ve it found are unlikely to have sufficiently low pH).

One extra twist to the story is that many chrysophytes are “mixotrophic”, meaning that they can switch between using photosynthesis as a means of getting the carbon they need to grow from inorganic sources, and “feeding” on other organisms.  Our Chrysocapsa epiphytica, in other words,  has parked itself beside a convenient supermarket of pre-packaged carbon in the form of decaying vegetation and associated bacteria which it then ingests by a process known as “phagotrophy”.

Phagotrophy is, in fact, a very ancient characteristic, insofar as the very first eukaryotic cells were the result of Cyanobacteria-type cells being ingested by larger heterotrophic cells and being retained as on-board “energy farms” rather than digested and treated as one-off vegetarian dinners.   However, the shift to a permanent role for chloroplasts within a eukaryotic cell involved a lot of rewiring of intercellular machinery, and effectively “switching off” the intercellular mechanisms involved in phagotrophy.   Retaining the ability to “feed” on bacteria alongside a capacity for photosynthesis is the cellular equivalent of a hybrid car: there is a lot more to cram under the bonnet.  Flexibility, in other words, comes at a cost.

On the other hand, phagotrophy does not just result in extra carbon for the Chrysocapsa cells in Cogra Moss.   In an oligotrophic tarn such as this, the extra nutrients that are obtained when the bacteria are absorbed will also be a useful boost.   Once again, though, you can see that, in environments where nutrients are more plentiful, the cost to the cell of maintaining the equipment required for phagotrophy outweighs the benefits.

I’m sure that a close inspection of the land around Cogra Moss would have revealed insectivorous plants such as Drosera(sundew) and we also recorded Utricularia minor, an aquatic insectivorous plant, in another tarn we visited whilst desmid-hunting (see “Lessons from School Knott Tarn”).  Chrysocapsa is, in many senses, a microscopic equivalent of these carnivorous plants.   OK, so it has a taste for bacteria rather than flesh but, somewhere out there, there must be a sub-editor in search of a headline …

References

Lund, J.W.G. (1949). New or rare British Chrysophyceae. 1.  New Phytologist48: 453-460.

Maberly, S. C., Ball, L. A., Raven, J. A., & Sültemeyer, D. (2009). Inorganic carbon acquisition by chrysophytes. Journal of Phycology 45: 1052-1061. https://doi.org/10.1111/j.1529-8817.2009.00734.x

Raven, J. A. (1997). Phagotrophy in phototrophs. Limnology and Oceanography 42: 198-205. https://doi.org/10.4319/lo.1997.42.1.0198

Saxby-Rouen, K. J., Leadbeater, B. S. C., & Reynolds, C. S. (1997). The growth response of Synura petersenii(Synurophyceae) to photon flux density, temperature, and pH. Phycologia 26: 233-243. https://doi.org/10.2216/i0031-8884-36-3-233.1

Saxby-Rouen, K. J., Leadbeater, B. S. C., & Reynolds, C. S. (1998). The relationship between the growth of Synura petersenii (Synurophyceae) and components of the dissolved inorganic carbon system. Phycologia 37: 467-477.  https://doi.org/10.2216/i0031-8884-37-6-467.1

Terrado, R., Pasulka, A. L., Lie, A. A. Y., Orphan, V. J., Heidelberg, K. B., & Caron, D. A. (2017). Autotrophic and heterotrophic acquisition of carbon and nitrogen by a mixotrophic chrysophyte established through stable isotope analysis. ISME Journal. https://doi.org/10.1038/ismej.2017.68

 

The desmid dilemma …

The second location we investigated during the Quekett Microscopy Club / British Phycological Society weekend was Cogra Moss, a small reservoir about four kilometres north of Ennerdale Water.  It is also  about a kilometre or so west of Lampleugh Green where I was staring mournfully at my flat tire whilst the advance party, unaware of my predicament, was out collecting samples.  They must have missed me by a matter of minutes.

As at Moss Dub they found some promising locations around the margins and, in the small tarn at the north-east corner, some patches of Sphagnum from which desmids could be squeezed, plus some floating vegetation.   Once again, I’ve illustrated some of those that we encountered, with a warning that this is a limited selection of the more photogenic ones and we’ve sent samples off to David Williamson for a more comprehensive analysis.   And, once again, the sheer diversity of desmids in the sample is a source of wonderment.   How can one small lake support so many variations on a one type of alga?  I’ve speculated on such issues in the past, drawing on G.E. Hutchinson’s “Paradox of the Plankton” (see “Baffled by the benthos (1)”).   In that post I suggested that it is partly a matter of scale and perception and, in this case, I suspect that the desmids we see in a Sphagnum squeezing are adapted to a wide range of microhabitats.  That means that the desmids would have had a three-dimensional arrangement within the Sphagnum whilst it is in situ but this is lost when we drag a handful of moss from the lake margin and squeeze it into a pot.

Desmids from Cogra Moss: a. Eurastrum crassum (length: 140 – 180 micrometres; width: 75 – 92 micrometres); Netrium digitus (length: 130 – 390 micrometres; width: 40 – 82 micrometres); Closterium kuetzingii (length: 300 – 550 micrometres) and Pleurotaenium trabecula (length: 277 – 600 micrometres; 22 -46 micrometres).  The photograph of Cogra Moss at the top of the post is by Judy Johns.

More desmids (and other algae) from Cogra Moss: e. Micrasterias thomasiana (length: 200 – 288 micrometres; breadth: 170 – 269 micrometres); Tetmemonus laevis(length: 67. 5 – 123 micrometres; breadth: 20 – 31.5 micrometres);
g. Schroederia setigera (85 – 200 micrometres long); h. Gonatozygon monotaenium (length: 90-327 micrometres; width: 6.2-12.5 micrometres); i. Staurodesmus extensus (width: 42-50 micrometres, including spines); j. Cylindrocystis gracile (length: 20 – 80 micrometres; width: 11 – 15 micrometres).

But, coming at this issue of desmid diversity from another direction, the term “desmid” is about as particular as the term “mammal”, insofar all belong to the same Class.   In “The big pictures …” I described how desmids were related to other green algae (acknowledging, in the process, that the term “green algae” is, itself, outdated).   This listed five separate families of desmids: four in the order Desmidales and one in the Zygnemetales (I’ve listed the examples from this and the previous post in the table below).   Think laterally and translate this level of organisation to the landscape around Cogra Moss and Ennerdale: the forests contain red squirrels (Rodentia), foxes (Carnivora) and deer (Artiodactlya) and there are otters in the River Ehen (another Carnivora but in the family Mustelidae rather than Canidae).   If we can appreciate how different mammals can interact within a landscape, then we should be able to apply the same principles on a much finer scale to organisms that are five orders of magnitude smaller.   It’s the principle behind fractals, but applied to biological  diversity rather than to geometry.

Earlier in the year, I published a paper with two colleagues that tried to explain how the way we study the microbial world can shape and, in many cases, impede our understanding (it’s open-access, so click on the link below if you want to read it).  We illustrated this with pictures that tried to demonstrate how microscopic algae interact with other organisms.  These included host plants, in the case of epiphytic algae, but also the protozoans that feed on them.  Most of our examples were diatoms, and there was a reasonable literature on which we could draw.  Curiously, I’ve never come across papers that provide this contextual information for desmids. Perhaps I just don’t look in the right places.   If it is out there and I’ve missed it, please do let me know.

Reference

Kelly, M. G., King, L., & Yallop, M. L. (2019). As trees walking: the pros and cons of partial sight in the analysis of stream biofilms. Plant Ecology and Evolution152: 120-130.

Organisation of the class Conjugatophyceae with examples encountered in Moss Dub and Cogra Moss.

Order / Family	Examples
Desmidales
Closteriaceae	Closterium
Desmidaceae	Desmidium, Euastrum, Pleurotaenium, Staurodesmus, Tememorus
Gonzatozygaceae	Gonatozygon
Peniaceae	No examples in these posts, but see “Desmid Diversity” for illustrations of representatives in Kelly Hall and Long Moss tarns.
Zygnemetales
Mesotaeniaceae	Cylindrocystis
Zygnemetaceae	Not desmids: Filamentous algae including Mougeotia, Spirogyra and Zygnema – examples from Ennerdale area are described in several other posts,

Desmids from Moss Dub

I’d like to say that this post is about an excursion I made beyond Ennerdale Water and along the valley of the River Liza in order to find some different habitats from those that I usually write about in this blog.  I’d like to but, in truth, I was sitting by a road about ten miles away waiting for roadside assistance whilst my compatriots on the Quekett Microscopy Club / British Phycological Society algae weekend went up the valley on a glorious mid-September afternoon without me.  Whilst I was sitting waiting for a tyre to be replaced in a garage in Egremont they were casting plankton nets and squeezing handfuls of Sphagnum beside Moss Dub, a small tarn set amidst woodland close by the River Liza.

Moss Dub is set within one of Britain’s oldest and most ambitious rewilding schemes, Wild Ennerdale, where nature is allowed to shape the landscape as far as possible free from human interference.  However, Moss Dub, as we found out, is far from a natural water body.  A path forks and the two arms act as bunds encompassing a shallow pond, now partly overgrown with aquatic vegetation.   There is evidence of past mining activity – for iron and copper – in the area and my guess is that Moss Dub was, in the far past, a reservoir associated with the Lingmell mine located on the hillside above the River Liza and active in the late 19^thcentury.     Whatever its history, it proved to be a rich location for desmids, and we spent a happy Saturday dipping Pasteur pipettes into the vials of peaty water that they collected and peering through our microscopes (If you want to know more about how to collect desmids, look at the post I wrote after our last excursion to the Lake District: “Desmid masterclass”).

There were some conspicuous green growths suspended in the water at the margin of the pond.  Even without a microscope, their filamentous nature was obvious.  When magnified, we saw chains of green cells set within a distinct mucilaginous sheath.  Each filament was composed of short cells with a distinct notch on either side.   This is a representative of Desmidium, one of a relatively small number of filamentous desmids.  We met D. schwartziion our previous excursion (see “Lessons from School Knott Tarn”); that species was present here along with D. grevillei, which is similar in many respects but the cross-section is lemon-shaped rather than triangular.

Desmidium grevillei from Moss Dub, Ennerdale Valley, September 2019.  a. shows a macroscopic view of filaments in a Petri dish; b. shows a filament of cells, along with a distinct mucilaginous sheath whilst c. shows a cell in cross-section.  I forgot to bring my graticule so cannot add scale bars to any of the images in this post.   Instead, I will quote dimensions from the Freshwater Algal Flora of Britain and Ireland to give an indication of size.   Cells of D. grevillei are 30 – 56 micrometres wide (50 micrometres is 1/20^thof a millimetre).   The photo at the top of this post is a view of Moss Dub, taken by Zeneb Henderson

There were numerous other desmids in the sample.  A couple are illustrated below, and we’ve sent the sample off to David Williamson for a more thorough examination, and some definitive names.  On the right-hand side of the plate there is a different green alga, Coelastrum pulchrum, a member of the Chlorophyceae that forms spherical colonies with a fixed number of cells (“coenobia”).  We met Coelastrum microporum in the River Wear last summer (see “More green algae from the River Wear”): cells of C. pulchrum, by contrast, have a blunt projection.

More algae from Moss Dub: d. Micrasterias radiosa (142 – 191 micrometres across); e. Euastrum pinnatum (65-75 micrometres across; 125 – 170 long); f. Coelastrum pulchrum (about 100 micrometres in diameter). 

The final desmid I’ve illustrated is Closterium lunula, large by desmid standards as it can reach half a millimetre or more in length.  Members of this genus have prominent vacuoles at each end of the cell within which small crystals can be seen.  Because C. lunula is so large it is easy to see both vacuole and watch Brownian motion move the crystals within.   Studies have shown that these are crystals of barium sulphate and also that the crystals are scattered throughout the cells, just happening to be easier to see I the vacuoles.  Quite what role they play remains speculation: barium is not required for plant nutrition and is, indeed, toxic in high concentrations.   It is also scarce in the soft waters where Closteriumis most often encountered, both in absolute terms and relative to other trace metals, which only adds further to the mystery.

That’s enough about Moss Dub for now.  A few words about Ennerdale Bridge, where we were based before I sign off from this post.  I usually stay at the Shepherd’s Arms when I am in the area and Keith and his staff hosted most of us and fed all of us.  It is a comfortable, unprententious inn, living mostly off walkers doing the Coast-to-Coast walk and with a menu that managed to put a smile on the faces of vegetarians and non-vegetarians alike.  Our daytime events took place in the community room of The Gather, a community-owned and run café and shop,  That gave us the satisfaction of knowing that the money we paid for the room was going to good use.   Their coffee keeps me going during long days of fieldwork in the area so I’m keen to make sure that they thrive!

Closterium lunula (400 – 663 micrometres long) from Moss Dub, showing the terminal vacuole (ringed) and (below) a close up showing rectangular crystals of barium sulphate inside the vacuole.

Reference

Brook, A. J., Fotheringham, A., Bradly, J., & Jenkins, A. (1980). Barium accumulation by desmids of the genus Closterium (Zygnemaphyceae). British Phycological Journal 15: 261-264. https://doi.org/10.1080/00071618000650251

Quekett Microscopy Club and British Phycological Society members getting stuck into analysis of samples from Moss Dub and the Ennerdale valley at The Gather, Ennerdale Bridge, September 2019.