Algae behaving selfishly …

My most recent trip to Ennerdale Water was on a wonderful windless winter day, offering perfect reflections of the snow-dusted peaks beyond the lake. It was a cold day but I was well wrapped-up and could enjoy both the long-distance views and the close-ups of nature around the lake’s margins.   One of the small streams that I crossed as I skirted the perimeter of the lake had patches of green algae growing on its submerged stones and even a quick examination showed it to be coarser than the green algae that covered most of the larger stones on the lake bed itself, as well on those in the River Ehen, just below the outfall.   When I managed to get specimens under my microscope I saw that the algae on the lake bed was Spirogyra (which I have seen here before; see “A lake of two halves”) whilst that in the inflow stream was Oedogonium.

I’ve written about Oedogonium before, and lamented the problems we face when we try to identify the species within this large genus (see “The perplexing case of the celibate alga”).   Ironically, a couple of weeks after I wrote this, I encountered a population of Oedogonium in another Cumbrian stream that did have sexual organs (see “Love and sex in a tufa-forming stream”).  However, this was the exception that proves the rule, as I have not seen a sexually-mature population of Oedogonium since.  The population I found beside Ennerdale was not sexually mature either but it did show a different, but equally effective, means of going forth and multiplying.

In the left hand diagram below we see a vegetative cell from an Oedogonium filament that has split open, allowing a vesicle to be extruded within which a single zoospore has formed.   This has a ring of flagella at one end, resembling a monk’s tonsure (you can just see these flagella in the photograph).   The other two photographs show the monk’s bald pate, though the fringe of flagella is not very clear.    The transparent vesicle swells and eventually ruptures, releasing the zoospore, which swim around for an hour or so, before settling on a new substratum and growing into new filaments.

Zoospores of Oedogonium from a stream flowing into Ennerdale Water, January 2018.   Scale bar: 25 micrometres (= 1/40th of a millimetre). 

In my material, the new filaments were mostly attached to mature Oedogonium filaments; however, this is probably partly an artefact and, in the field, they would almost certainly also settle on rocks and other surfaces too.   You can see, in the diagram below, how the “bald” end of the zoospore has started to differentiate into a holdfast that will secure the cell to the substrate whilst, over time, the other end will start to divide to produce the first cells of the new filament.  The whole process is described in a series of papers by Jeremy Pickett-Heaps (see reference list below).

Why did I see zoospore formation in this particular sample?   I don’t know for sure but it may be because I let a longer than usual time elapse between collecting and examining the sample.   This one had sat around in a cool box and fridge for four days, whereas I usually manage to check them within 24 hours.   Neglect can be a useful tool in the phycologist’s arsenal, as many freshwater algae see no need to indulge in anything more taxing than routine cell division for as long as the habitat keeps them replenished with whatever light, nutrients and other resources that they need.   Only when this is no longer the case do the algae start to channel resources into survival strategies.

Oedogonium zoospores germinating into new filaments, both epiphytic on mature filaments.   From a stream flowing into Ennerdale Water, January 2018. .   Scale bar: 25 micrometres (= 1/40th of a millimetre). 

Although I used the phrase “go forth and multiply” in an earlier paragraph, these Oedogonium cells are actually “going forth” rather than “multiplying” as the process we are watching only produces a single new cell.  However, were this zoospore to be released in a stream rather than a sample bottle, then there is a good chance that it would have been washed downstream and that a few of the many zoospores might have settled on a suitable habitat away from the constraints of their former home.   Asexual reproduction is a dispersal mechanism that results in the spread of genetically-identical copies of the parent cell.  For a sessile organism, this strategy allows a single genotype to move on from less-favourable locations and to exploit the potential of nearby locations.

The word “reproduction” is misleading as the mixing of genetic material that we associate with sex doesn’t take place.  The end product is a clone of a successful Oedogonium filament growing somewhere else.   However, taking the “sex” out of “asexual” removes a huge potential for innuendo, and readers who have battled this far through a post on nondescript green filaments deserve a reward.  So let’s finish with Woody Allen’s definition of masturbation as “sex with someone you love” and suggesting that the cytological huffing and puffing involved in zoospore production may not have the romance of sex but nor does it lead to any of the complications which result from sex either.   The alga gets offspring that are 100% identical to itself, just slightly further downstream and there is no risk of mixing with inferior genotypes.   That’s about as “selfish” as the “selfish gene” can get.


Pickett-Heaps, J. (1971).   Reproduction by zoospores in Oedogonium. I. Zoosporogenesis.   Protoplasma 72: 275-314.

Pickett-Heaps, J. (1971).   Reproduction by zoospores in Oedogonium. II. Emergence of the zoospore and the motile phase. Protoplasma 74: 149-167.

Pickett-Heaps, J. (1972).   Reproduction by zoospores in Oedogonium. III. Differentiation of the germling.  Protoplasma 74: 169-173.

Pickett-Heaps, J. (1972).   Reproduction by zoospores in Oedogonium. IV. Cell division in the germling and the possible evolution of the wall rings.   Protoplasma 74: 195-212.

See also “The River Ehen in March” for some further perspectives on asexual reproduction in algae.

View from near our sampling site on Croasdale Beck, looking towards Ennerdale Bridge, January 2018.



Change is the only constant …

The diatoms I saw in my sample from the littoral of Lake Popovo (described in the previous post) reminded me of an assemblage that I had seen in another lake which, apart from its location, has much in common with Popovo. This lake is Wastwater, in the western part of the English Lake District (see “The Power of Rock …”).  Like Popovo, it is situated in a remote a region of hard volcanic rocks and, as such, has very soft water and is subject to few of the pressures to which most of our freshwaters are subject.  The photograph above shows me sampling Wastwater in about 2006 (more about this photograph, by the way, in “A cautionary tale …”).

I wrote about Wastwater when I was writing my book Of Microscopes and Monsters, the precursor of this blog.   I focussed, in particular, on an experiment that my friend Lydia King had performed as part of the research towards her PhD.  Her previous work had established that there were relationships between the types of algae that she found in lakes in the Lake District and the amount of nutrients that they contained.  She also saw that the types of algae she found depended upon how acid or alkaline the water was.  But the water chemistry only explained a part of the variation in the algae and now she wanted to find out about the variation that was not explained by this.   In particular, she wanted to know how much of the variation was due to the way that the algae interacted with each other.

Lydia’s experiment involved putting clay pots into the shallows at the edge of Wastwater and then watched how the algal communities changed over the course of six weeks.  She also examined small parts of the pots at extremely high magnifications using a scanning electron microscope.   These micrographs, and subsequent conversations with her, had inspired some of my early paintings and I returned to this subject several times, finally producing a series of three pictures that showed changes in the algae over time.

The microbial world of the littoral zone of Wastwater after two weeks of colonisation showing unidentified small unicellular blue-green alga,  unidentified small unicellular green alga; thin filaments of Phormidium,  Achnanthidium minutissimum and Gomphonema parvulum.

The first of these shows the surface of the plant pot after being submerged in Wastwater for two weeks.   You could think of this as a patch of waste ground that was, at the start of the experiment, bare of vegetation.   If we watched this patch over a number of weeks, we would notice some plants appearing: scattered stalks of grass, perhaps some rosebay willow herb, dock or plantains. A gardener might dismiss these as “weeds”, although this term has no ecological meaning but ecologists prefer to think of these as “pioneers”: plants adapted to colonising new habitats, growing quickly (which might mean producing lots of seeds in a short space of time or producing rhizomes or runners) and covering the ground.  This same process has taken place on Lydia’s plant pot in Wastwater: the “weeds” in this case are scattered thin filaments of the blue-green alga Phormidium, the diatoms Achnanthidium minutissimum and Gomphonema parvulum plus a number of spherical green and blue-green cells that she couldn’t identify.   Such is the scale that we are working at that this open landscape still contains about 92000 cells per square centimetre.

The microbial world of the littoral zone of Wastwater after three weeks of colonisation.   The composition is similar to that in the previous figure but the density of cells is greater.

When she came back a week later, much of the empty space had been infilled; there were now about 300,000 cells per square centimetre.  These mostly belonged to the same species that she had found the week before.  The difference is that they are now rubbing up against each other and this has some important consequences.  All plants need light and nutrients to grow and algae are no exceptions.   Sunlight provides the energy for photosynthesis but now, at week three, the density of algae is such that there is a chance that some of the light will be intercepted by a neighbouring cell.   The total amount of sunlight that filters through the water to the pot surface is already much lower than that available at the lake surface; now it has to be shared out between many more cells.   At this point, properties such as fast growth rates that helped our pioneers to colonise the plant pot become less relevant, and it is algae that are better adapted to capturing the limited light that will survive.

So when Lydia came back to Wastwater after six weeks, she saw a very different community of algae on her pots.   There was still a lot of Achnanthidium minutissimum, but rising above these was the elegant art deco shape of Gomphonema acuminatum (also found in Lake Popovo) which, importantly for our story, grows on a long stalk.  There are also cells of “Cymbella affinis” (the correct name at the time that Lydia was working but see comments in the previous post about the nomenclatural history of this species).   This, too, grows on a long-stalk, the better to grow above the Achnanthidium and other pioneers.   If we continue to use the analogy of a patch of wasteland, then it has now reached the point where it has been invaded by shrubs such as hawthorn and blackthorn.   However, in a terrestrial habitat this would happen two or three years after the first pioneers had arrived, not six weeks as Lydia had observed for the algae.   She also found the diatom called Tabellaria flocculosa which forms filaments.  These often start out loosely-attached to the substratum but more often break free and become entangled around the other algae.   In our “wasteland” analogy, these would be the brambles.

The microbial world of the littoral zone of Wastwater after five weeks of colonisation.  Gomphonema acuminatum, “Cymbella affinis” and Tabellaria flocculosa have now joined the assemblage seen in the two earlier dioramas.

The experiment finished shortly after this, terminated when the apparatus was overturned.  Whether by a wave or by vandalism, Lydia will never know but this event is, itself, a metaphor for the harsh world in which benthic algae have to survive.  In real life, the many cobbles in the littoral zone will be rolled by wave action or, as we have seen in other posts, invertebrate grazers could have removed much of the “shrubbery”, leaving a “pasture” composed of the tough, fast-growing species such as Achnanthidium minutissimum to dominate samples.   The “successions” we see in the microscopic world not only take place much more quickly than those in the macro world, but they also rarely have a stable “climax”: just a brief pause before the next onslaught from the physical, chemical and biological processes that shape their existence.


King, L., Barker, P. & Jones, R.I. (2000). Epilithic algal communities and their relationship to environmental variables in lakes of the English Lake District. Freshwater Biology 45: 425-442.

King, L., Jones, R.I. & Barker, P. (2002). Seasonal variation in the epilithic algal communities from four lakes of different trophic state. Archiv für Hydrobiologie 154: 177-198.

The underwater world of Ennerdale Water …

I’ve tried to capture the world of microscopic benthic algae many times but never, until now, attempted the same effect with plankton.   The picture below illustrates the problem that I face: whereas the benthic flora are organised with, for the most part, a clear three-dimensional structure and known dependencies amongst organisms (species A, for example, being epiphytic on species B), plankton are randomly distributed in a very dilute solution.   My picture  below, which is based on four phytoplankton samples collected by the Environment Agency in the summers of 2014 and 2016.

A representation of the phytoplankton of Ennerdale Water with cells of Rhodomonas and Kephyrion depicted at a realistic density (c. 1000 – 2000 cells per millilitre).

I had to address two issues in producing this image, which is based on four phytoplankton samples collected by the Environment Agency in the summers of 2014 and 2016: depicting the phytoplankton cells at approximately the correct density and making sense of the list of names that appeared on the list.  Ennerdale Water is a very nutrient-poor lake and cell concentrations during the summer are in the order of 1000 to 2000 per millilitre.  That sounds a large number until you consider the scale at which we are working.   For simplicity, I assumed spherical cells of about 20 micrometres diameter (= 1/50th of a millimetre) at a density of 1000 cells/ml.    That equates to one cell per micrometre which is 1 mm x 1 mm x 1 mm.   Using these assumptions, each cell is 50 diameters distant from its nearest neighbour, which means the foreground of a picture should contain only two small cells and a lot of blue paint.

Next, I need to know what algae to paint and the problem here is that 85 per cent of the cells in the Environment Agency phytoplankton analyses were described as “picoplankton < 2 micrometres diameter” or “nanoplankton 2-20 micrometres diameter” (the latter divided into flagellates and non-flagellates).  There are, apparently, big difficulties in naming many of the cells found as preservation with Lugol’s Iodine coupled with the long time in storage before analysis can lead to loss of useful diagnostic features.   Cells in the nanoplankton category can, in theory, belong to any one of a number of groups of algae but If I focussed just on those organisms that could be named, I see that the Cryptophyta Rhodomonas lacustris var nannoplanctica (formerly R. minuta var. nannoplanctica) predominates, followed by Chrysophytes, of which Kephyrion is the most abundant.   So these are the two cells that I have put in the foreground.

I subsequently turned up a paper from 1912 by the father and son team of William and George West who looked at the phytoplankton of Ennerdale Water and a number of other lakes in the Lake District and Scotland.  The range of taxa that they found was quite different to that recorded in these recent surveys with samples dominated by desmids and almost no Chrysophytes or Cryptophytes recorded at all. That may, in part, be due to differences in methods – they collected samples using a “silken tow net”, which would probably have missed the very small Chrysophyta and Cryptophyta (an earlier paper by them tells us of the size of the nets but not the mesh itself) .  Some desmids that they found were found in the recent surveys but in much smaller quantities and it is possible that this was partly an artefact of the differences in sampling technique.  The idea of comparing count data from old papers with modern records is appealing but, in most cases, separating genuine changes in composition from differences introduced by sampling and analytical methods is always difficult.

Excuse these ramblings … there is, as you can see, not a lot of pictorial interest in the underwater world of an oligotrophic lake.   If you want excitement, tune into Blue Planet II, David Attenborough’s latest series for the BBC You will find sex and violence galore there.  The underwater world of Ennerdale Water is a quieter, more serene and certainly less televisual place.  Maybe that’s not such a bad thing …


Lund, J.W.G. (1948) A rarely recorded but very common British alga, Rhodomonas minuta Skuja. British Phycological Bulletin, 2:3, 133-139.

West, W. & West, G.S. (1909). The British freshwater phytoplankton, with special reference to the desmid-plankton and the distribution of British desmids.   Proceedings of the Royal Society of London Series B 81: 165-206.

West, W. & West, G.S. (1912).  On the periodicity of the phytoplankton of some British lakes.  Journal of the Linnaean Society, Botany 40: 395-432.

What a difference a storm makes …

I was back at Croasdale Beck last week and noticed a rather dramatic change to the meander just upstream from our regular sampling spot.   If you look at the photograph that heads the post “A tale of two diatoms …”, you’ll see the stream flowing around this meander.  Now, however, it has cut a new, shorter channel that bypasses the meander altogether.   We visited the stream just a few days after Storm Ophelia had passed through although, judging by the grass growing on the gravel of the abandoned meander, it was not necessarily this particular event that reshaped the stream.

Croasdale Beck is an unruly tributary of the River Ehen, rising on the fells above Ennerdale Water and tumbling down across rough grazing land and some semi-improved pasture (as in the picture above) before joining the Ehen in Ennerdale Bridge.   This is not the first time that we have seen conspicuous changes in the channel after a storm.  The magnitude of the flood is illustrated by the hydrograph below, which went off-scale for a period, as the discharge exceeded 3000 mega litres per day (300 MLD is the approximate limit for safe wading, in my experience).   I noticed that there was much less green algae present than we usually record at this time of year, although the diatom film was still quite thick.   Some of the stones that I picked up to sample had the slimy biofilm on the underside, suggesting that they had been recently rolled by the flooded river.   Croasdale Beck has no lake to buffer the rise and fall of the floodwaters and a huge amount of energy is carried down in a short period of time as the water surges downstream.

By the time we had arrived, the floodwaters had subsided and the sheep were contentedly grazing the surrounding land.  The stream itself was almost back to base flow (in contrast to the River Ehen which was still only just wadable).  Only the meander looked different …

The hydrograph for the River Ehen, as the aftereffects of Storm Ophelia make their way downstream.

Buffers for duffers …

In Ecology in the hard rock café I wrote about the challenges of living in an aquatic world where carbon – one of the raw materials for photosynthesis – was in short supply.   What I did not write about in that post is that this carbon also gives freshwater some useful additional properties.   In brief, rainwater is not pure water, but absorbs carbon dioxide from the atmosphere.  This, in turn, makes rainwater slightly acidic and, when it falls onto rocks, this weak acid dissolves the minerals from which the rock is made.  This adds two other forms of carbon to the water – bicarbonate and carbonate (the latter, particularly, from limestone).

Each of these three types of carbon in freshwater can convert to either of the other two types, with the speed of the reaction depending on the balance between the forms (the “law of mass actions”).  In essence, the reactions proceed until equilibrium is obtained, and this equilibrium, in turn, depends upon the pH of the solution.  These processes are summarised in the diagram below.

Relationship between pH and the proportion of inorganic carbon as free carbon dioxide (or carbonic acid, H2CO3 – orange line), bicarbonate (HCO3 – green line) and carbonate (CO32- – blue line).

The chemistry behind this is not easy to explain but a consequence is that any attempt to shift the pH (e.g. by adding acid) causes an automatic adjustment in the balance between the different forms of carbon.  Some of the hydrogen ions that could make the water acid are, instead , bound up as bicarbonate, and the pH, as a result, does not change.  The greater the quantity of inorganic carbon in the sample, in other words, the greater the capacity of the water to resist changes in pH.   The carbonate, bicarbonate and free carbon dioxide together act as a “buffer”, a chemical shock absorber.   Think of it as equivalent to the responsible use of a credit card or savings account to defer the cost of an unexpected bill (a car repair, for example) so that your current account does not go overdrawn.

Because life largely evolved in well-buffered marine systems, the enzymes that run our cells generally work best within a narrow range of pH (approximately 6-9).   Cells – unicellular life forms in particular – get stressed if pH strays outside this range, so the greater the buffering capacity, the easier it is for cells (life at high pH can bring additional complications, but we don’t have time to go into those here).  “Alkalinity”, as I mentioned in the earlier post, is the measure that ecologists use to assess the strength of the buffer system in a lake or river.  The principle of the measurement is straightforward: we add a dilute acid very slowly and watch what happens to the pH.   At first, nothing happens but, as soon as the water’s natural buffering capacity has been exceeded, pH drops rapidly.

I have a small portable alkalinity titration kit which involves adding drops of bromophenol blue indicator to a sample of stream or lake water.  This gives the water a blue colour when the pH is greater than 4.6.  As the pH falls, the solution becomes colourless and, eventually, turns yellow.   If you look at the graph above you will see that, at pH 4.6 most of the bicarbonate (HCO3) has been converted to carbon dioxide so the buffering capacity is pretty much non-existent.  This means that I can use the quantity of acid that is needed to make the bromophenol blue change colour as a measure of the buffering capacity of the water.

Alkalinity titrations beside Ennerdale Water (see top photograph) using a Hanna HI 3811 alkalinity test kit.  The right hand image shows acid being added to the water sample with a 1 ml pipette.  The blue colour shows that pH has not yet dropped below 4.6.

All this talk of chemical equilibria seems to be a long way from the natural history that is the core business of this blog.  Yet, at the same time, these reactions describe natural phenomena every bit as real as the plants and animals that attract the interest of naturalists.   Geology and chemistry ultimately create the context within which biology flourishes, but it is rare to meet a chemist who can talk with a naturalist’s passion.  I think that this is partly because chemistry tends not to describe tangible features of the landscape but, instead, quickly gets lost in abstract equations.  However, it is also a matter of culture: chemists need clinical separation from the mud and filth to maximise precision, whilst ecologists feel the lure of the field.  There is, nonetheless, a very basic and necessary link between the chemistry and ecology of aquatic systems.   Geology may shape a landscape but chemistry is one of the key mediators that determines the types of plants that cloak the hills and vales.  We ignore it at our peril.

Different tarn, different desmids …

Geoff and Chris, two of our band of desmid hunters, chose to stay in the FBA’s brand new holiday apartments and, rather than cross the lake to join us on Saturday morning they headed out to Moss Eccles Tarn, in the area between Esthwaite Water and Windermere.   One of Dave’s first dips into one of their samples yielded an almost pure monoculture of another filamentous desmid, Spherozosma vertebratum which presented some beguiling abstract patterns on my computer monitor.

Spherozosma vertebratum from Moss Eccles Tarn, September 2017.   Scale bar: 25 micrometres (= 1/40th of a millimetre).

Curiously, after our first encounter with Spherozosma vertebratum we did not see it in any of our other dips into the Moss Eccles samples although there were plenty of other desmids on display.   The most abundant of these was Staurastrum productum and, usefully, there were examples showing both apical and side views.   The three arms are distinctive (and distinguish it from relatives such as S. arachne which have five) and you can also see the knobbly “verrucae” on the spines as well as a broad mucilaginous envelope around the cells.

Staurastrum productum in side (left) and apical (right) views.  Images photographed from a computer monitor so apologies for their poor quality.  Scale bar: 25 micrometres (= 1/40th of a millimetre).

Another desmid with spines and mucilage was quite common.  This was Staurodesmus bulnheimii.  Spines slow the rate of sinking so are associated with several genera of predominately planktonic desmids.   The star-shaped arrangement of colonies of the diatom Asterionella formosa play a similar role (see “Little bugs have littler bugs upon their backs to bite ‘em”).   There were also several cells  of a small Cosmarium species, including some that had recently divided and the image shows how one cell has split down the central isthmus and a new semicell is growing back on each of the two daughter cells.   Finally, I have included an illustration of Micrasterias radiosa.  To the uninitiated this may look little different to M. compereana, illustrated in the previous post, but if you look closely you will see that the incisions between the lobes are much deeper in M. radiosa.

One sample from Moss Eccles Tarn kept me busy for half the morning and this account describes only part of the diversity.   Note how the differences between this and the School Knott Tarn sample are not just in the genera and species present but also in the life-forms I found.  The School Knott sample was from a Sphagnum squeezing whilst the Moss Eccles sample was from a plankton net.  That explains why I saw more spine-bearing desmids in the latter.  If I had looked at a plankton sample from School Knott and a Sphagnum squeezing from Moss Eccles, I might have found a different balance of life-forms between the two tarns.   But time was running out and I had to move on …

More desmids from Moss Eccles Tarn, September 2017: a. Staurodesmus bulnheimii; b. Cosmarium quadrifarium var. hexastichum; c. Euastrum cf. gemmatum.   Scale bar: 25 micrometres (= 1/40th of a millimetre).

Micrasterias radiosa from Moss Eccles Tarn, September 2017.   Scale bar: 25 micrometres (= 1/40th of a millimetre).

Lessons from School Knott Tarn …

As not everyone could join us on our excursion on Friday afternoon, we repeated the exercise on Saturday morning, heading to a small tarn just a short walk from Windermere and Bowness.   Despite its proximity to two of the busiest towns in the Lake District, there were very few other people around to disturb our peace whilst we collected samples.   As at Kelly Hall and Long Moss Tarns, Dave had his plankton net out, but we also explored a boggy region at one end, finding more patches of Sphagnum but also extensive growths of Utricularia minor (Lesser Bladderwort), one of a small number of aquatic carnivorous plants.   Dave was particularly pleased by this find as he associates this particular plant with rich hauls of desmids.

It was tempting to linger in the sunshine beside School Knott Tarn but the green tinge of the water that dripped out of the Sphagnum squeezings in particular was enough to lure us towards the Freshwater Biological Association’s laboratories in order to start examining our samples.

Utricularia minor (Lesser Bladderwort) from School Knott Tarn, near Windermere, September 2017.   Several of the spherical bladders which trap small invertebrates are visible on the plant.

My selection of photographs below shows just a part of the diversity that we encountered during our microscopic examinations.  I was using a borrowed set-up and the images are all from photographs of the desmids displayed on computer monitor, which is far from ideal.   Some of the larger desmids – one large Closterium species in particular – were too large to fit onto the screen and have had to be omitted from this account.  There were also a number of cells of Eremosphaera (see “More from Loughrigg Fell”) and some Cyanobacteria (Merismopedia was quite common) so this is a very partial description of our microscopical adventures in School Knott Tarn.

The first two desmids, Spirotaenia condensata and Cylindrocystis gracilis, belong to a group of desmids called “saccoderm desmids”.  These are more closely related to filamentous green algae of the Zygnemetaceae that are old friends of this blog (see “Concentrating on Carbon, for example) and, in fact, we could think of these genera as being unicellular analogues of their filamentous cousins.   Spirotaenia, with its helical chloroplast, for example, recalls Spirogyra whilst Cylindrocystis’ two star-shaped chloroplasts is reminiscent of Zygnema.  Mesotaenium, which we did not see in this sample, has a plate-like chloroplast similar to that in Mougeotia.

The next two illustrations both show species of Micrasterias.  Of these, M. compereana generated a vigorous discussion amongst our experts. This would have been described as M. fimbriata using the latest British floras but a paper has been published recently which uses molecular data to demonstrated the need to split the species. Finally, we have representatives of Euastrum and Haplotaenium, two genera that we also met at Dock Tarn (see “Damp days in search of desmids …”) although the species are different.   Haplotaenium differs from Pleurotaenium in the number and form of the chloroplasts and also because it lacks a terminal vacuole.

Desmids from Sphagnum squeezings from School Knott Tarn, September 2017: a. Spirotaenia condensata; b. Cylindrocystis gracilis; c. Micrasterias compereana; d. Micrasterias crux-meltensis; e. Euastrum oblongum; f. Haplotaenium rectum.  Scale bar: 25 micrometres (= 1/40th of a millimetre).

Four more desmids are illustrated on the lower plate.   Of these, we have seen Netrium digitus in Dock Tarn and the illustration there is better than this one, showing the undulating nature of the chloroplast margins quite clearly.   The desmid below this, Closterium closterioides caused some confusion at first.   We usually associate Closterium with lunate (moon-shaped) cells (see “More from Loughrigg Fell”) but this species is straight, sending me towards the section on Netrium in my Flora.  However, Netrium lacks terminal vacuoles whereas this specimen has prominent vacuoles at both ends.   We also found a variety, C. closterioides var. intermedium, in the same sample.

The final desmid that I have illustrated is a filamentous form: Desmidium schwartzii.  In contrast to Hyalotheca dissilens (see “Desmids from the Pirin mountains”) there is no obvious mucilaginous sheath around this specimen, but this may be an anomaly of this population or an artefact of the microscopy set-up.   We are looking at the side view of a chain of cells but if we were to look at the end view of one cell it would be triangular in this particular species.  The chloroplast fills most of the cell and has projections running into the corners of the cells.  However, as the filaments of the cells are slightly twisted, these projections appear to shift in position from cell to cell, giving a helical appearance.  I’ve tried to illustrate this with a schematic diagram.

More desmids from Sphagnum squeezings from School Knott Tarn, September 2017: g. Netrium digitus; h. Closterium closterioides var. closterioides; i. C. closterioides var. intermedium; j. Desmidium schwartzii Scale bar: 25 micrometres (= 1/40th of a millimetre).

This short post gives some idea of the diversity in a single sample from a single Tarn.   Dave handed all the samples we collected over to David Williamson on his way back south and we’ll get a fuller list of their diversity in due course.  This one sample occupied me for the latter part of Saturday morning and all of the afternoon.   On Sunday, I moved on to look at another sample and I’ll write about that in another post very soon.

A schematic view of a chain of Desmidium cells, showing the arrangement of the chloroplast seen in apical view (k.) and the implications of slight twisting of the filament on appearance (l.).  Diagram adapted from John et al. (2011).


John, D.M., Whitton, B.A. & Brock, A.J. (2011). The Freshwater Algal Flora of the British Isles. 2nd Edition. Cambridge University Press, Cambridge.

Neustupa, J., Šťastný, J. & Škaloud, P. (2014). Splitting of Micrasterias fimbriata (Desmidiales, Viridiplantae) into two monophyletic species and description of Micrasterias compereana sp. nov.  Plant Ecology and Evolution 147: 405-411.