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.

Desmid masterclass …

My chance encounter with desmids in the Pirin Mountains (described in the previous post) was a serendipitous insofar as, less than a fortnight after my return from Bulgaria I find myself standing beside a soft water tarn in the Lake District noted for its rich desmid flora.  I’m here for a joint field meeting of the British Phycological Society and Quekett Microscopical Club and, having decided on a venue, realised that the location of the Freshwater Biological Association’s laboratory meant that desmids were the obvious focus of our investigations.

So, on Friday afternoon, a small convoy of vehicles made its way from the FBA, on the west side of Windermere, via Hawkshead and Coniston, to two small tarns for an initial collecting trip.  Dave John, our in-house desmid expert, had been directed to these by David Williamson, the UK’s leading expert on desmids (but, alas, too elderly and infirm to join us), who had recorded large numbers of desmids from both Kelly Hall Tarn and Long Moss Tarn.   Fortunately, the weather today was rather more amenable to sampling than my previous desmid-hunting trip in this part of the world (see “Damp days in search of desmids”).

The predilection of desmids for boggy areas means that there is always a risk of damp feet when –collecting samples, especially when grabbing a chance sample whilst out walking.   Last time, my problem was that footwear appropriate to getting to Dock Tarn was not really appropriate for getting into Dock Tarn; today, however, I could tramp the short distance from our vehicles in wellies.  Dave noted wryly that most of David Williamson’s best sites were close to car parks, and that many of the sites sampled by the Wests, writers of a significant desmid flora of Britain in the late 19th century, were similarly placed closed to train stations.   Such practicalities probably lie behind more of our decisions about where to sample than we care to admit.

Once by the edge of the Kelly Hall Tarn, Dave gave us a demonstration of sampling desmids, starting with David Williamson’s preferred method using a plankton net.   There are two options – either casting a net attached to a piece of rope, then drawing it back through the water  or using a small plankton net attached to a telescopic handle.   Dave’s has a mesh of 33 micrometres (1/30th of a millimetre) which is fine for relatively large algae such as desmids.   He took care to avoid disturbing the bottom, in order to collect only phytoplankton but also pulled the through patches of submerged plants in order to dislodge the algae that live around their stems and leaves.  With care, it is also possible to brush the net gently against the bottom, dislodging some of the heavier desmids that typically sit just on the surface of the bottom sediments.  Too much disturbance will mean that the net quickly clogs with fine sediment particles.

Dave John demonstrating how to sample desmids with a plankton net at Kelly Hall Tarn, Cumbria, September 2017.

From Kelly Hall Tarn it was a short uphill walk to Long Moss Tarn for our second demonstration.   There is usually a part of the perimeter of tarns in this part of the world that is marshy, often dominated by Sphagnum.  The hummocks of Sphagnum were of no great interest except as stepping stones to the edge but, once there, we could see the semi-aquatic Sphagnum cuspidatum living at the margins of the lake (moss cognoscenti describe S. cuspidatum as looking like “drowned kittens”).   Dave took a handful of this soft, damp moss, flicked it a couple of times to remove surplus water, then gently squeezed it, catching the water that oozed out of the moss in a small sampling bottle.   Holding this up to the light, we could see a distinct green tinge which brought a big smile to Dave’s face, anticipating the rich haul of desmids that awaited us.

Dave John demonstrating how to collect desmids by squeezing Sphagnum at Long Moss Tarn, Cumbria, September 2017.

A patch of Sphagnum cuspidatum at Kelly Hall Tarn: perfect desmid habitat.

Kelly Hall Tarn had one further attraction for me.   It is perched on a hillside overlooking Coniston Water and from a small knoll a couple of hundred metres from the tarn I had a wonderful view southwards down the lake.  In the middle distance I could see Peel Island which was the inspiration for “Wild Cat Island” in Arthur Ransome’s Swallows and Amazons, a book that I read and re-read many times as a child.   At the north end of the lake you will find Bank Ground Farm, which was the setting for “Holly Howe” in the book and which, I notice, still offers accommodation and can be booked via Airbnb.   Finally, on this theme, the Steamboat Museum in Windermere has the original “Amazon”, the sailing dinghy that is the centre of many of the adventures.

Enough of these digressions.   We now have samples from two tarns and the next post will start to explore the contents of these and other samples that we collected.

The view down Coniston Water from Long Moss Tarn.  Peel Island is in the centre of the picture.



Desmids from the Pirin mountains

Our travels in southern Bulgaria took us south from Rila Monastery to Bansko, on the edge of the Pirin Mountains and, from here, via a chair lift and a rather more strenuous walk than we had expected to Popovo Lake  situated in a corrie overlooked by rugged mountains soaring up to over 2800 metres.

The outflow from the lake cascades over the lip of the corrie and down the hillside to a series of smaller lakes, before merging with some other small streams to form the Mesta river which flows south from Bulgaria and through northern Greece to the Aegean Sea.   The footpath that took us back from Popovo took a gentler route down the hillside but brought us close enough to the first of this series of lakes for the bright green areas at the margins to pique our interest.  Getting closer, we found ourselves on soft, yielding Sphagnum bog, more familiar to us from the moorlands of northern England (see, amongst other posts, “Back to the bog“) than in southern Europe.   A lot of the rock that we had passed on our hike up from the chair lift terminus had been granite, so the water around us would clearly have been soft enough for Sphagnum and, I guess, the marshy land was testimony to a higher level of precipitation than the cloudless skies that we encountered suggested.

The first of the “Fish Popovski” lakes below Popovo Lake in the Pirin Mountains, southern Bulgaria, with the marshland area clearly visible in the foreground.

One of the pools in this bog attracted my attention: a mat-like portion of the substratum, had floated to the surface whilst still being loosely attached at one corner.   This is a good clue that the substratum is jam-packed with algae, doing a double job of binding the silty particles together into a cohesive whole and, at the same time, pumping out oxygen as a by-product of photosynthesis in order to make the mat buoyant.   I last wrote about this in 2013 when I found some mats of Oscillatoria limosa in my local river (see “More from the River Wear”).   The same phenomena seem to be at play here although, on closer investigation, it was desmids rather than Cyanobacteria which were responsible for the mat.   There were, in fact, far fewer filamentous algae in this particular mat, something that I quickly noticed as there was very little physical integrity to the mat, and it dissolved into a suspension of fine particles as soon as I tried to remove a piece.   Had I stayed until nightfall, I expect the mat would have gradually sunk again as the rate of photosynthesis declined and oxygen production ceased.

The desmid mat floating up from the bottom of the bog pool beside the first of the Fish Popovski lakes in the Pirin Mountains.

The most abundant desmids in the mat were Hyalotheca dissilens and Closterium baillyanum.  The former is a filamentous desmid, whose chains of cells are enclosed in a broad mucilage sheath and, whilst there were many fewer of these filaments than I have seen in more cohesive mats, I suspect that these played a role in trapping the other algae, plus organic and inorganic particulate matter to form the structure that I saw.  Closterium baillyanum, by contrast, has large, robust cells and, in this case, the cell wall has a distinct brown colour.  Other desmids that I found in my brief examination included Tetmemorus granulatus, which has cylindrical cells with a narrow incision in the broadly rounded apex (most clearly visible at the left-hand side of the illustration) and two species of Euastrum: E. humerosum and E. ansatum.  There were also a few large cells of Eremosphaera, a green alga though not a desmid (see “More from Loughrigg Fell”), and various assorted unicellular algae.  I’ll write about the diatoms in a separate post.

Some common desmids from the Pirin mountains: a. Hyalotheca dissilens; b. Closterium baillyanum prox.; c. Tetmemorus granulatus.  Scale bar: 25 mm (= 1/40th of a millimetre).

Unfortunately, the chloroplasts in these illustrations are not at their best.   My preference is always to keep algal samples fresh for as long as possible but, as I was moving around southern Europe in August I adopted my usual practice when travelling and added some vodka to the sample as a temporary preservative.   Nonetheless, Dave John, who identified the species, could find enough in the general morphology and cell wall characteristics to guide him.   Desmid species are considered to be cosmopolitan so he was able to use identification literature from Northwest and Central Europe in order to do this.   The search terms “desmid” and “Bulgaria” yielded no papers when I looked on Web of Science, so this looks like a relatively unexplored corner of Europe, as far as this group is concerned.   Having said that, all five of the species illustrated here are listed in the checklist of Romanian algae, and it is quite likely that there are local publications that have not made it onto the major bibliographic databases.

Incidentally, it was only after I had bought a miniature of vodka that I realised that I should have used this as an opportunity to buy a bottle of rakija, the local spirit.  For some reason, I had assumed that this was an ouzo-type spirit and that it would give the water an opaque milky-white appearance (caused by the lower solubility of essential oil of anise in water compared to alcohol).   Experimental studies later that same evening showed that the local rakija was closer to the Romanian ţuică, prepared from grapes, plums or apricots (the latter is especially fine), and would have made a fine preservative.   I am older and wiser although, in the immediate aftermath of my experiment, that wisdom may not have been immediately apparent.

More desmids from the Pirin mountains: d. Euastrum humerosum; e. E. ansatum. Scale bar: 25 mm (= 1/40th of a millimetre).


Cărăuş, I. (2017). Algae of Romania.  A Distributional Checklist of Actual Algae.  Version 2.4.  Original print edition published by University of Bacău, Romania.  Latest version available online [https://www.researchgate.net/publication/285888477_The_algae_of_Romania]

The art of icons …

A week off from algae, as I travel around Bulgaria on holiday.  In between exploring mountains (and, I admit, pulling a toothbrush from my knapsack on a couple of occasions for a sneaky diatom sample), I have been learning about the intricacies of Eastern Orthodox icon painting, as a break from my normal scientific and artistic routines.  My interest was piqued by a visit to the superb icon gallery at the National Museum of Art of Romania in Bucharest last year, though this mostly served to demonstrate how little I knew, either about icons or their context in Orthodox worship.

Context is important because, in our secular age, we are most likely to encounter religious art in a gallery rather than a church.  My initial response to an icon, such as that in the image below, is to place it into a Western art historical context.   I note the relatively simple modelling of the features, depicting archetypes of religious figures and the flat background.  There is no attempt to place the figure in three-dimensional space, as most religious painters from the Renaissance onwards would have tried to do.  They were trying to draw the viewers in, creating space inside the picture that encouraged them to engage with the subject matter.  Painters of the Counter-Reformation, such as Rubens, went further, painting the protagonists in their religious paintings life size and dressing them in contemporary clothes to encourage viewers sense of participation.

An icon of Christ Pantocrator from the Bankso school of icon painters (late 18th / early 19th century) in southern Bulgaria.  The image at the top of the post shows the iconostasis at Mānāstirea Stavropoleos, Bucharest, Romania.

By contrast, by flattening everything but the subject’s physiognomy, the Orthodox icon painter projects his subjects into our space, encouraging a different type of engagement.   Orthodox Christianity has a strong tradition of contemplative prayer, in which knowledge of God is attained through meditative practices such as repetition of a meaningful word or short phrase.  In this context, icons can serve as objects that help viewers to concentrate their minds while they step away from the everyday world and towards the divine realm.  One manifestation of this is that there is typically more activity in an Orthodox Church, compared to a Catholic or Protestant church, outside of organised services, as worshippers make their own private devotions in front of icons.

This use of repeated phrases suggests parallels with eastern religions – the Hindu incantation “Om mani padme hum” being the best-known example.  Look, too, at the right hand of Christ in the icon below.  That, too, resembles the symbolic hand gestures – mudras – found in Hindu and Buddhist contemplative practices.   Whether there is more than a superficial resemblance, in this particular instance, is a moot point.  Christ’s hand is raised to confer a blessing on the viewer and the position of the fingers is related to this.  They spell out “ICXC” –  IhcoyC XpictoC, or “Jesus Christ”.   The confusion with eastern practices arises, I suspect, from the way that the fourth finger is bent over to touch the thumb, similar to the Chin Mudrā.

On the other hand, there would have been ample opportunity for exchange of ideas along the Silk Road.  Early Christianity extended much further east, and Buddhism further to the west before the rise of Islam. Diarmaid MacCulloch has suggested that the principle of monasticism, for example, may have been brought into the church by early missionaries returning from the east and, if this is the case, then it is possible that practices associated with monasticism would also have flowed east.  And, equally, there is no reason to assume that the movement was entirely one-way or solely between Christianity and Buddhism.  Our first reaction on walking into Rila monastery in southern Bulgaria was to notice the physical similarities with the huge Madrassas that we saw in Uzbekistan earlier this year (see “Reaching for the stars …“).

What we can see an Orthodox icon, in other words, is a product of time and place, only if we also recognise that time and place are continua, that ideas can flow and that there is a ‘natural selection’, of sorts, that selects and shapes these to fit local circumstances.  Traveling broadens the mind, without a doubt, but sometimes you need to unload your preconceptions in order to free up the mind to see the world through fresh eyes.

The courtyard of Rila monastery in southern Bulgaria with the Church of the Nativity on the right.

How to make an ecosystem

I visited Scotland vicariously last week (meaning that I did not actually cross the border but a little bit of Scotland made its way south to me).   In this case, my wife had been reconnoitring potential sites for a field course along the Fife coast and had visited the sand dunes at Tentsmuir National Nature Reserve to see if the plant succession there would make a suitable exercise for her students.  Behind the sand dunes there was a low lying area of saltmarsh and, within that, large areas of algal mats.  I’m guessing that, having brought me a bottle of Highland Park whisky as a reward for not killing her houseplants during her previous trip to Scotland, she thought that my liver deserved a break.

Tentsmuir is a dynamic ecosystem, with sand dunes on the coastal side and, in places, a complete succession from colonising grasses on the seaward side to mature forest on the land.   However, there are also slacks behind the dunes which are periodically inundated by seawater, leading to the development of saltmarshes.   The periodic wetting and drying of saltmarshes is ideal for filamentous algae and these, in turn, create a mesh of interlocking filaments that binds the sand grains and traps organic matter.   Over the course of many tidal cycles, conditions become suitable for higher plants such as glasswort and sea asters.   I have a soft spot for saltmarshes and sand dunes as these are the habitats where I made some of my first forays as an ecologist (see “How to be an ecologist #4”); however, I have never looked in detail at the algal mats before.   So, I poured myself a glass of Highland Park, turned on my microscope and teased out a few of the filaments from my present.

Algal mats from saltmarsh at Tentsmuir National Nature Reserve.  The left hand picture shows a plant of Salicornia europaea agg. (Common Glasswort, or “samphire”) surrounded by algal mats (photo: Heather Kelly).  The right hand picture demonstrates how the mat retains its integrity after being removed from the saltmarsh.

The mat, in this case, seemed to be made up predominately of two types of alga: the yellow-green alga Vaucheria and the Cyanobacterium Microcoleus chtonoplastes.   I often see Vaucheria in freshwaters so it was the Microcoleus that attracted my attention.   It belongs to the same family as the Phormidium that we met in Mallerstang (see “The stresses of summertime …”) and we can see several of the same features: rows of almost identical cells and, in particular, no “heterocysts”, specialised cells that are responsible for nitrogen fixation.   Technically, the chain of cyanobacterial cells is referred to as a “trichome” and these are enclosed in a “sheath” (seen most clearly in genera such as Scytonema: see “Tales from the splash zone …”).  In Phormidium there is a single trichome per sheath but each sheath of Microcoleus contains several, often twisted around each other to form rope-like bundles.

Microcoleus cf chthonoplastes from the saltmarsh at Tentsmuir National Nature Reserve, August 2017.  Scale bar: 10 micrometres (= 100th of a millimetre).

Although I mentioned that Microcoleus lacked heterocysts, this does not mean that it is not capable of nitrogen fixation.   The reason that cyanobacterial cells need heterocysts is that the nitrogenase enzyme only works in anerobic conditions.  The oxygen that is produced as a result of photosynthesis is, therefore, a toxin that needs to be kept away from nitrogenase. Heterocysts have thick cell walls and less chlorophyll as means of keeping the nitrogenase in an oxygen-free environment.  However, some non-heterocystous cyanobacteria, including Microcoleus, are able to fix nitrogen at night (when the photosynthetic apparatus is not pumping out oxygen) .   As there seems to be no protection for the enzyme inside the cells, it is possible that the daily destruction of enzyme is offset by renewed synthesis when light levels fall and there is no more oxygen being produced internally.   Nitrogen-fixation is already an expensive process for cells, requiring a large amount of energy, and this will increase the cost further.  However, in the case of the saltmarsh at Tentsmuir, there is a large amount of habitat available and few other organisms capable of exploiting it, so perhaps this is an investment worth making?

The benefits of that investment then “trickle down” (or up, depending on your point of view) through the ecosystem.   The cyanobacteria “fix” carbon and nitrogen and, in effect, create the soil within which other organisms thrive.  Janet Sprent, of the University of Dundee, calculated that, assuming nitrogen to be the limiting nutrient, then the fixation by Microcoleus and other cyanobacteria in such habitats could probably support the biomass of higher plants that is usually observed.  They are, in other words, a self-perpetuating “green manure” that creates a habitat within which other organisms can thrive.  In turn, by binding sand, they help to stabilise coastal features and, in turn, protect other coastal habitats and the communities that live amongst these.


Malin, G. & Pearson, H.W. (1988).  Aerobic nitrogen fixation in aggregate-forming cultures of the nonheterocystous Cyanobacterium Microcoleus chthonoplustes. Journal of General Microbiology 134: 1755-1763.

Omoregie, E.O., Crumbliss, L.L., Bebout, B.M. & Zehr, J.P. (2004).  Determination of nitrogen-fixing phylotypes in Lyngbya sp. and Microcoleus chthonoplastes cyanobacterial mats from Guerrero Negro, Baja California, Mexico.  Applied and Environmental Microbiology 70: 2119-2128.

Sprent, J.I. (1993).  The role of nitrogen fixation in primary succession on land.  pp. 209-219.  In: Primary Succession on Land (edited by J. Miles & D.W.H. Walton), Blackwell Scientific Publications, Oxford.

Sroga, G.E. (1997).  Regulation of nitrogen fixation by different nitrogen sources in the filamentous non-heterocystous cyanobacterium Microcoleus sp.  FEMS Microbiology Letters 153: 11-15.

How to win the Hilda Canter-Lund competition (4)

Daniella Schatz’ image of the coccolithophore Emiliania huxleyi is one of a relatively small number of electron micrographs to have made it to the shortlist of the Hilda Canter-Lund prize and, though not an outright winner, it offers some useful lessons to anyone considering submitting an image in next year’s competition.

The first point to note is that Daniella has not submitted a single image, but a montage of two separate images. The competition rules state that “basic image enhancement (i.e. cropping, adjustment of contrast, colour balance etc) is permitted, along with focus stacking and stitching. However, excessive image manipulation is not acceptable.”   “Excessive image manipulation” is not easy to define; however, Daniella’s montage worked for the judges because the two elements together tell a story about the life of this alga.  The left- and right-hand images are the “before” and “after” cases of a major factor controlling the ecology of Emiliania huxleyi.  Daniella wanted to tell the story of the decline and fall of E. huxleyi blooms in the oceans; in the process she also evoked a long tradition of memento mori – artworks that remind viewers of their own mortality, and of the fragility of all life on earth. Another montage, this time by Alizée Mauffey, made it to the short list in 2017; again, the images were not selected and placed for aesthetic reasons, but to illustrate the range of functional traits within intertidal macroalgae.

Daniella piles on a little more “image manipulation” by using false colour to highlight the tiny EhV201 virus cells that are scattered across the right hand cell and which are responsible for its sorry state.  A couple of SEMs that have been enhanced by false colour are submitted each year but the artificiality of the medium rarely results in a major improvement to the image.  The stark monochrome of SEMs places them in a long and noble tradition of black and white photography that should not need this type of enhancement.   She, however, challenges this by using false colour very sparingly and to draw attention to an important element of her story.

And so to the “story”: we now ask all entries to the competition to be accompanied by a legend of about 100 words explaining a little more about the picture.   Most experienced phycologists will recognise the left hand image as a coccolithophore but many viewers will see these as abstract geometric shapes. The legend is important to help the viewer decode these shapes and place them into a broader context; in this case, by emphasising their role in global carbon cycling.  Having said that, most of the shortlisting takes place without reference to the legend with initial screening based primarily on the quality of the images.  I do remember, however, that Daniella’s image was one where we did need the legend in order to understand what she was trying to say.

A detail from Daniella Schatz’ Scanning Electron Micrograph (SEM) of the coccolithophore Emiliania huxleyi showing the large dsDNA Emiliania huxleyi virus (EhV201, coloured orange). EhV is a large dsDNA virus that is responsible for the demise of vast oceanic blooms of E. huxleyi. During viral infection the cells undergo programmed cell death and shed their coccoliths, important components of the carbon cycle.  The individual viruses are each about 100 nanometres (1/10000th of a millimetre) in diameter.

We also encourage photographers, particularly those submitting microscopic images, to include a measure of scale in the legend, particularly for microscopic images.  This is important, as lay audiences will have little idea about the size of the objects that are being portrayed.   When images are used as illustrations, then a scale bar is appropriate (see “The stresses of summertime …” for a recent example); however, a scale bar is likely to be an unwelcome intrusion in an otherwise balanced composition so a sentence in the legend is usually more appropriate.   Remember that the term “micrometre” might not be easily understood by many viewers, and it is a good idea to explain dimensions in millimetres as well.

When the votes were counted in 2015, Daniella’s image lost out to Günter Forsterra’s stunning view of the Beagle Channel off the coast of Chile.  However, it stands as a fine example of conceptual approach to the Hilda Canter-Lund competition – with several different elements combining to convey an idea that is more than the sum of its parts.   The photographer of the microscopic world rarely has the luxury of the “decisive moment” and, instead, the quality of the final image often lies as much in post-production as it does in image capture.