The big pictures …

If you read this blog regularly you will, I hope, have some sense of just how varied are the algae that live in our freshwaters.   It occurred to me, however, that, in cataloguing this diversity, I don’t often step back and give you some idea of how these many forms relate to one another. I drop terms such as “diatom” and “green algae” into my posts but have not, perhaps, discussed the meaning of these terms in very much detail for some time.

One of the problems is that the meaning of these terms can vary, as knowledge unfolds.  For the early part of my career, for example, I could define “green algae” quite easily, and point to several authoritative textbooks to support my case.   Depending on who wrote the book (and when), green algae were either a separate division (“Chlorophyta”) or a class (“Chlorophyceae”).  There was some dispute about whether Chara and relatives belonged in this group or formed a separate group (“Charophyta”) but that was pretty much the end of the story and taxonomists then got down to arguing about how the many genera and species of green algae should be arranged within this broad heading.

Opinion has, however, shifted over the last couple of decades, with the green algae now split between two separate phyla within the kingdom Plantae.   One of these phyla is the Chlorophyta and the other is the Charophyta, which includes not just Chara and relatives but also some quite important Classes of green algae.    We have met representatives from many of the Classes from both of these phyla in this blog over the years, with the exception of the Prasinophytes, which is an important group of marine plankton with only a few freshwater representatives, and the Trebouxiphyceae.


The organisation of the “green algae” subkingdom (“Viridiplantae”) showing division into two Phyla, and the major Classes found in freshwaters within each Phylum.   The organisation follows Algaebase and the Tree of Life website (see also Lewis & McCourt, 2004). 

Back in the summer I described a number of green algae that I found in the River Wear.   In “Summertime blues …” I wrote about algae that belong to the Chlorophyceae whilst, later in the summer, I explained how these had been joined by a number of desmids, which belong to the Conjugatophyceae (see “Talking about the weather …”).  The plate in that post includes a cell of Pediastrum boryanumbeside some of the desmids; if I was to put together a plate of animals sharing a similar level of kinship, I might include a human and a slug – representatives of two separate phyla within the same kingdom, Animalia (see “Who do you think you are?”).  That is a remarkable amount of diversity to pack into a group of microscopic cells.

The next figure shows the organisation within the Conjugatophyceae, one of the Classes of Charophyta.  The biggest group, in terms of number of species, is the Desmidales, which have featured in quite a few posts (see “Desmid diversity …”), but this class also includes Mougeotia and Zygnema, which we met in the previous post.  Again, just to give you some idea of the scale of the differences, Mougeotia and Zygnema are as closely related as we are to chimpanzees (different genera, same family), whilst their kinship to a desmid is on a par with ours to a warthog (different families, same order).

If you think that you are rather more different to a warthog than one microscopic green alga is to another, there are two things you need to remember: the first is that humans are, relatively speaking, rather good at knowing what features set different types of mammal apart, and that the absence of two short tusks protruding from the sides of the mouth, coupled with a bipedal gate, are highly relevant factors when struggling to decide whether or not the organism in front of you is a man or a warthog.  When trying to understand microscopic organisms such as algae, there are fewer obvious characters, and some of the most useful (such as the presence of flagellae during the reproductive stages) may be present only for a short period of the life cycle.   Straightforward observation, quite simply, is not so useful when trying to determine relationships between microscopic organisms.


Organisation within the Conjugatophyceae, showing division into two Orders and Families.  After Algaebase and the Tree of Life website.

The other point to bear in mind is that algae having had far longer to evolve than mammals.   The two green algae lineages may have separated before the end of the Precambrian era, whilst the primates, the Order to which humans belong, split from other mammals only 65 million years ago.   That means that the green algae have had eight times as long to evolve subtle differences as humans have had to ensure no confusion with warthogs.   Just because these differences are not manifest in obvious features such as tusks does not mean that they are not there.

This brief overview of the green algae has had a side-benefit for me, as it has highlighted a couple of groups I have not previously written about.  One of these groups (the Prasinophytes) is uncommon in freshwaters but the other (Trebouxiphyceae) is quite common and I can even see a green patch formed by a member of this Class from my window as I write this post.   At least I know now what I should write about next …


Lewis, M.A. & McCourt, M.M. (2004). Green algae and the origin of land plants.  American Journal of Botany91: 1535-1556.

Leliaert F, Smith DR, Moreau H, Herron MD, Verbruggen H, Delwiche CF & De Clerck O (2012) Phylogeny and molecular evolution of the green algae. Critical Reviews in Plant Sciences 31: 1-46.


Links to posts describing representatives of the major groups of green algae.  Only the most recent posts are included but these should have links to older posts.

Group Link
Chlorophyceae Keeping the cogs turning …

Summertime blues …

Ulvophyceae Includes many important filamentous and thalloid genera from freshwaters:

Chaetophorales: Life in the colonies …

Cladophorales: Cladophora and friends

Oedogoniales: More about Oedogonium

Trentepoliales: Fake tans in the Yorkshire Dales

Ulothrichales: Spring in Ennerdale

Ulvales: Loving the low flows

Trebouxiphyceae Watch this space …
Prasinophyta Watch this space …
Charophyceaee Life in the deep zone …
Conjugatophyceae Desmidiales: Desmid diversity

Zygnemetales: Fifty shades of green

Klebsormidiaceae The River Ehen in November



Everything is connected …

I’ve written about a curious group of algae called stoneworts (or charophytes) on a couple of occasions (see “The desert shall rejoice and bloom” and “Croft Kettle through the magnifying glass”. The significance of the name “stonewort” becomes obvious when you pick up a Chara plant, expecting it to be soft and pliable, and are struck by the rough texture of the axes, caused by the deposition of lime.


Chara hispida, photographed by Chris Carter.   Note the main axis and branches, from which whorls of branchlets arise at intervals.

The stoneworts are asssociated with hard water, so this deposition should not be a great surprise (the process by which kettles are coated with lime scale in hard water areas is very similar) however, most of the other plants in these habitats don’t share this property, so what is so special about Chara?   The answer is that, in hard waters, the carbon dioxide that plants need for photosynthesis is in short supply, but much more carbon is available as the bicarbonate ion.   Some aquatic plants can absorb the bicarbonate and then use an enzyme, carbonic anhydrase, to convert this bicarbonate to carbon dioxide. Chara, however, has a different strategy, actively pumping out hydrogen from inside the cells which, in turn, react with the bicarbonate and release carbon dioxide, which can then be absorbed by the plant.   However, as the water is also rich in calcium, a further series of reactions produces insoluble calcium carbonate, generating some additional carbon dioxide in the process As this series of reactions occurs very close to the cells from which the hydrogen ions are leaking, the precipitates end up on the plant surface, creating the rough texture.   The chemistry is way beyond this blog (meaning “… this blogger”) but you can follow it up in the references below.


Marcroscopic view of Chara intermedia showing an internode with a whorl of branchlets, along with spine cells and cortex cells (photograph: Chris Carter).

Another ion that is not very soluble is phosphate and this often gets caught up with the precipitating lime to form calcium phosphate.   This can be beneficial, as this phosphorus might otherwise fuel growth of phytoplankton which, in turn, would shade the Chara.   This means that Chara meadows should be resilient to artificial enrichment of nutrients to a limited extent at least.   However, there is some evidence that this capacity might be much less than was previously thought.   Hawes Water, a small tarn in Lancashire (not to be confused with Haweswater in Cumbria), for example, used to have rich and diverse communities of Chara spp, even in the deepest parts, but now the Chara and other submerged aquatic plants are confined to the shallow margins of the lake.   There is also good evidence of artificial enrichment in this catchment. The surprise is that concentrations of phosphorus in the water are still relatively low, yet the Chara meadows are much reduced compared with their condition fifty years ago.   The team that did this work also looked at another small marl lake, Cunswick Tarn, near Kendal in Cumbria, and found very similar changes.

It suggests a sensitivity to eutrophication that, perhaps, has previously been under-estimated, but it also points to the importance of balancing mechanisms in nature. On the one hand, Chara has some inbuilt capacity to counter-act increased nutrient concentrations. But others have shown that the ability of Chara to precipitate calcium carbonate is, itself, based on the photosynthesis rate.   The Chara meadows will reach a point when their natural capacity to absorb this extra phosphorus will be exhausted and then, as the phytoplankton take advantage of this, the water will get more turbid, reducing the amount of light reaching the Chara.   Less light means less photosynthesis and that will reduce the need for bicarbonate and, in turn, mean less carbonate deposition and less phosphorus removed. The evidence from Hawes Water is that this change happens very quickly: an ecological “domino effect”, if you like. As ever, everything is connected; sometimes in surprising ways.


Chara virgata (with oospores) from the Isle of Skye, photographed by Chris Carter.


McConnaughey, T. (1991). Calcification in Chara corallina: CO2 hydroxylation generates protons for bicarbonate assimilation. Limnology and Oceanography 619-628.

Pentecost, A. (1984). The growth of Chara globularis and its relationship to calcium carbonate deposition in Malham Tarn. Field Studies 6: 53-58.

Walker, N.A., Smith, F.A. & Cathers, I.R. (1980). Bicarbonate assimilation by freshwater charophytes and higher plants: I. Membrane transport of bicarbonate ions is not proven. Journal of Membrane Biology 57: 51-58.

Wiik, E., Bennion, H., Sayer, C.D., Davidson, T.A., McGowan, S., Patmore, I.R. & Clarke, S.J. (2015). Ecological sensitivity of marl lakes to nutrient enrichment: Evidence from Hawes Water, UK   Freshwater Biology 60: 2226-2247.

Wiik, E., Bennion, H., Sayer, C.D., Davidson, T.A., Clarke, S.J., McGowen, S., Prentice, S., Simpson, G.L. & Stone, L. (2015). The coming and going of a marl lake: multi-indicator palaeolimnology reveals abrupte cological change and alternative views of reference conditions.  Frontiers in Ecology and Evolution 3:82. doi: 10.3389/fevo.2015.00082.

Croft Kettle through the magnifying glass …


Cymbella-dominated community of epiphytic algae around Chara hispida stems in Croft Kettle, May 2015.

The final stage of my journey of discovery through Croft Kettle is a three-dimensional diorama in which the various components that I have described in posts since my visit on 29 May are reassembled into something approaching their natural state.   In this image, I have tried to show how much of the yellow-brown gunk which you can see in my post of 1st June is, in fact, the stalks of Cymbella cymbiformis, which form a dense matrix around the Chara stems. This, in turn, creates a habitat within which other diatoms can move around. In my illustration, I have included cells of Navicula radiosa and Amphipleura pellucida as well as Rhopalodia gibba and several crystals of calcite. The Rhopalodia intrigues me: Chris Carter’s photographs in my post from 7 June show it attached to Chara stems but I did also see it moving around in samples dominated by the Cymbella stalks.   Adhering strictly to a sessile lifestyle when Cymbella stalks are growing all around you and creating a light-capturing canopy is probably not a great survival strategy and the capacity to move amidst this forest of stalks must give the Rhopalodia more opportunities.   On the other hand, Rhopalodia is not really optimised for motility, as both of it’s raphe slits are on the same side, in contrast to Navicula and relatives where they are on opposite sides. Diatoms exude mucilage through the raphe which attaches to the surface and then gives them something to push against. In this Cymbella forest, these stalks will, presumably, provide that point of contact.   As the diatom pushes against the stalk, it will need to connect with another stalk before it can progress. Having two raphe slits on opposite sides of the valve increases the chances of this happening (in much the same way as a climber using both arms and both legs to work his or her way up a narrow chimney). Having both on the same side, presumably, reduces the chance of successfully adhering to another stalk.   It is, I suspect, a case of “needs must”: limited motility is still better than no motility at all.   If you look carefully, you’ll see that I’ve also included some cyanelles in the Rhopalodia cells.

It is slightly disingenuous for me to suggest that this image is purely the result of my own observations.   They are, for sure, the starting point but I find myself referring to several books as I constructed the image. The view down the microscope has a very flattened perspective, which means that it can be difficult to get an impression of the three-dimensional appearance of a diatom such as Rhopalodia.   For this, I referred to scanning electron micrographs in The Diatoms: Biology and Morphology of the Genera. However, these show the diatom frustules as opaque so I then need to refer back to my own images in order to build up a view of the cell interior.   I can start from my own observations but, after a few days, the chloroplasts of some species start to degrade, so I also turned to Eileen Cox’s book on identification of live diatoms. This is good for details of the plastids, but not so good for stalks so, for these, I am back to peering down my microscope at fresh material.   Finally, I found an excellent new book on charophytes that had some great illustrations that helped me understand the structure of the stem of Chara.

This interplay between direct observation and existing knowledge is necessary and, indeed, there are noble precedents (see “I am only trying to teach you to see …”). However, it also carries the possibility that we promulgate the errors of the past; we look at the natural world through eyes conditioned by the opinions and interpretations of others.  But, then, my picture is no more than an accumulation of my own opinions and interpretations.   In science, as in history, we always walk backwards into the future …


Cox, E.J. (1996). Identification of Freshwater Diatoms from Live Material. Chapman & Hall, London.

Round, F.E., Crawford, D.M. & Mann, D.G. (1990). The Diatoms: Biology and Morphology of the Genera.   Cambridge University Press, Cambridge.

Urbaniak, J. & Gąbka, M. (2014). Polish Charophytes: An Illustrated Guide To Identification. Wroclaw University of Environmental and Life Sciences. Wroclaw.

More about Croft Kettle

In my post on Croft Kettle, I commented on the long stalks possessed by Cymbella cymbiformis. These were difficult to capture with my camera, partly because the Cymbella cells readily detach themselves from their stalks and partly because the tangle of stalks exceeds the depth of field available to the microscopist. Instead, I have tried to capture the view through the microscope eyepiece in a drawing.


Croft Kettle, epiphytic algae associated with Chara hispida stems, May 2015. Drawn at x400 magnification.   The narrow stalks on the left hand side are about five micrometres in diameter.

There is a tangle of stalks on the left hand side, along with two cells each of Cymbella cymbiformis, Navicula radiosa and Rhopalodia gibba. Note, too, the narrow filament of Oedogonium, complete with oogonia (see “Love and sex in a tufa-forming stream …”).   The Rhopalodia cells have glided free from the tangle of stalks.

As I looked at these rich communities of algae I started to wonder if would make a good subject for a painting, so I have been continuing to examine the material I collected in order to build up a sense of what the three-dimensional community around the Chara stems would have looked like. One interesting observation came when I had a look at some of the narrow branchlets of Chara. I wanted to see which algae were directly attached to the Chara surface (more about this in a moment) but the feature that was most noticeable was the quantity of calcite crystals deposited around the stems.   These give Chara its characteristic stiff stems that are rough to the touch.   The calcite is deposited as a by-product of photosynthesis; intriguingly, Chara shares this property with many tufa-forming algae and bryophytes but not with its close relative Nitella, which is much softer to the touch (see “Finding the missing link in plant evolution…”).


Calcite crystals deposited around the tip of a branchlet of Chara hispida from Croft Kettle, May 2015.   Image composed using Helicon Focus stacking software. Scale bar: 10 micrometres (= 1/100th of a millimetre).

One of the questions that was puzzling me was the habit of the diatom Rhopalodia gibba within the community of algae on and around the Chara stems.   In many of my specimens, Rhopalodia seemed not to be attached to the Chara but, instead, glided amidst the tangle of Cymbella stalks growing around the Chara stems; however, I also saw a few cells directly epiphytic on the Chara stems, and this also seems to be the habit that Chris Carter has captured in some of his images of Rhopalodia. I suspect that Rhopalodia, and many other diatoms are opportunistic and can adopt slightly different habits depending upon the prevailing conditions. There is no point, for example, in doggedly sticking to an epiphytic habit if this m

eans sitting in the deep shade cast by a Cymbella forest. Whatever the textbooks say.


Rhopalodia gibba associated with Chara hispida stems in Croft Kettle, May 2015. b. is a valve view; e. is a girdle view and a.,c. and d. are intermediate between the two positions. Scale bar: 25 micrometres (= 1/40th of a millimetre).


Rhopalodia growing on Chara. Photographs by Chris Carter.

Rhopalodia is a genus with an unusual morphology. The raphe follows the dorsal margin (i.e. the left hand side of b. in the figure above) but this means that, in girdle view (i.e. looking from above), both raphes are on the same side of the valve.   I have often assumed that having raphe slits on opposite sides assists motility, by giving the cell two planes by which it may attach (much like a climber working his way up a narrow chimney).   It is possible that being attached to the surface is the preferred habit; motility would become an advantage only when the energy that this process consumes is offset by that supplied by the extra photosynthesis that can take place when it moves away from the shaded areas and into the canopy.   I have never seen any work done to address this topic, but it would make an interesting study.

Another interesting feature of Rhopalodia is the presence of cyanelles, organelles derived from cyanobacteria or similar prokaryotic algae. We also encountered these in Epithemia (see “A return to Cassop”) where I also mentioned that they may be involved in nitrogen fixation.   Cyanelles deserve a post all of their own at some point in the future, so I will just leave you for now with another of Chris Carter’s excellent photographs, in which the cyanelles of a Rhopalodia sp. are highlighted.   They are near-transparent, with thin membranes and are easy to confuse with the vacuoles that contain the polysaccharide chrysolaminarin (these tend to be more refractive). Very easy to overlook.

As I was putting this post together, I noticed that West and Fritsch noted that Rhopalodia gibba was “common in all kinds of localities.”   This surprised me, as I have only ever seen it at a handful of sites in the UK. It did make me wonder if West and Fritsch, writing in 1927, were right, and that it has declined significantly subsequently. A species that has a competitive advantage at low nitrogen concentrations will not have had an easy life in the period after West and Fritsch wrote, as agricultural intensification and widespread use of fertilisers led to increases in the concentration of nitrogen in surface water.


A girdle view of Rhopalodia sp. with cyanelles indicated by arrows.   Note, too, the characteristic lobed chloroplast. Photograph by Chris Carter.


West, G.S. & Fritsch, F.E. (1927). A Treatise on the British Freshwater Algae.   Cambridge University Press, Cambridge.

The desert shall rejoice and blossom …


The Nordic Diatom Meeting, Ratnieki, Latvia, May 2015.  

The motivation for my recent trip to Latvia (see “Following in Arthur Ransome’s footsteps …”) was to attend the Nordic Diatom Meeting, which took place in the University of Riga’s conference centre, Ratnieki, set in the Latvian countryside. It was a small, informal and very friendly meeting, fuelled by enormous quantities of food and enlivened by an excursion that managed to compress an overview of Latvian history from the Iron Age to the present into one afternoon of sightseeing.

It is a little unfair to pick out one of the presentations, but there was a moment on the first afternoon when I sat up with a start as the story of a fossil lake in the Saharan desert unfolded.   There is, in the middle of the Western Great Erg (a huge sand-covered area of the Sahara in southern Algeria), within which there are several depressions. Some of these contain deposits that suggest that they were once lakes.   An Algerian PhD student, Nassima Yahiaoul, told us about her study of an outcrop in one of these depressions, Guern Touil, which was composed largely of diatomite, a rock consisting largely of the remains of dead diatoms.   This is good evidence that, in a moister period perhaps 7000 years ago (the precise date is not yet known), this area was not a bleak, unforgiving desert, but a freshwater or brackish lake.

What made me take particular notice, however, was the diatoms that she found when she analysed these deposits.   These included Cymbella cymbiformis, Epithemia argus, Denticula tenuis, three species of Mastogloia and Navicula oblonga, a very large and distinctive species.   None of these are particularly common in the streams and lakes that I study in temperate Europe but, curiously, several of these occur together in a small pond about 30 kilometres away from where I lived. This pond is, itself, botanically quite distinctive, and it was a strange sensation to sit in the Latvian countryside and hear about another with such an uncanny resemblance but which is so far distant in both space and time.


The outcrop of diatomite in the Guern Touil depression, Western Great Erg, Algeria, studied by Nassima Yahiaoul.

The place that Nassima’s description evoked for me is Croft Kettle, a small pond is fed by subterranean springs emanating from the Permian limestone.   It is fringed by the saw sedge, Cladium mariscus but the edges of pond then shelve very steeply and the submerged vegetation is dominated by Chara hispida and C. vulgaris.   Whether Nassima’s pond ever looked like the illustration below is debatable (there are fossil forests near Guern Touil so the idea of a tree-fringed oasis at some point in the Holocene is not wholly fanciful). The bare evidence that palaeoecologists produces often needs to be catalysed by the imagination, and the imagination, in turn, feeds off analogies. So long as we treat these speculations with a healthy dose of caution, all is good.


Croft Kettle, a Site of Special Scientific Interest in County Durham, just south of Darlington, photographed in May 2015.

As the pond is fed by subterranean springs, the water in Croft Kettle is very clear, allowing the dense Chara beds to extend into the depths.   I could only reach the very edge of these beds when I visited a few days ago, but I was struck by the large quantity of yellow-brown diatom growths that smothered the Chara.   Under the microscope, these proved to be composed of a dense tangle of a stalked diatom, probably Cymbella cymbiformis, within which other diatoms such as Rhopalodia gibba and Navicula radiosa were moving.   The Cymbella is the same one that Nassima found in Guern Touil and I could also see representatives of three of the other genera that she described. I have recorded some of the other species that Nassima recorded from here, but they were not showing themselves today.


An underwater view of the margins of Croft Kettle, showing the dense beds of Chara, smothered by growths of diatoms, May 2015.

The quantity of diatoms that I saw in Croft Kettle was surprising, especially as I normally expect grazers to be very active at this time of year.   The yellow-brown growths resembled those that I reported from the River Ehen in April (see “Diatoms and dinosaurs”). Those were of a Gomphonema species which, like Cymbella cymbiformis, grows on the end of long stalks. These, in turn, create a tangled matrix within which other species of diatom can live.

The Cymbella cells become detached from their stalks very easily, which means that it is easier to photograph isolated cells than the complete stem plus stalk complex.   The tangle of stalks is also difficult to capture in a photograph due to the very shallow depth of field available when you are using medium-and high-magnifications.   That brings me back to the topic of imagination: the microscopist needs this just as much as the palaeoecologist, if s/he is to gain an insight into the nature of communities that have been wrenched out of their natural habitat and squashed under a cover slip.   More so, indeed, for the diatomist, who habitually marinades samples in a sauce of oxidising agents to leave just the silica frustule behind.   But here I go again … droning on about the need to understand diatoms in their living state.   Forgive me …

Enough for today: Croft Kettle is a pond with many fascinating – and one or two very unexpected – stories to tell.   Plenty to keep me going for a few more posts …


Cymbella spp. growing on Chara in Croft Kettle, May 2015; a. – c.: Cymbella cf. cymbiformis; d. – e.: two as yet unknown Cymbella sp. Scale bar: 10 micrometres (= 1/100th of a millimetre).


Wheeler, B.D. & Whitton, B.A.(1971). Terrestrial and Sub-aquatic vegetation. The Vasculum 56: 25-37.

Hudson, J.W., Crompton, K.F. & Whitton, B.A. (1971). Ecology of Hell Kettles; 2. The Ponds. The Vasculum 56: 38-45.


Hilda Canter-Lund photography award 2013 winner

Much of the pioneering work on the fungal parasites of algae such as Asterionella was performed by Hilda Canter-Lund during her time at the Freshwater Biological Association, which makes a nice link with this post, as the winner of the 2013 Hilda Canter-Lund photography award has just been announced on the British Phycological Society website.  Hilda Canter-Lund was an extremely accomplished photographer of the microscopic world, producing pictures that combined high technical and aesthetic merits and was a Fellow of the Royal Photographic Society.  The award was set up in her memory by the British Phycological Society.

I was extremely pleased that Chris Carter won this year.  He made it to the shortlists in 2010 and 2011 and, as readers of this blog will already know produces pictures of an extremely high standard (see posts of 1 March and 14 May).   Chris’ winning entry shows the reproductive organs of a stonewort, Chara virgata from a pond in Northamptonshire, where he lives.   The visual focus s the bright orange antheridium, about 0.4 millimetres across, with interlocking shield cells caught just before they rupture.


Chara virgata: Chris Carter’s winning image in the 2013 Hilda Canter-Lund photography award.

Chris’ career was spent developing infra-red sensors in the electronics industry, with natural history and photomicroscopy as spare-time enthusiasms.  Now he has retired from the electronics industry he has more time to spend on these enthusiasms, with some spectacular results.   Despite all the advances in optical technology and digital imaging over the last decades, good microscopic images require an extraordinary amount of patience and technical know-how.   His winning image encapsulates perfectly the standards that Hilda Canter-Lund set herself.


Chris Carter, out in the field collecting algae.