A twist in the tale …


After my sojourn in East Durham, described in the previous post, I have travelled back to the Pennines for this one, crossing the River Wear at Wolsingham before driving up onto the fells and finally dropping down to the woodlands that are Hamsterley Forest.  This is a large man-made plantation, dating from the 1930s and popular for recreation. In January, however, the forest is quiet, and I only have a few mountain bikers and a lone dog walker for company as I peer into the peaty waters of Euden Beck.   This stream rises on the open fells of Hamsterley Common, between Weardale and Teesdale, before flowing through the forest and joining Spurl’s Wood Beck just downstream from where I am standing, to become Hamsterley Beck.  This then joins the Wear a few kilometres downstream from Wolsingham.


Euden Beck, just above the forest drive in Hamsterley Forest, January 2019.  The photograph at the top of the post shows a view towards Hamsterley Forest. 

There is a mixture of diatoms growing on the stones here but I am most interested in the genus Fragilaria today.   One of the curiosities of this genus is that we often find several representatives growing at the same site at the same time, reminiscent of the old adage about London buses (“you wait ages, and then three come along at once”).   I’ve written about this before (see “Baffled by the benthos (2)” and “When is a diatom like a London bus?”) and Euden Beck is another good example of this conundrum in practice.

Today, I could see quite a few cells of Fragilaria teneraand smaller numbers ofF. gracilisplus a newly-described species that I will talk more about later in the post.  Fragilaria teneraforms long, needle-like cells, often clustering together to form sea urchin-like masses growing out from either a filamentous alga or particle to which they are attached (see “Food for thought in the River Ehen” for an illustration).  Most of the ones that I saw in my samples from Euden Beck were either single cells or pairs of cells, presumably following a recent division. Note how the second cell from the left in the figure below is not as straight as the others.   This is something that I often see with Fragilaria populations in streams in the northern Pennines, and indicates that there may be heavy metal pollution in the water.  There are a lot of abandoned lead mines in the northern Pennines and, sure enough, when I looked at a large scale map, I found one that I had not previously noticed in the upper part of Euden Beck’s catchment.


Live cells of Fragilaria tenera(a. – d.) and F. heatherae from Euden Beck, January 2019.   a., b. and e. are valve views; c. and d. are girdle views.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 

The next image shows these valve abnormalities even more clearly, with almost all of the cells showing aberrations in their outline.   These images are from an older sample; the curiosity here is that whilst most of the Fragilaria tenera valves were twisted, fewer of the valves of Fragilaria gracilisare twisted, whilst few of the valves of the third Fragilaria species show any abnomality in their outline at all.   This species is very common in northern Pennine streams, and I have often seen distorted valves of this species in streams polluted by mine discharges.  This makes the discrepancy between the outlines of this and Fragilaria tenera in Euden Beck particularly intriguing.


Fragilaria tenera from a sample collected from Euden Beck in June 2012.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   Photographs: Lydia King.

I say “Fragilaria gracilis” with a modicum of trepidation as a recent study in which I have been involved, suggests that there may well be at least two species.  These are, as far as we can tell, indistinguishable using characteristics that can be seen with the light microscope though we know that they are genetically quite distinct, and both are widespread, turning up not just in the UK but in other parts of Europe too.

The third species, to the best of our knowledge, does not match the description of any other Fragilaria species, and we are in the process of publishing it as a new species, Fragilaria heatherae.   We have found it a number of samples, not just from the UK but also from sites elsewhere in Europe.   These, by comparison with the other two species, show very little distortion at all.   Whilst several authors have noted this phenomenon in the past, the physiological cause is still not understood. My guess is that the metal ions are displacing a metal co-factor in an enzyme that is involved in the process of laying down the silica cell wall.   Fragilaria seems to be particularly susceptible, but this may be because their long needle-like cells show the distortions more clearly than in some genera but, based on the evidence from Euden Beck, there are clearly differences in susceptibility between species.

Once again, I seem to be ending a post having asked more questions than I have answered. That is always frustrating but another way of looking at this is to realise that the frontiers of ecology are only ever a short drive away from where you are now.  It is very nice to cross oceans to visit rain forests and coral reefs, but there are adventures to be had closer to your doorstep.


Fragilaria gracilis from a sample collected from Euden Beck in June 2012.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   Photographs: Lydia King.


Fragilaria heatherae” from a sample collected in Euden Beck in June 2012.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   Photographs: Lydia King


Duong, T.T., Morin, S., Herlory, O. & Feurtet-Mazel, A. (2008). Seasonal effects of cadmium accumulation in periphytic diatom communities of freshwater biofilms.  Aquatic Toxicology90: 19-28.

Falasco, E., Bona, F., Ginepro, M., Hlúbiková, D., Hoffmann, L. & Ector, L. (2009). Morphological abnormalities of diatom silica walls in relation to heavy metal contamination and artificial growth conditions.  Water SA35: 595-606.

McFarland, B.H., Hill, B.H. & Willingham, W.T. (1996). Abnormal Fragilaria spp. (Bacillariophyceae) in Streams Impacted by Mine Drainage. Journal of Freshwater Ecology 12: 141-149.


Castle Eden Dene in January


The story so far: in 2018 I made bi-monthly visits to the River Wear, my local river and tried to capture, in my posts, the changes in the algae that occurred over the course of 12 months (follow the links in “A year in the life of the River Wear” to learn more).  It was an interesting exercise, partly because last summer’s exceptional weather led to some intriguing changes over the course of the year.   Consequently, as 2019 dawned, I thought I should find a different type of stream within a short drive from my home and try again.  So, bearing in mind that Wolsingham is south and west from where I live, I turned in the opposite direction and drove due east instead, stopping on the edge of the brutal concrete housing estates of Peterlee, a most unprepossessing location for a National Nature Reserve.

My journey has brought me right across the Permian limestone that dominates the eastern Durham landscape. Its escarpment rises up close to my home, and I have written about the algae that live in the ponds at the foot of it (see “A hitchhiker’s guide to algae…”).  On the other side, however, the limestone ends in a series of cliffs overlooking the North Sea and small streams have cut into the limestone to create a series of wooded valleys, or “denes”.   I’ve come to Castle Eden Dene, the largest of these: if you want a cultural reference point, watch the film “Billy Elliott”, set just a few miles further north along the coast, or read Barry Unsworth’s The Quality of Mercy.

We made our way down the footpath into the dene on a crisp and very cold winter morning, past the old yew trees from which the name is derived, and myriad ferns.   A deer bounded across the path ahead and disappeared into some scrub, and then we turned a corner and looked into Castle Eden Burn, which runs along the bottom of the dene.   To my surprise, the stream was dry.   This is a valley that cuts through limestone, so it is common for the stream to be dry in the summer, but I had not expected it to be dry in the middle of winter.  Thinking back, however, I realised that there has not been much rain for some weeks, and this may have meant that the water table, still low, perhaps, after last summer’s dry weather, is too low for the stream to flow.


Diatoms and cyanobacterial colonies in Blunt’s Burn, Castle Eden Dene, January 2019.   The top photograph shows diatom growths on bedrock; the lower image shows Phormidium retzii colonies, each about two millimetres across.   The photograph at the top of the post shows a yew tree overhanging Castle Eden Burn. 

A few hundred metres further down the dene, we finally heard the sound of running water where a small tributary stream, Blunt’s Burn, joined the main burn.  Judging from my OS map, it drains a good part of Peterlee so it might not have very high water quality.  It was, however, a stream and it did, as I could see with the naked eye, have some distinct diatom-rich growths.    These, I discovered later, were dominated by the diatoms such as Navicula tripunctataand N. lanceolata which are typical of cold weather conditions (see, for example, “The River Wear in January”).   A closer look showed that the orange-brown diatom growths were, in places, flecked with dark brown spots.  Somehow, I managed to get my cold fingers to manipulate a pair of forceps and pick up a few of these spots for closer examination.


Diatoms from Blunt’s Burn, January 2019: a. Navicula tripunctata; b. N. lanceolata; c.Gyrosigma cf. acuminatum; d. Nitzschiacf. linearis (girdle view); e. N. linearis(valve view).  Scale bar: 10 micrometres (= 1/100thof a millimetre).

I had a good idea, when I first saw these spots, that they were colonies of a filamentous cyanobacterium and, peering through my microscope a few hours later, once I had warmed myself up, I was relieved to see that I was right.  I picked out a dark patch and teased it apart before putting it onto a slide with a drop of water.  Once I had done this, I could see the tangle of filaments along with a mass of organic and inorganic particles and lots of diatoms.   The filaments themselves were simple chains of cells (a “trichome”) of Phormidium retzii, surrounded by a sheath.   There were also, however, a few cases, where I could see the sheath without the Phormidium trichome, and in some those I could also see diatom cells.

There are some diatoms that make their own mucilage tubes (see “An excuse for a crab sandwich, really …”) but Nitzschia is not one of those most often associated with tube-formation (there are a few exceptions).    On the other hand, there are some references to Nitzschiacells squatting in tubes made by other diatoms.   Some of those who have observed this refer to Nitzschia as a “symbiont” but whether there is any formal arrangement or is just a by-product of Nitzschia’s ability to glide and seek out favourable microhabitats, is not clear.  There are, as far as I can see, no references, to diatoms inhabiting the sheaths of Cyanobacteria, though Brian Whitton tells me he has occasionally seen this too.

We made our way back along the dry bed of Castle Eden Burn.  Many of the rocks here were quite slippery, suggesting that there had been some water flowing along it in the recent past.  That encouraged me to scrub at the top surface of one with my toothbrush and I managed to get a sample that certainly contains diatoms though these were mostly smaller than the ones that I found in Blunt’s Burn, and there was also a lot of mineral matter.   I’ll need to get that sample prepped and a permanent slide prepared before I can report back on just what diatoms thrive in this tough habitat.  Watch this space …


Cyanobacterial filaments from Blunt’s Burn, Co. Durham, January 2019: a. a single trichome of Phormidium retzii; b. and c. empty sheaths colonised by cells of Nitzschia; d. aPhormidiumfilament with a sheath and a trichome but also with epiphytes and adsorbed organic and inorganic matter.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   


Carr, J.M. & Hergenrader, G.L. (2004).  Occurrence of three Nitzschia(Bacillariophyceae) taxa within colonies of tube-forming diatoms. Journal of Phycology23: 62-70.

Houpt, P.M. (1994). Marine tube-dwelling diatoms and their occurrence in the Netherlands. Netherlands Journal of Aquatic Ecology28: 77-84.

Lobban, C.S. (1984). Marine tube-dwelling diatoms of the Pacific coast of North America. I. BerkeleyaHasleaNitzschia, and Navicula sect. Microstigmaticae.  Canadian Journal of Botany63: 1779-1784.

Lobban, C.S. & Mann, D.G. (1987).  The systematics of the tube-dwelling diatom Nitzschia martiana and Nitzschia section Spathulatae. Canadian Journal of Botany.  65: 2396-2402, 


A year in the life of the River Wear …

After six bimonthly visits to the River Wear at Wolsingham during 2018, I can now step back and have a look at the complete dataset to see what patterns emerge.   Over the course of the year, I have visited the site six times and recorded a total of 107 species: 5 Cyanobacteria, 32 green algae, 69 diatoms and one red alga.  The true figure is probably higher than this, as the green algae include a number of “LRGT” (see “Little round green things …”) and certainly did not receive the same level of attention as the diatoms.

This crude enumeration of species, however, disguises some interesting seasonal patterns with, as I described in “Summertime Blues” and “Talking about the weather …”, abundant growths of green algae during the heatwave and associated low flow periods.  This can be seen clearly in the bar chart showing the seasonal changes in the river: diatoms predominate in the early part of the year whilst green algae are very scarce.  The bloom of the green filamentous alga Ulothrix zonata that I expected to see in March was missing due, I suspected, to the hard weather we experienced in late Feburary (see “The mystery of the alga that wasn’t there …”) but, by the summer, the river had taken on a very different complexion and was dominated by small green algae.   The last sample of the year, collected in November, showed a return to diatom dominance with a late autumn showing of Ulothrix zonata(see “The River Wear in November …”).


Relative proportions (by approximate biovolume) of the main groups of algae found in the River Wear at Wolsingham during 2018.  

Looking back at records of a similar exercise in 2009, I see that the beginning and end of the year were quite similar, with thick biofilms dominated by diatoms; however, the algae in the summer of 2009 were very different to those I found in 2018.  My 2009 exercise involved visits every month rather than every other month and I see that I recorded more Cyanobacteria in June and July 2009 than I found in Summer 2018.  These were mostly filaments of Phormidium retziiand tufts of Homoeothrix varians, which I assumed to be a consequence of intense grazing (there is evidence that invertebrates find Cyanobacteria to be less palatable than other algae).  By July, Cyanobacteria comprised over half the total biovolume of algae; however, there was a major spate soon after my visit.  I was surprised to find, when I visited in August, a noticeably thicker biofilm smothering the rocks and, when I looked closely, this was dominated by the small motile diatom Nitzschia archibaldii.   The Cyanobacteria had disappeared almost completely.   I attributed this change to the invertebrate grazers being washed away by the spate, allowing the algae to grow unhindered.  As the biofilm grew in thickness, so the algal cells start to shade each other, and a diatom that can glide through the biofilm has an advantage over any that are stuck to one place.  Diatoms remained dominant for the remainder of the year, although my November sample came just after another storm and the stones I sampled were completely bare.


Relative proportions (by approximate biovolume) of the main groups of algae found in the River Wear at Wolsingham during 2009.   A sample was collected in November but no living algae were recorded from it.

Overall, however, the similarities between the years outweighed the differences in the summer assemblages, whilst the composition of communities between late autumn and late spring was remarkably similar across the two years.   The changes in summer 2018 extended beyond just a shift in the balance of algae in favour of greens: there were also changes in the composition of diatoms too.  In fact, the changes in diatoms proved to be quite powerful mirrors of the changes in the community as a whole.  I have demonstrated this in datasets spanning a number of sites in the past but it is reassuring to see that they are also reflecting patterns within one site.   On the other hand, if I only had examined the diatoms, I would have missed some of the most interesting changes in the river over the course of the year.

Another observation is that no single sample from 2018 contained more than a quarter of the total algal diversity that I recorded over the course of the year.  Every month saw some new arrivals and some departures (or, more likely in some cases, a few taxa that were present had dropped below my analytical detection limit).  Some of these were expected (the seasonal dynamics of Ulothirx zonata, for example); others not (e.g. dominance by Keratococcus bicaudatusin the summer).  I discussed this in “A brief history of time-wasting …” and, in honour of that post, am not going to repeat myself here. In an age when our environmental regulators are cutting back on the amount of data that they gather, I shall go into 2019 reflecting on Yuval Noah Harari’s comment that “the greatest scientific discovery was the discovery of ignorance”.

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


A day out in Wasdale


A few days after my trip to Weardale I found myself beside the River Irt, a few hundred metres below the point where it flows out of Wastwater, in the western part of the Lake District.   Whereas the River Wear drains a catchment underlain by Carboniferous rocks, including a high proportion of limestone (see “Co. Durham’s secret Karst landscape”), the Irt’s catchment is largely underlain by ancient volcanic rocks, resulting in much softer water.   I was curious to see how different the algae were here compared to those in the Wear.

The river bed at this point is dominated by boulders of granite, which host a patchwork of mosses, filamentous algae and discrete growths of diatoms (visible on the right-hand side of the figure below).  Between these there were areas of pebbles and gravels, suggesting good habitat for freshwater mussels.   The patches of filamentous algae (mostly no more than a couple of centimetres in length) were a mixture of Mougeotiaand Zygnema, similar to the forms that I find in the River Ehen, a 30 minute drive to the north.   These two species differ in the form of their chloroplasts (Mougeotiahas a flat plate whilst Zygnemahas two star-shaped chloroplasts, attached by thin cytoplasmic strands to resemble an animal skin stretched on a frame) but are closely-related, both belonging to the family Zygnemtaceae.


An underwater photograph of the substratum of the River Irt in November 2018 showing patches of filamentous green algae, mosses and (on the right-hand side) diatoms growing on granite boulders.


Filamentous green algae from the River Irt, November 2018.   The upper photograph shows cells from a filament of Mougeotiawhilst the lower image shows two filaments of Zygnema. Scale bar: 20 micrometres (= 1/50thof a millimetre).

In between the tufts of filamentous algae were apparently bare patches of rock (they almost certainly had a very thin biofilm that would be hard to sample in isolation from the lusher algal growths that shared their habitat) and some conspicuous orange-brown growths of colonial diatoms.  These turned out to be almost pure growths ofGomphonema hebridense, or a close relative (I can’t give a definitive answer until I have examined cleaned material), growing on long mucilaginous, sometimes branched, stalks to create a veritable “bush” of diatoms.  There were a few other species of diatom growing within this bush, most notably some cells of Achnanthidium (cf.) caledonicumthat seemed to be growing on short stalks attached to the Gomphonemastalks, but also a few cells of Gomphonema capitatum(which also grows on long stalks) and some chains of Tabellaria flocculosa.

Gomphonema hebridenseis a diatom that I have written about several times before, as it is also common in the River Ehen, and also presents a number of interesting challenges to taxonomists (see “Diatoms and dinosaurs”). Whatever future studies reveal, however, the presence of colonies of this (or these) species that are visible with the naked eye is something I associate with only the cleanest rivers in the country during the cooler times of year.  It should not have been a great surprise to me to find it flowing out of one of the most pristine lakes in England (see “The Power of Rock …”).


A close up of cells within a colony of Gomphonemacf hebridense.  Several mucilaginous stalks are also visible as well as (top left) a cell of Achnanthidiumcf caledonicum.   Scale bar: 10 micrometres (= 100th of a millimetre).

The predominance of boulders over smaller, more easily moved stones, suggests a river that has more energy than the River Ehen, one of my regular Lake District haunts.   Both flow out of lakes whose catchments include some of the wildest and most mountainous terrain in the country.   Lakes tend to act as shock absorbers in catchments, slowing down the water that pours off the fells after heavy rain.   Streams in this part of the world that have no such impediments to flow tend to have rocky, mobile beds and relatively sparse algal communities.   By contrast, the Irt and Ehen just below their respective lakes have relatively lush growths of algae.   The substrates of the two rivers, however, are very different: the Ehen having very few boulders in comparison to the Irt, due to the presence of a weir at the outfall. This allows Ennerdale Water to be used as a water supply for the towns of north west Cumbria but, at the same time, turns the lake into an even more effective hydrological shock absorber.  Yet more of the energy that should be washing smaller stones down the river is no longer available except after the most exceptional storms.

That’s my working hypothesis, then: the Irt is a river that is subject to just enough high energy events to move the rocky substrates around yet no so many that rich algal communities cannot develop between these.  The Ehen, by contrast, has fewer events, leading to fewer opportunities for the algae to be scoured away, whilst unregulated streams such as Croasdale Beck (see “What a difference a storm makes …”) have such regular scouring spates that the algal communities are usually sparse.   I might be wrong, of course and I might be back in a years time with a better hypothesis.  Until then …



The River Wear in November


I was back at the River Wear last week for my final visit of the year.   The heatwave that dominated the summer seems like an aeon ago as I plunged my arm into the cold water to find some stones and take some photographs.  I’m curious to see what is here, though.   The river has surprised me several times already this year.  Has it reverted to type as the British climate has regained a semblance of normality, or will the changes that we saw in the summer (see “Summertime blues …” and “Talking about the weather …”) still have consequences for the algae growing on the river bed?

The river bed itself had many patches of green filamentous algae which, on closer examination, turned out to be my old friend Ulothrix zonata, an alga that is common in these parts and which has a distinct preference for early spring conditions (see “Bollihope Bavakakra” and references therein).   A closer look showed two types of filament present: the normal vegetative ones with a single chloroplast encircling the cell but also some where the cell contents have divided to produce zoospores which are released and which, if they land on a suitable surface, will produce new vegetative filaments.   The “parent” filaments, themselves, are produced as zygotes, produced back in the spring, germinate.  The zygotes are the product of sexual reproduction, triggered by lengthening days (see reference in earlier post) and are dormant through the summer, only germinating once day length shortens and temperatures start falling.


The river bed of the River Wear at Wolsingham, November 2018, showing conspicuous growths of Ulothrix zonata.


Magnified views of Ulothrix zonatafilaments from the River Wear at Wolsingham.  The upper image shows a vegetative filament and the lower image shows filaments where the cell contents have divided up prior to the release of zoospores.  Scale bar: 20 micrometres (= 1/50thof a millimetre).

The areas between the patches of Ulothrix zonatawere covered with a thick film, composed primarily of diatoms, in contrast to the situation on my last two visits when non-filamentous green algae predominated.  This time, it was Achnanthidium minutissimumdominated my count (about 70% of cells) although, because they are relatively small, they comprised just under half of the total volume of algae present.   Other diatoms bumped this up to about 70 per cent of the total volume, with motile cells of Navicula and Nitzschia, which were so abundant at the start of the year, beginning to appear in numbers again.   The green cells that dominated my counts in July and September now only constitute about five per cent of the total.   The River Wear, in other words, has shaken off the effects of the summer, just as a healthy human gets over a winter cold, and is now back to its old self.


A view down my microscope whilst examining samples from the River Wear at Wolsingham showing the predominance of Achnanthidium minutissimum with (on the right-hand side) a filament of a narrow Ulothrix (not U. zonata).  

Entomoneis in three dimensions

I’ve written about the genus Entomoneison a few occasions in the past (see “A typical Geordie alga …”).   It is a challenging species to understand partly because the cells often do not survive digestion in the strong oxidizing agents that we routinely use to understand the structure of diatom cell walls, and partly because of its unusual three-dimensional architecture.   I’ve commented on this before, using some of Chris Carter’s photos to illustrate this (see “The really rare diatom show”).  Now, thanks to yet more careful work from Chris, we have a new set of photos with which to understand this species.

The underlying problem of a complicated geometry (the frustule [cell wall] is actually twisted in two planes) is compounded by the shallow depth of field that is available when viewing organisms at high magnifications. The first of Chris’ images shows how most diatomists will encounter Entomoneis: as a cleaned cell mounted on a slide and shows how the girdle bands bands (the silica “spacers” between the two valves) seem to present a particular problem.  Look, in particular, at the arrangement of these in the left-hand image, focused on the top of the cell, and note how they appear to cross over one another.  Compare this to image that is focused on the bottom of the cell.  By contrast, a cell that has not been subjected to the strong oxidising agents that we use to “clean” diatoms prior to observation presents quite a different view, as seen in the second set of three photographs.   The contrast is poorer here, as the cell is not mounted in a high-resolution mountant (the reason diatomists “clean” their samples in the first place) but we can, nonetheless, see the girdle bands.   When Chris focuses on the top of the cell. the girdle bands are clearly visible, not criss-crossed, and diagonal across the cell. At the other extreme (focus on bottom of cell) the bands are still just visible, sloped the other way somewhat obscured by the cell contents but, most importantly, not presenting a gaping hole.

B Entomoneis naphrax mount.jpg

A cell of Entomoneisthat has been cleaned and mounted in Naphrax before being photographed at three focus levels using simple brightfield microscopy.  The left-hand image is focussed on the top of the cell and shows how the girdle bands appear to cross one another whilst the right hand image is focussed on the bottom of the cell and shows a chasm in the centre of the cell where the girdle bands have collapsed. The middle image shows an intermediate focal plane where the apices are in focus: this is where the girdle bands are attached.

C Entomoneis alcohol mount.jpg

A cell of Entomoneisthat has been mounted in alcohol before being photographed at three focus levels. The contrast is much poorer here but at one extreme (focus on top of cell ie towards observer) the bands are clearly visible, not criss-crossed, and diagonal across the cell. At the other extreme (focus on bottom of cell) the bands are still just visible, sloped the other way but somewhat obscured by the cell contents.

What we think is happening is that the girdle bands are so weak that they collapse as soon as the frustule is dried or hits hot Naphrax; this collapse can be either towards the observer or away from the observer, creating a slightly different optical effect in each case.   Most of the time, however, the bands detach completely leaving isolated valves – sometimes with some straggly bits attached.  Chris thinks that almost all the published images of this taxon are misleading: usually flattened either optically or by software in order to give a sharp image for presentation and, in the process, disguising this detail.

These images all show us what Entomoneis looks like in girdle-view, the way we are most likely to encounter an intact cell when looking down a light microscope.  The next two plates show it from above (“valve view”) and in apical view (i.e. looking at the cell from one end), both of which are not often seen during routine observation.    The pair of valve views show the outline at different focal levels, and we can see how the thin wing (keel) is twisted towards the viewer; this twist is also present in the main (cylindrical) part of the cell but is not visible in these photographs.   The series of photographs in the next plate takes this further: the sequence along the top shows an apical view at several points of focus.  Some particulate matter is caught within the open structure of the frustule and acts as a reference point when comparing the two views. The thin keel with its thickened edge (containing the raphe) shows clearly. The body of the cell is not symmetrical because of the twist; the girdle band section is at the bottom of the inverted U section and is demarcated by ridges associated with each band: the number of bands can be estimated as shown on the enlarged fourth section. The other valve must have detached without holding onto any girdle bands.

A Entomoneis valve view in alcohol.jpg

Valve view of an alcohol mounted celul of Entomoneisat two focus levels.

D Entomoneis semicell in apical view in alcohol.jpg

An alcohol mounted semicell of Entomoneis caught in both apical (top row, showing several points of focus) and girdle views (bottom right).  The image at the bottom left shows a slightly magnified version of the fourth apical view indicating the location of the girdle bands on the opposite sides of the valve (indicated by the vertical red lines).

Entomoneis highlights the limitations of using two-dimensions to portray algae.  The answer, Chris and I agree, would be a three-dimensional model (see “Taking desmids to the next dimension …”) that we could pick up and view from all angles.  Another option is to use a scanning electron micrograph (SEM), and the two references at the end of this article contain several useful images.   However, most of us are still going to encounter Entomoneisprimarily via the light microscope.  Having a sense of the three-dimensional form of an alga lodged in your mind makes it much easier to interpret the flattened two-dimensional images that we routinely encounter.  Prior to the era of SEMs, the three-dimensional form of Entomoneis, and, indeed, its true taxonomic position, was very difficult to appreciate.   Both the 1930 and 1990s editions of the Süsswassflora von Mitteleuropaplaced it with Naviculawhereas we now understand enough about the form of the raphe to know that Entomoneis is more closely related to Surirella(see Round et al.,referenced below).  It is a good reminder that the study of diatoms has always been limited by the technology available.   Our toys may have changed enormously over the past hundred years but the gaps in our understanding remain …


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

Dalu, T., Taylor, J.C., Richoux, N.B. & Froneman, P.W. (2015).  A re–examination of the type material of Entomoneis paludosa(W Smith) Reimer and its morphology and distribution in African waters.  Fottea15: 11-25.