Welcome back to the art-science interface

My most recent visit to the River Ehen stimulated me to continue my explorations of what the subaquatic microbial worlds would look like in close up.   Before showing my latest creation, here’s a view of my working space which, as befits my positon in the no-man’s land between art and science, is neither a “laboratory” nor a “studio”.   My microscope is on the right of the desk, with a monitor for displaying specimens at the back. To the left is the bookcase where I keep my identification guides, within easy reach.   The desk has been cleared of clutter to make space for my watercolour pad, paints and pencils.   The main attraction of my study for painting is the good natural light, which floods through the south-facing window.


My study / studio / laboratory.

I have usually worked out a rough design for a painting in a sketchbook before I have started, working both from direct observations down the microscope or photographs, with occasional dips into my books to check features or dimensions.   I then build up the picture gradually, starting with the foreground, and then slotting the other organisms into the spaces around the individuals at the front of the picture.   Working with watercolour and gouache means that I have bursts of activity, followed by pauses whilst a wash dries, then another burst.   Or, to put it another way, I get on with real work (the stuff I get paid for doing) and take occasional breaks to add the next piece of detail to my picture.   The one I have shown here has taken about five days from start to finish, with painting slotted around other activities.

The finished picture is below.   I’ve written much about the River Ehen over the past eighteen months or so and have been constantly surprised at how much variation we have seen in the algal communities over that time.   Each time I visit, I pick up a stone covered in green algae from the stream bed but when I look at these apparently identical growths under the microscope, I se that several different species have appeared and disappeared over the course of the study. Back in winter 2013, these communities were dominated by Spirogyra (see “The River Ehen in February”); other times of the year, the most abundant alga has been Bulbochaete (see “The River Ehen in August”).   Just a few kilometres downstream, we saw completely different algae again (see “At last a red alga that really is red …”). The structure of the communities when green algae predominated has, however, always been similar: the green filaments form a distinct layer over the top of a diatom-dominated understory.   This is what I have tried to capture in this picture. I have also added some filaments of the cyanobacterium Calothrix sp (see “Looking is not the same as seeing …”).


The River Ehen in September 2014.   The biofilm on stones has an understory of diatoms including Tabellaria flocculosa and Fragilaria tenera (right foreground) below a “canopy” of Bulbochaete.   Other algae living in the biofilm include the cyanobacterium Calothrix (left foreground).   Both Calothrix and Bulbochaete have long hairs, which release enzymes to capture phosphorus bound to organic particles suspended in the water. The Bulbochaete filaments are about 10-15 micrometres (1/100th – `/67th of a millimetre) across.

The presence of both Bulbochaete and Calothrix at the same location tells an interesting story. If you look back through my earlier posts, you’ll see that the long hairs characteristic of Bulbochaete and some other algae we’ve seen in the Ehen are thought to be adaptations that allow these species to thrive in situations where phosphorus is very scarce. But Calothrix also has the capacity to fix nitrogen, which is a useful adaptation when nitrogen is the scarcest nutrient. We know that nutrient levels in the River Ehen are low but the presence of both types of adaptation simultaneously suggests the organisms may be subject to both phosphorus and nitrogen limitation, perhaps reflecting short term oscillations in the relative availability of each over the year.

We’ve now got heaps of data telling us what species of algae are present, and how much algae there are over the courseof a year.   What we have done is akin to lifting up the bonnet of a car and naming all the parts of the engine. Our next task is to try to work out what all these different parts are doing (interpreting the role of hairs is part of this) and then, more importantly, to work out why the River Ehen’s engine doesn’t seem to be running as smoothly as we think it should.   I’ll come back to this in a future post.


A very happy pearl mussel

I could not resist including this photograph of a pearl mussel grinning at my camera, partly because it reminded me of a paper I read a few months ago on the problems of conserving species such as pearl mussels which are not “charismatic”.   Though pearl mussels lack characters, such as large eyes that make it easier for humans to empathise with them, there is, nonetheless, a layer of language used by conservation professionals which sits over the usual objective language of science when discussing the plight of mussels.   This, the authors argue, helps us to think of species in human terms, even though they are “rhetorically challenged”.   They point to one officer in a conservation body who referred to pearl mussels as “poor souls” who needed our help – the classic language of charity. And, indeed, there must be something about pearl mussels that raise them so much higher on the conservation agenda than most other invertebrates and way much higher than the lower plants which interest me the most. And, looking at this fellow leering at me from the bed of the River Ehen, I could see that they maybe had a point. Who would not want to conserve such an anthropomorphic little mollusc?


A happy pearl mussel in the River Ehen


Carrithers, M., Bracken, L.J. & Emery, S. (2011). Can a species be a person? A trope and its entanglements in the Anthropocene Era. Current Anthropololgy 52: 61-685

Pearl mussels with some unexpected bedfellows …

I was thinking about my recent encounters with epiphytic algae (see “Cladophora and friends“) during my most recent visit to the River Ehen, when I noticed algae attached to some of the pearl mussels on the stream bed.   We’ve seen this a few times before but today I had my Olympus TG2 camera at the ready and was able to take a few photographs.   There have been occasions when the quantity of algae on the shells has been greater than this, and there has been concern about how this algae may affect the mussel’s ability to feed.   It is one of several possible stresses on the pearl mussel population that we are investigating at the moment.

When I put a small piece of algae that I removed from the mussel shell onto a slide and had a look at it through my microscope, I saw that it was Oedogonium, which we have already met in some posts from earlier this year (see “More about Oedogonium” and links therein).   My guess is that this is not a sophisticated relationship between host and alga, but simply that the mussel shells represent a convenient and relatively stable substratum for opportunistic filamentous algae, which seem to thrive in the River Ehen at this time of year.


Pearl mussels (Margaritifera margaritifera) on the bed of the River Ehen, several with bright-green growths of Oedogonium attached to the shells.   Note, too, the moss Fontinalis antiypretica in the upper part of the picture.

The description of an alga growing on an animal shell makes a nice counterpoint to the recent posts on algae growing on other algae.   There are other accounts of epizoic algae, though nowhere near as many as for epiphytic algae. This might simply mean that we have not looked in the right places.   The most spectacular of the few records that I do know about comes from my Belgian friend, Luc Denys, who scraped the backs of beached whales and found two previously unknown genera of diatoms that seem to grow exclusively on this rather unusual habitat.


Epizoic Oedogonium from the River Ehen, September 2014 at low (x100, left) and medium (x400, right) magnification. Scale bar: 10 micrometres (1/100th of a millimetre).


Denys, L. (1997). Morphology and taxonomy of epizoic diatoms (Epiphalaina and Tursiocola) on a sperm whale (Physeter macrocephalus) stranded on the coast of Belgium.   Diatom Research 12: 1-18.

More about Oedogonium

Having written about a population of Oedogonium that I found in an enriched lowland stream a couple of weeks ago (see “The perplexing case of the celibate alga”), I arrived at the River Ehen last week to find much of the stream bed covered with a dense growth of filamentous green algae that, once again, turned out to be composed largely of Oedogonium.   This soft water site differs in many ways from Stockerley Burn, but a species of Oedogonium seems to be thriving here too. Looking back at my notes, I see that I also found it at the same location this time last year.   The quantities are such that colleagues are concerned for the health of the pearl mussels which also grow here (see “Pearl mussels in the River Ehen”).


Wefts of filamentous algae (mostly Oedogonium) on the bed of the River Ehen, July 2014. Note, too, the flocculant material on the surfaces of the boulders.

The photomicrographs of Oedogonium also show very nicely the reticulate (net-like) nature of the chloroplast.   You can also see two cells of the diatom Tabellaria growing epiphytically on the lower filament.


Photomicrographs of Oedogonium from the River Ehen, July 2014.   Note the cap cells (arrowed) and the reticulate (net-like) chloroplasts.   Scale bar: 25 micrometres (1/40th of a millimetre).

We do not yet have a definitive explanation for the proliferation of Oedogonium in the River Ehen.   The most likely explanation is that the river levels have been low for some time and this means that the gradual accumulation of algae proceeds without the sloughing that occurs when the river is in spate.   The longer the algae can grow without disturbance, the higher the biomass.   When there is a spate and stones are rolled over, the algae that were on top of the stone are deprived of light and gradually die and decompose.   This means that the space between the stones of the substrate, through which oxygen-rich water flows to sustain the young pearl mussels becomes a fetid aquatic “compost heap” which will, in turn, extract this oxygen and make life for the young pearl mussels more difficult. As this is the last healthy pearl mussel population in England, my colleagues in conservation bodies are probably the only people in a region largely dependent on tourists to sustain its economy who are hoping for rain.

Diminishing with age …

Peering down my microscope following my latest trip to the River Ehen, I saw the characteristic curved outlines of cells the diatom Hannaea arcus. It is a species that is most abundant in the spring time and, then, only in relatively unpolluted streams. What surprised me was that it was far less abundant in my sample this year than from samples collected at the same time last year. Of course, as I only visit once a month and this species only thrives for a few weeks, I may have missed the peak of its growth. Or some as-yet unknown combination of the organism’s life-cycle and local environmental fluctuations may have conspired to keep numbers lower than last year.


Live cells of Hannaea arcus from the River Ehen, near Ennerdale Village, May 2014. Scale bar: 10 micrometres (100th of a millimetre).

One other feature that struck me is that the diatoms I was looking at this year seemed to be shorter than those I saw last year. I dug out an old slide to check this and the difference is quite striking. Last year, I saw cells that were 100 micrometres or even longer in a few cases. This year, the longest I saw was 70 micrometres. Such fluctuations in size are common in diatoms and relate to the way the cells divide. The silica cell wall, the frustule, is in two parts, which overlap in the manner of the two halves of a Petri dish or old-fashioned date box. When the cell divides, each of these halves becomes the larger half of one of the two daughters so the average size of the population drops. This is repeated many times until, eventually, cell size diminishes to a threshold whereupon sexual reproduction is initiated.


Cleaned cells of Hannaea arcus collected from the same site on the River Ehen as the live cells photographed above, but a year earlier, in spring 2013. Scale bar: 10 micrometres (100th of a millimetre).

We do not know much about the specifics of the life-cycle of Hannaea arcus but a colleague, David Jewson, suspects that many freshwater benthic diatoms have a two-year life cycle. The reduction in size between 2013 and 2014 would support this assertion. The big question, then, is what size will the cells be when we return to the river in spring 2015?

One aside on Hannaea arcus: a couple of months ago, I wrote about the late John Carter (“remembering John Carter”) and I recalled seeing a short paper that he wrote in 1946. In those days, Hannaea arcus was known as Ceratoneis arcus and he wrote about a sample he had collected in 1927 which was an almost pure growth of this species. That, in itself, is uncommon but it was the location that surprised me: he had found it in a water-filled hole, nearly twenty feet (6.5 metres) up an oak tree. Having only known him as a staid old man, the image of the younger John Carter clambering through the branches of an oak tree in search of algae brought a wry smile to my face.


Carter, J.R. (1946). Diatom notes: the importance of records. The Microscope 6: 70-73.

Rotifers in the River Ehen

I promise that this is the final post on the River Ehen for a while. There is often quite a lot of the aquatic moss Fontinalis antipyretica at one of the sites we visit and it proved too tempting a subject for my new underwater camera. However, the real surprise came when I put a couple of leaves under the microscope on my return. I thought I should check that this really was F. antipyretica and not the related F. squamosa, and this necessitated a quick check of the leaf structure. What I saw when I looked down was that every leaf had several tiny rotifers attached to the surface, busily whirring their cilia to suck food particles into their gullets.


Fontinalis antipyretica in the River Ehen, March 2014.
Rotifers really are beautiful organisms to watch under the microscope. The ones I was looking at were mostly about 0.1-0.2 mm long (it is hard to give exact dimensions as their shapes were constantly changing). They were attached by their “foot end” to the leaf surface whilst the other end bears a simple mouth surrounded by a ring of cilia. These beat in synchronous waves to sweep food particles into the mouth; the impression to the viewer is of a rotating ring though, in fact, the cilia remain stationary. The rotifers I was looking at, Bdelloidea, actually possess two of these rotating “wheels” of cilia.
I wanted to take some photographs, and even videos, of these rotifers, because the visual effect of these beating rings of cilia is quite mesmerising. However, the rotifers were constantly in motion, moving through three dimensions making it impossible to keep the wheels in focus for long enough. There are ways of slowing rotifers down but all require experimentation to get it right. I tried adding alcohol and then some gum arabic, which should have made the water just viscous enough to slow down the cilia. Neither worked for me but it is probably just a matter of time and experience to get the concentrations and exposure times right. There is also a commercial preparation, ProtoSlow, which has much the same effect. In the end, I resorted to my pencils and paintbrushes though this does not really capture the full glory of the spinning wheels of cilia around the rotifer’s mouth.


A sketch of a Bdelloid rotifer feeding on the leaf of Fontinalis antipyretica in the River Ehen, March 2014. The main picture shows the rotifer in an upright position, extracting food particles from the water; the smaller picture, bottom right, shows the rotifer bent over to “hoover” up diatoms and other algae from the leaf surface. The scale bar is 25 micrometres (1/40th of a millimetre).

The presence of so many rotifers tells me a little more about the story of the River Ehen. So many organisms adapted to capturing tiny food particles from the water flowing past must be a sign that there is a plentiful supply of such particles. Perhaps these are washed out of the sediment, perhaps from the surrounding catchment. I suspect, too, that the rotifers are not the only bugs feeding on these particles, and that concentrations will be highest just after spates (which will flush much more of these particles into the river). There was a spate just a few days before we visited. And, I suspect, these particles also act as micro “compost heaps” for the algae which are my main interest in the river. This might partly explain the conundrum of why so many algae are growing in a river that apparently contains so few nutrients.

Lemanea in the River Ehen

The rocks in the fastest-flowing sections of the lowermost of our four sample sites on the River Ehen were all smothered with the coarse filaments of Lemanea fluviatilis. Lemanea is another red alga (see “The schizophrenic life of red algae …”) but one that grows to a much larger size than Audouinella which I wrote about back in early February. I wrote about Lemanea last year (“The River Ehen in April”) but that was before I had an underwater camera. However, most of the Lemanea is attached to large, stable boulders located in sections of the river where the fast current made it almost impossible to photograph safely. Instead, I hunted around and found a smaller stone that was wedged in amongst these, and moved this to a shallow area where it was easier to photograph.


Lemanea fluviatilis from the River Ehen in March 2014. Scale bar: one centimetre.
If you look closely you will see that each of the filaments has a series of nodes along its length. Under the microscope, these nodes form darker patches, composed of smaller cells than the rest of the filament. These are, in fact, the reproductive structures, spermatangia, of the plant as Lemanea has a similar life-cycle to that of Audouniella, which I described in my earlier post. There is also a closely-related genus, Paralemanea, which looks like Lemanea but which has these spermatangia in rings rather than in patches. Older books do not recognise the distinction between Lemanea and Paralemanea.


Lemanea fluviatilis from the River Ehen, March 2014. a. low-power image showing the knobbly stems; b. close-up of a single stem showing the spermatangia patches associated with these protruberences (scale bar: 20 micrometres; 1/50th of a millimetre); c. close-up of a patch of spermatangia.
Lemanea is, in my experience, a very useful indicator of good quality aquatic ecosystems. Looking back through my own records, I see 88 per cent are associated with “high status” or “good status” conditions and the few instances where it is found associated with poorer quality conditions, it is always quite sparse. There is a site quite close to Durham where we used to find Lemanea despite the water being quite enriched with nutrients: though low down in the catchment there was an extensive riffle area with fast currents and, I suspect, few other organisms able to compete for Lemanea’s favourite conditions. Remember, too, that there are enormous reserves of Lemanea in the upper catchment of the River Wear, and plenty of opportunities for this to be scoured off boulders and carried downstream. The wise ecologist always works on “balance of evidence”, rather than making categorical judgements on the presence or absence of a single organism. There is, simply, too much that we still don’t know about the biology of these species.