A very hungry chironomid …

One of the features of the upper River Ehen is thick brown growths, visible to the naked eye on the submerged stones.   Diatoms often just form part of a homogeneous, slimy layer on the top of stones and it is unusual to see growths that are so distinctive. Under the microscope, these resolve into masses of diatoms – mostly the mass of Gomphonema on their long stalks and zig-zag filaments of Tabellaria that we have already met but, as you can see in the photograph below, they are not always covered by green algae such as Spirogyra (see post).   Perhaps – and this is just a hunch – this site has heavier shade from the surrounding trees which favours the diatoms over the green algae?

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The left hand image shows a diatom-covered stone in the upper River Ehen, February 2013; the right-hand image is a low-power view of one of these growths under the microscope showing the mass of diatoms (mostly Gomphonema and Tabellaria).  The scale bar is 50 micrometres (1/20th of a millimetre).

Amongst this dense forest there were a few chironomid larvae, greedily feeding amongst the diatom stalks.  These are the young stages of the non-biting midges often seen swarming close to rivers on summer evenings.   I don’t know if these cold-blooded larvae would have been so vigorous in the freezing cold water of the Ehen but as soon as they were exposed to the warmth from the microscope lamp they became surprisingly active, grabbing the mass of stalks with the finely-hooked “toes” on their front prolegs and using their mandibles to shovel the diatoms into their mouths.

http://youtu.be/Fni7HhTPFco

To give you some idea of scale, the head of the larva is about 100 micrometres (1/10th of a millimetre) across, and the whole organism is between two or three millimetres long.   The video was taken at 100x magnification; none of my videos taken at higher magnifications were successful due to the shallow depth of field.  However, you can still just about make out the motion of the mandibles, the larva’s jaws, which, unlike vertebrate jaws, work from side to side rather than up and down.

I have made sporadic attempts to incorporate grazing animals into my pictures but have not yet produced an image that I am happy with.  It is partly a problem of my own unfamiliarity with insect mouthparts, which makes portraying them in a “lifelike” manner very difficult, and partly simply a matter of scale: the invertebrates, though still only a few millimetres in length, are enormous in comparison to the algae that have generally formed the foreground of my art.   It is the equivalent of trying to incorporate both a blade of grass and a cow into the same photograph.   But I need to persist.   Aquatic ecology too often seems to default to lists of species, whether algae, invertebrates, higher plants or fish.   We can get a good sense from such lists of the health of an ecosystem, but we never get a complete picture.  Today I was watching the ecosystem functioning: energy, originally from the sun, passing from one trophic level (the algae) to another (the invertebrates).  In due course, this chironomid might itself be eaten by a trout, or die and decompose, letting bacteria and fungi feast on the energy it contained.   The “nouns” that make up an ecosystem are easier to describe, but it is the “verbs” that drive it along and it is good to be able to peer down my microscope and be reminded of this.

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The River Ehen in March

There were flurries of snow as I pulled on my waders beside the River Ehen last week.  This is, somewhat counter-intuitively, a good omen for fieldwork.   Rain flows off the land and into the river whereas snow stays where it falls, at least until the thaw comes.  The water was cold – so cold that I could barely decipher the notes that I made with my frozen hands – but it is high flows that are the real problem when working in rivers.

The algae living in the Ehen were much the same as on the last visit though my first impression is that the overall quantities were lower.  In particular, I was fairly sure that the gelatinous growths of Draparnaldia (see earlier post) were less prolific.  They looked different under the microscope too, with the filaments surrounded by swarms of tiny pear-shaped cells, all moving around.  Each has two flagella for motility though these are too small to be visible here.

These tiny cells are zoospores, produced by Draparnaldia as a means of dispersal.  I had not looked at the sample immediately on my return and the following day it had sat in a plastic bag in my fridge whilst I got on with other jobs.  So almost two days elapsed between removing the alga from the stream and looking at it under the microscope.  My experience is that Draparnaldia and relatives often form zoospores when subjected to such indignities and it gives us a useful insight into the biology of these algae.

I’ve previously I mentioned that algae such as Spirogyra rarely form reproductive organs in the field, which makes it hard to identify the exact species.  Those of us who study freshwater algae accept this as a fact of life but it is at odds with much of what is written about the advantages of sexual reproduction to organisms (Matt Ridley’s otherwise excellent book The Red Queen is a case in point).   The pessimistic view is that the best you can get from sex is a 50 per cent dilution of your DNA.  So if all is good in your world, invest your energy into adding more biomass to what you already have.   This is especially the case for an alga stuck to a rock in a fast-flowing stream.   The zoospores I found were the alga responding to an unfavourable environment (my fridge) by emigrating. Or, at least, trying too.  A tiny minority of these, in the wild, would settle on a surface and grow into new filaments genetically identical to the parent.   True sexual reproduction does happen in Draparnaldia but, as is the case for Spirogyra, it is rare.

If you do want to see sexual reproduction in Spirogyra, the best thing to do is let a sample dry out slowly.   In other words, simulate the worst possible conditions that the alga would expect to encounter. Why? Because the outcome of conjugation is a resistant zygospore that can sit out the hard times, waiting until the environment is again suitable to produce a new filament.  Curiously, science writers such as Ridley don’t hint at this as one possible advantage of sexual reproduction.  This is doubly curious because, as algae are amongst the most primitive of organisms, these are the situations where sex must have first evolved.  We view sexual reproduction from the opposite end of the evolutionary scale and, thanks to Petrarch, Shakespeare, Freud and others, give it far more prominence than perhaps should be the case.

The amazing jelly-pit of doom ….

My recent conversion to the delights of YouTube has encouraged me to dig out another video clip from my iPhone and to upload it.   This clip was filmed last August, after I had a call from the Highways Department at Durham County Council about a mystery alga that was growing prolifically on a kerbside in Pittington, a village a few kilometres away from me.   A utility company had dug a pit to work on some pipes beside this road and, when they had finished, filled it in again.   Somehow, perhaps as a result of the disturbance, a mucilage-producing alga started to grow and the result was a quaking mass of soil and jelly.

The video clip shows the scale of this growth quite clearly.   I found a stick in the hedge with which to poke at the growth and, though it was almost a metre long, still could not feel the bottom.  An unwary pedestrian would have got quite a shock had they had stumbled into this unawares.

Pittington_Schizochlamys

Schizochlamys gelatinosa (probably) from a road verge in Pittington, County Durham, August 2012.  Photograph by Chris Carter.

Under the microscope, the goo resolved itself into small green cells in a mucilaginous matrix.  So far we have not been able to identify this with total confidence.   The most likely candidate is Schizochlamys gelatinosa.  West and Fritsch’s British Freshwater Algae (1927) describes this as forming “very extensive gelatinous masses, often several cms. in diam., in ditches, ponds etc.” which fits with this specimen.    Compare the image above to that in the AlgaeVision collection.   You can also go to AlgaeBase, a very useful website for anyone interested in algae, and find more information about this genus as well as some more images (including an anaglyph – a three dimensional image) of the population shown here.

More from Coxhoe …

A note before I start: the last post (“Coxhoe”, March 15 2013) contained a short YouTube video that did not display properly when viewed as part of an email.  It is better to view the post through a browser.  Following this link will take you straight to it.

There is a series of lagoons along the side of the landfill site, the first of which has a striking milky-white appearance, due to the presence of what I presume to be suspended clay particles.   There is a narrow channel leading into a second pool, where the water is clearer but the clay minerals form a distinct layer on the bottom and, finally, another channel to the third lagoon in the series, again with clearer water.   We dipped a pH probe into each of these and recorded an extraordinarily high value of pH 11.1, dropping to pH 9.2 in the middle pool and pH 8.6 in the final pool.   Most of the surface waters in this part of Durham have a neutral or slightly alkaline pH, with values greater than about 8.5 being rare so these lagoons are very unusual.

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The lagoons at Coxhoe landfill site.   The photograph was taken from the mid-point of the middle lagoon, looking up towards the milky-white (and extremely alkaline) waters of the first lagoon.

The bottom of the middle lagoon was covered with bright green flocs of algae which, when I put them under my microscope, turned out to be Spirogyra, with distinctly-helical chloroplasts.   Interestingly, this is a different species to the one that I found a few kilometres away, in the pond at Cassop (see post) but, once again, I cannot name the species because there are no reproductive organs present.   That it is present in this pool, but absent from the first suggests that it must be able to survive in water up to about pH 9.5: a very high value and one that must put some extreme stresses on the physiology of the organism.  That said, there are a few microorganisms that can thrive at even higher pHs – between 10 and 11 – but not at more normal pH values.

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Spirogyra growing on the bed of the middle lagoon at Coxhoe landfill site.  Note the fine, white, clay substratum. 

Ecologists have a name for these habitats where we topple off the limits of “survival of the fittest” and into the realms of “survival of the specially adapted”.  These “extreme environments”.  They encompass hot springs, soda lakes, highly saline environments and Antarctic deserts but, also, various man-made habitats.   These ponds are extremely alkaline but I can take you to springs associated with abandoned mines in County Durham that are highly acidic or which have very high concentrations of toxic metals.  The sobering fact about this particular “extreme environment” is that this is where all my own domestic waste ends up.  Just to peer into the streams and ponds and make a few simple measurements is enough to see the long-term consequences of consumer culture.     To be fair to Durham County Council, these environments were in lagoons constructed especially to keep the runoff from polluting surrounding streams.  Nonetheless, my visit still served a useful purpose by turning the abstract generalisations that we tend to make about “pollution” into something very local and personal.

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Filaments of Spirogyra from the middle lagoon at Coxhoe landfill site.   Each is about 35 micrometres wide. 

Coxhoe

I spent this afternoon mooching around an old landfill site in Coxhoe, about four kilometres from my house, and a similar distance from the pond at Cassop that I visited in January.   The site lies at the foot of the Magnesian limestone escarpment that runs roughly north-south through County Durham.   Because of this rock’s value as an aggregate in road-building and the construction industry, this part of Durham is pot-marked with quarries and, as the quarries are exhausted, so the holes left behind become landfill sites, slowly filling up with the detritus of twenty-first century consumer culture.  The point where I was standing was a landfill site until about ten years ago, since when it has been capped off and the lorries trundle, instead, to an even larger hole in the ground on the other side of the B6291.

Meanwhile, County Durham’s seemingly incessant rain falls onto the closed landfill site, percolates through the topsoil and then through the layers of waste, gradually becoming polluted as it does so.   Some of this water stays underground as groundwater, a portion appears at the surface, either collecting in lagoons dotted around the edge or forming small springs and seepages.   I was exploring the site this afternoon with two students who were wondering whether the organisms that lived in these habitats would make a good subject for a dissertation.

We followed a tiny stream down from where it collected the overflow from two settlement lagoons down to the point where it joined a natural stream in the valley bottom.   The pebbles on the stream bed here had dark brown, almost black, patches of diatoms that are very common at this time of year.  What was interesting was that these patches of diatoms disappeared as soon as the water from the tiny stream bearing the landfill’s effluent joined the natural stream in the valley bottom.   And these dark patches were not obvious on the stones in the tiny stream either.

Coxhoe_Navicula_growths

Growths of diatoms – principally Navicula lanceolata – growing on submerged pebbles in a small stream draining Crow Trees Local Nature Reserve, Coxhoe, County Durham in March 2013.

These growths are sufficiently common in streams at this time of year that I could make a fairly confident guess about what to expect when I put a small sample under the microscope.  And I was not disappointed: a mass of tiny boat-shaped cells all moving about in irregular patterns across the slide.   The majority belong to a species called Navicula lanceolata, though there were also several belonging to a smaller species, Navicula gregaria.   I find these particularly in rivers in great abundance at this time of year – so long as the water is neither very soft nor acidic, they will grow across a wide range of conditions, from clean to fairly polluted.

Navicula lanceolata (along with a couple of cells of Navicula gregaria) from the stream draining Crow Trees Local Nature Reserve.   The largest cells are about 60 micrometres long.  

I was intrigued by the response of the students when they looked down the microscope.   Perhaps their surprise is the more natural reaction. Having first approached algae via botany courses, I was conditioned to think of these non-green moving organisms as “plants”.  Their brown colour is easy enough to rationalise when you know that they are related to the seaweeds on our coasts.  Their movement is less easy to explain, though all of us are familiar with higher plants growing towards the light.  The movement we could see under the microscope is just another response to a stimulus, albeit in a single-celled, rather than multicellular, organism.  I was actually pleased to be able to share their enthusiasm for a few minutes.  I spend too long staring at computer screens where the vitality of the microscopic world is abstracted to numbers and it is therapeutic to reconnect with the living organisms for a few minutes.

An inordinate fondness for … algae*

Looking back through my records I counted 152 different algae species from my observations and samples from the five kilometre of the River Ehen between the outflow from Ennerdale Water and the village of Ennerdale Bridge.   This might sound like a large number until you look at the relative numbers of algae compared to other types of plant.   Adding together all the flowering plants, native conifers, ferns, mosses and liverworts recorded from Britain and Ireland gives a figure of 2333 species.   By contrast, just under 6000 species of algae have been recorded.   This figure is almost certainly an underestimate for two reasons.  First, some habitats, particularly offshore coastal waters, have not been as thoroughly documented as others.  Second, there has been a recent rapid increase in numbers of records of algae not because people are looking in different places but because our idea of what a “species” of alga is has changed dramatically.

The classic definition of a “species” is a group of organisms capable of interbreeding and producing fertile offspring.  However, sex is hard to observe in many algae, so we assume that organisms capable of interbreeding will look the same, because they are sharing the same genes.   However, this brings human perception into the equation, exacerbated in the case of microscopic organisms by the limitations of optical technology (see earlier post).   Recently, we have discovered that many algae which look almost identical (and which were classified as a single species in the past) are, in fact, genetically-distinct species (see illustration below).

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The diversity of freshwater diatoms.  Each image shows a valve (half a cell wall) that would have been classified as “Sellaphora pupula” in most 20th century diatom Floras but which has subsequently been shown to be a genetically-distinct species in its own right.  Images are from the ADIAC database (http://rbg-web2.rbge.org.uk/ADIAC/db/instruct.htm)

This means that, as well as 6000 species of freshwater algae that we already know about, there is also a category of “known unknowns” – species that will, undoubtedly, need to be split into several distinct entities at some point in the future.   Doubling the figure of 6000 would not be unreasonable.   By contrast, the figure of 2333 higher plants is probably not going to change that much over the next few years, as these organisms have generally been more thoroughly studied and recorded.   This means that algae could represent over 80% of all plant diversity in Britain and Ireland.

But here’s the problem: there is almost no-one doing this work in the UK at the moment.  24 people contributed to the second edition of the Freshwater Algal Flora of the British Isles.  Of these, nine were not from Britain or Ireland, a further ten are retired, and two of the remainder are not currently in research roles.   Whilst most of the retirees are still active, this situation raises yet another spectre: that of knowledge loss.   These are people that could look at a sample and notice subtle differences in an organism that may indicate a new species.  These are people that keep up to date with literature from around the world that might help them match this new species to one described elsewhere.  The book they wrote encapsulates the “known knowns” (to use Donald Rumsfeld’s terminology) but we need active (and funded) researchers to explore the “known unknowns”, let alone the “unknown unknowns”.   Not far behind the “cutting edge” of research is a trailing edge where knowledge is no longer challenged by experience and, I’m afraid, some areas of algal taxonomy in the UK may already fall into that category.

* the title is an adaptation of J.B.S Haldane’s response when asked if anything could be concluded about the creator from the study of creation.  Haldane replied: “an inordinate fondness for beetles.”   Donald Rumsfeld’s quotations come from a press briefing in 2002 about the presence (or otherwise) of weapons of mass destruction in Iraq

More from Upper Teesdale

There are several points along the path beside the River Tees where bogs have developed on the flat area between the base of Falcon Clints and the river. My eyes were drawn to the bright red hummocks of Sphagnum capillifolium partly from a sense of nostalgia as I did my undergraduate dissertation on this moss and got to learn something of the ecology of the many species of Sphagnum that are found in the British uplands. Many of the species have quite distinct habit preferences, with S. capillifolium being a species that can quite happily live on the dryer parts of the bog. These red hummocks therefore represent a good way of crossing a bog without getting your boots too wet.

Teesdale_Sphagnum

A hummock of Sphagnum capillifolium (left) and a handful of Sphagnum cuspidatum (right) from a boggy pool in upper Teesdale.

Today, however, I was more interested in the boggy pools, where, once I had broken the two centimetres of ice with the heel of my boot, I found a different species of Sphagnum, S. cuspidatum, with limp, flaccid stems. I squeezed a handful of stems gently and collected the stream of brown-coloured water into a small sampling vial. Sphagnum is a species that creates its own habitat. The secret lies in the structure of the leaves, whose photosynthetic cells are juxtaposed with empty hyaline cells that, combined with the densely packed stems, allows it to absorb and retain huge quantities of water.

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Leaf cells of Sphagnum cuspidatum showing the slender “chlorophyllose” cells (approximately 10 micrometres, or 1/100th of a millimetre wide) between the dead hyaline cells with their thickened bands of supporting tissue.

Under the microscope, this brown liquid yields a rich assortment of algae, mostly diatoms but also a number of cells which share the symmetry which we associate with diatoms but which are larger and are bright green in colour.  These are the desmids and they are the only group of freshwater algae that come close to rivaling the diatoms in their diversity.   It did not take long to find six distinct species living within the Sphagnum cuspidatum.   Look at their structure: each is a single cell mostly divided into two “semi-cells”, each with a separate chloroplast (there are some exceptions to this rule: Netrium [a. in the figure below) does not have semi-cells and a few have a single chloroplast].  The desmids are closely related to Spirogyra which we met in earlier posts from Cassop and the River Ehen.

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Desmids from a boggy pool in upper Teesdale.   a. Netrium digitus; b. Micrasterias truncata; c) Micrasterias oscitans (var. oscitans); d) Closterium intermedium/striolatum; e) Eurastrum insigne forma; e) Eurastrum didelta.  The scale bar is 50 micrometres (1/20th of a millimetre) long.

You can find a lot of information about desmids on the internet.  Two sources that are worth exploring are http://www.desmids.nl/index.html which focuses on desmids from the Netherlands but is written in English and has many useful links and http://rbg-web2.rbge.org.uk/DIADIST/ww_intro.htm which has a facsimile of W. and G.S. West’s five volume Flora of British desmids published between 1904 and 1923 which is well worth a look.