Spheres of influence

Back to Moss Dub for this post because Chris Carter has sent me some stunning images of the filamentous desmid Desmidium grevillei that I talked about in my earlier post.   I mentioned that it is surrounded by a mucilaginous sheath, which was just apparent in my brightfield image.   Chris has added Indian ink to the wet mount.  The ink forms a dense suspension in the water but is repelled by the mucilage around the desmid cells, resulting in a much better impression of the extent of the sheath around the cell than is otherwise possible.

Desmidium-grevillei_CCarter_#1_Sept19

Desmidium grevillei from Moss Dub, photographed by Chris Carter using Indian ink to highlight the mucilage sheath around the cells. 

Indian ink is a negative stain, which means that it is the background, rather than the specimen itself, which takes up the colour.   This, in turn, alters the passage of light through the sample and appears to improve the contrast of the final image.   Chris’ images of the apical view show this well, and also illustrates the complicated three-dimensional arrangement of the chloroplasts within each semi-cell.   His photographs also show the pores through which the mucilage is secreted.

The curious thing about this negative stain is that, whilst it appears to emphasis a halo of nothingness around the Desmidium filament, it is actually drawing our attention to something important.   In his presidential address to the British Phycological Society in 1981 A.D. Boney referred to mucilage as “the ubiquitous algal attribute” and goes on to list the many functions that the slimes produced by a wide range of algal groups may perform.  Not all will apply to our Desmidium but Boney does use desmids as examples of some of the roles slime may play: it can be, for example, a buoyancy aid, keeping the desmids in the well-lit regions of a lake or pond and it can protect cells against desiccation if a pond or lake dries out.  It may also play a role in helping desmids adhere to their substrates and there is also evidence that mucilage layers may help to protect algae from toxins.

Desmidium-grevillei_apical_view_CCarter_Sept19

Apical view (at four different focal planes) of Desmidium grevillei from Moss Dub, photographed by Chris Carter, September 2019.

But that’s only part of the story.   There is two-way traffic across the membranes of algal cells, with essential nutrients moving into the cell but, in some cases, enzymes moving in the opposite direction.  If nutrients are in short supply then these enzymes can help the cell by breaking down organic molecules in order to release nutrients that can then be absorbed. Those enzymes take energy to manufacture, and the sheath of gunk around the filament means that there is a lower chance of them diffusing away before doing their job (see “Life in the colonies …”).   The same principle applies to sexual reproduction too, with mucilage serving, in some cases, as “sperm traps” or simply as the phycological equivalent of KY Jelly.

It is not just the algae that benefit from this mucilage: the outer layers, especially, can be colonised by bacteria which will also be hoovering up any spare organic molecules for their own benefit with, no doubt, some collateral benefits for the organisms around them.  The connection is probably too tenuous to count as a symbiosis with the desmids but we could think in terms of mutual benefits.

So that “nothing” really is a “something”, and that is before we consider the role of these extracellular compounds in the wider ecosystem.  I mentioned the role of similar compounds in consolidating the fine sediments on coastal mudflats in “In the shadow of the Venerable Bede” to give a flavour of this.   The least prepossessing aspect of the least prepossessing plants can, given time, change landscapes.  That should give us all pause for thought.

Desmidium-grev_apical_pore_CCarater

Close-up of Desmidium grevillei filament with focus on the left-hand cell adjusted to show the apical pores.   Photographed by Chris Carter from material from Moss Dub collected in September 2019.

Reference

Boney, A.D. (1981). Mucilage: the ubiquitous algal attribute.  British Phycological Journal 16: 115-132.

Domozych, D. S., & Domozych, C. R. (2008). Desmids and biofilms of freshwater wetlands: Development and microarchitecture. Microbial Ecology https://doi.org/10.1007/s00248-007-9253-y

Sorentino, C. (1985). Copper resistance in Hormidium fluitans (Gay) Heering (Ulotrichaceae, Chlorophyceae). Phycologia 24: 366-388. https://doi.org/10.2216/i0031-8884-24-3-366.1

 

The little tarn of horrors …

In addition to desmids, we found several other algae in the samples collected from Cogra Moss.  One of these consisted of colonies of cells in mucilaginous masses attached to floating mats of vegetation (which looked like terrestrial grasses).  We decided that these were probably Chrysocapsa epiphytica, the second representative of the Chrysophyta I’ve described in this blog this year (see also “Fade to grey …”).  As is the case for Chromulina, much of what we know about Chrysocapsa epiphytica is down to the patient work of John Lund who first described this species back in 1949.

Chrysocapsa_epiphytica

Colonies of Chrysocapsa epiphytica growing on submerged vegetation at Cogra Moss, Cumbria, September 2019.  Cells are 7.5 – 15 micrometres long and 7.5 – 12 micrometres wide. 

He described the various mucilaginous lobes as “reminiscent of the …. human brain”.  The spherical, oval or ovoid cells form a layer, two to four cells deep, at the surface of the colony.   The cells have the typical yellow-brown colour of chrysophytes and, though it is hard to see the chloroplasts in this photograph, John Lund says that there are usually two, sometimes four, in mature cells.

Its presence in a soft-water lake probably means that it is a species that relies on dissolved carbon dioxide rather than bicarbonate as its raw material for phytosynthesis (see “Concentrating on carbon …” for some background on this).   We know, from laboratory studies, that most chrysophytes rely exclusively on carbon dioxide, and lack the capacity to use bicarbonate.  This confines them to water where the pH is low enough to ensure a supply of carbon dioxide (the chemistry behind this is explained in “Buffers for duffers”. It may also explain why Chromulina lives in surface films rather than submerged in the pond (the locations where we’ve it found are unlikely to have sufficiently low pH).

One extra twist to the story is that many chrysophytes are “mixotrophic”, meaning that they can switch between using photosynthesis as a means of getting the carbon they need to grow from inorganic sources, and “feeding” on other organisms.  Our Chrysocapsa epiphytica, in other words,  has parked itself beside a convenient supermarket of pre-packaged carbon in the form of decaying vegetation and associated bacteria which it then ingests by a process known as “phagotrophy”.

Phagotrophy is, in fact, a very ancient characteristic, insofar as the very first eukaryotic cells were the result of Cyanobacteria-type cells being ingested by larger heterotrophic cells and being retained as on-board “energy farms” rather than digested and treated as one-off vegetarian dinners.   However, the shift to a permanent role for chloroplasts within a eukaryotic cell involved a lot of rewiring of intercellular machinery, and effectively “switching off” the intercellular mechanisms involved in phagotrophy.   Retaining the ability to “feed” on bacteria alongside a capacity for photosynthesis is the cellular equivalent of a hybrid car: there is a lot more to cram under the bonnet.  Flexibility, in other words, comes at a cost.

On the other hand, phagotrophy does not just result in extra carbon for the Chrysocapsa cells in Cogra Moss.   In an oligotrophic tarn such as this, the extra nutrients that are obtained when the bacteria are absorbed will also be a useful boost.   Once again, though, you can see that, in environments where nutrients are more plentiful, the cost to the cell of maintaining the equipment required for phagotrophy outweighs the benefits.

I’m sure that a close inspection of the land around Cogra Moss would have revealed insectivorous plants such as Drosera(sundew) and we also recorded Utricularia minor, an aquatic insectivorous plant, in another tarn we visited whilst desmid-hunting (see “Lessons from School Knott Tarn”).  Chrysocapsa is, in many senses, a microscopic equivalent of these carnivorous plants.   OK, so it has a taste for bacteria rather than flesh but, somewhere out there, there must be a sub-editor in search of a headline …

References

Lund, J.W.G. (1949). New or rare British Chrysophyceae. 1.  New Phytologist48: 453-460.

Maberly, S. C., Ball, L. A., Raven, J. A., & Sültemeyer, D. (2009). Inorganic carbon acquisition by chrysophytes. Journal of Phycology 45: 1052-1061. https://doi.org/10.1111/j.1529-8817.2009.00734.x

Raven, J. A. (1997). Phagotrophy in phototrophs. Limnology and Oceanography 42: 198-205. https://doi.org/10.4319/lo.1997.42.1.0198

Saxby-Rouen, K. J., Leadbeater, B. S. C., & Reynolds, C. S. (1997). The growth response of Synura petersenii(Synurophyceae) to photon flux density, temperature, and pH. Phycologia 26: 233-243. https://doi.org/10.2216/i0031-8884-36-3-233.1

Saxby-Rouen, K. J., Leadbeater, B. S. C., & Reynolds, C. S. (1998). The relationship between the growth of Synura petersenii (Synurophyceae) and components of the dissolved inorganic carbon system. Phycologia 37: 467-477.  https://doi.org/10.2216/i0031-8884-37-6-467.1

Terrado, R., Pasulka, A. L., Lie, A. A. Y., Orphan, V. J., Heidelberg, K. B., & Caron, D. A. (2017). Autotrophic and heterotrophic acquisition of carbon and nitrogen by a mixotrophic chrysophyte established through stable isotope analysis. ISME Journal. https://doi.org/10.1038/ismej.2017.68

 

The desmid dilemma …

Cogra_Moss_Sep19_JJohn

The second location we investigated during the Quekett Microscopy Club / British Phycological Society weekend was Cogra Moss, a small reservoir about four kilometres north of Ennerdale Water.  It is also  about a kilometre or so west of Lampleugh Green where I was staring mournfully at my flat tire whilst the advance party, unaware of my predicament, was out collecting samples.  They must have missed me by a matter of minutes.

As at Moss Dub they found some promising locations around the margins and, in the small tarn at the north-east corner, some patches of Sphagnum from which desmids could be squeezed, plus some floating vegetation.   Once again, I’ve illustrated some of those that we encountered, with a warning that this is a limited selection of the more photogenic ones and we’ve sent samples off to David Williamson for a more comprehensive analysis.   And, once again, the sheer diversity of desmids in the sample is a source of wonderment.   How can one small lake support so many variations on a one type of alga?  I’ve speculated on such issues in the past, drawing on G.E. Hutchinson’s “Paradox of the Plankton” (see “Baffled by the benthos (1)”).   In that post I suggested that it is partly a matter of scale and perception and, in this case, I suspect that the desmids we see in a Sphagnum squeezing are adapted to a wide range of microhabitats.  That means that the desmids would have had a three-dimensional arrangement within the Sphagnum whilst it is in situ but this is lost when we drag a handful of moss from the lake margin and squeeze it into a pot.

Cogra_Moss_desmids_#1

Desmids from Cogra Moss: a. Eurastrum crassum (length: 140 – 180 micrometres; width: 75 – 92 micrometres); Netrium digitus (length: 130 – 390 micrometres; width: 40 – 82 micrometres); Closterium kuetzingii (length: 300 – 550 micrometres) and Pleurotaenium trabecula (length: 277 – 600 micrometres; 22 -46 micrometres).  The photograph of Cogra Moss at the top of the post is by Judy Johns.

Cogra_Moss_desmids_#2

More desmids (and other algae) from Cogra Moss: e. Micrasterias thomasiana (length: 200 – 288 micrometres; breadth: 170 – 269 micrometres); Tetmemonus laevis(length: 67. 5 – 123 micrometres; breadth: 20 – 31.5 micrometres);
g. Schroederia setigera (85 – 200 micrometres long); h. Gonatozygon monotaenium (length: 90-327 micrometres; width: 6.2-12.5 micrometres); i. Staurodesmus extensus (width: 42-50 micrometres, including spines); j. Cylindrocystis gracile (length: 20 – 80 micrometres; width: 11 – 15 micrometres).

But, coming at this issue of desmid diversity from another direction, the term “desmid” is about as particular as the term “mammal”, insofar all belong to the same Class.   In “The big pictures …” I described how desmids were related to other green algae (acknowledging, in the process, that the term “green algae” is, itself, outdated).   This listed five separate families of desmids: four in the order Desmidales and one in the Zygnemetales (I’ve listed the examples from this and the previous post in the table below).   Think laterally and translate this level of organisation to the landscape around Cogra Moss and Ennerdale: the forests contain red squirrels (Rodentia), foxes (Carnivora) and deer (Artiodactlya) and there are otters in the River Ehen (another Carnivora but in the family Mustelidae rather than Canidae).   If we can appreciate how different mammals can interact within a landscape, then we should be able to apply the same principles on a much finer scale to organisms that are five orders of magnitude smaller.   It’s the principle behind fractals, but applied to biological  diversity rather than to geometry.

Earlier in the year, I published a paper with two colleagues that tried to explain how the way we study the microbial world can shape and, in many cases, impede our understanding (it’s open-access, so click on the link below if you want to read it).  We illustrated this with pictures that tried to demonstrate how microscopic algae interact with other organisms.  These included host plants, in the case of epiphytic algae, but also the protozoans that feed on them.  Most of our examples were diatoms, and there was a reasonable literature on which we could draw.  Curiously, I’ve never come across papers that provide this contextual information for desmids. Perhaps I just don’t look in the right places.   If it is out there and I’ve missed it, please do let me know.

Reference

Kelly, M. G., King, L., & Yallop, M. L. (2019). As trees walking: the pros and cons of partial sight in the analysis of stream biofilms. Plant Ecology and Evolution152: 120-130.

Organisation of the class Conjugatophyceae with examples encountered in Moss Dub and Cogra Moss.

Order / Family Examples
Desmidales  
     Closteriaceae Closterium
     Desmidaceae Desmidium, Euastrum, Pleurotaenium, Staurodesmus, Tememorus
     Gonzatozygaceae Gonatozygon
     Peniaceae No examples in these posts, but see “Desmid Diversity” for illustrations of representatives in Kelly Hall and Long Moss tarns.
Zygnemetales  
     Mesotaeniaceae Cylindrocystis
     Zygnemetaceae Not desmids: Filamentous algae including Mougeotia, Spirogyra and Zygnema – examples from Ennerdale area are described in several other posts,

Desmids from Moss Dub

Moss_Dub_Sep19_ZHenderson

I’d like to say that this post is about an excursion I made beyond Ennerdale Water and along the valley of the River Liza in order to find some different habitats from those that I usually write about in this blog.  I’d like to but, in truth, I was sitting by a road about ten miles away waiting for roadside assistance whilst my compatriots on the Quekett Microscopy Club / British Phycological Society algae weekend went up the valley on a glorious mid-September afternoon without me.  Whilst I was sitting waiting for a tyre to be replaced in a garage in Egremont they were casting plankton nets and squeezing handfuls of Sphagnum beside Moss Dub, a small tarn set amidst woodland close by the River Liza.

Moss Dub is set within one of Britain’s oldest and most ambitious rewilding schemes, Wild Ennerdale, where nature is allowed to shape the landscape as far as possible free from human interference.  However, Moss Dub, as we found out, is far from a natural water body.  A path forks and the two arms act as bunds encompassing a shallow pond, now partly overgrown with aquatic vegetation.   There is evidence of past mining activity – for iron and copper – in the area and my guess is that Moss Dub was, in the far past, a reservoir associated with the Lingmell mine located on the hillside above the River Liza and active in the late 19thcentury.     Whatever its history, it proved to be a rich location for desmids, and we spent a happy Saturday dipping Pasteur pipettes into the vials of peaty water that they collected and peering through our microscopes (If you want to know more about how to collect desmids, look at the post I wrote after our last excursion to the Lake District: “Desmid masterclass”).

There were some conspicuous green growths suspended in the water at the margin of the pond.  Even without a microscope, their filamentous nature was obvious.  When magnified, we saw chains of green cells set within a distinct mucilaginous sheath.  Each filament was composed of short cells with a distinct notch on either side.   This is a representative of Desmidium, one of a relatively small number of filamentous desmids.  We met D. schwartziion our previous excursion (see “Lessons from School Knott Tarn”); that species was present here along with D. grevillei, which is similar in many respects but the cross-section is lemon-shaped rather than triangular.

Desmidium_grevillei_MossDub

Desmidium grevillei from Moss Dub, Ennerdale Valley, September 2019.  a. shows a macroscopic view of filaments in a Petri dish; b. shows a filament of cells, along with a distinct mucilaginous sheath whilst c. shows a cell in cross-section.  I forgot to bring my graticule so cannot add scale bars to any of the images in this post.   Instead, I will quote dimensions from the Freshwater Algal Flora of Britain and Ireland to give an indication of size.   Cells of D. grevillei are 30 – 56 micrometres wide (50 micrometres is 1/20thof a millimetre).   The photo at the top of this post is a view of Moss Dub, taken by Zeneb Henderson

There were numerous other desmids in the sample.  A couple are illustrated below, and we’ve sent the sample off to David Williamson for a more thorough examination, and some definitive names.  On the right-hand side of the plate there is a different green alga, Coelastrum pulchrum, a member of the Chlorophyceae that forms spherical colonies with a fixed number of cells (“coenobia”).  We met Coelastrum microporum in the River Wear last summer (see “More green algae from the River Wear”): cells of C. pulchrum, by contrast, have a blunt projection.

Moss_Dub_algae

More algae from Moss Dub: d. Micrasterias radiosa (142 – 191 micrometres across); e. Euastrum pinnatum (65-75 micrometres across; 125 – 170 long); f. Coelastrum pulchrum (about 100 micrometres in diameter). 

The final desmid I’ve illustrated is Closterium lunula, large by desmid standards as it can reach half a millimetre or more in length.  Members of this genus have prominent vacuoles at each end of the cell within which small crystals can be seen.  Because C. lunula is so large it is easy to see both vacuole and watch Brownian motion move the crystals within.   Studies have shown that these are crystals of barium sulphate and also that the crystals are scattered throughout the cells, just happening to be easier to see I the vacuoles.  Quite what role they play remains speculation: barium is not required for plant nutrition and is, indeed, toxic in high concentrations.   It is also scarce in the soft waters where Closteriumis most often encountered, both in absolute terms and relative to other trace metals, which only adds further to the mystery.

That’s enough about Moss Dub for now.  A few words about Ennerdale Bridge, where we were based before I sign off from this post.  I usually stay at the Shepherd’s Arms when I am in the area and Keith and his staff hosted most of us and fed all of us.  It is a comfortable, unprententious inn, living mostly off walkers doing the Coast-to-Coast walk and with a menu that managed to put a smile on the faces of vegetarians and non-vegetarians alike.  Our daytime events took place in the community room of The Gather, a community-owned and run café and shop,  That gave us the satisfaction of knowing that the money we paid for the room was going to good use.   Their coffee keeps me going during long days of fieldwork in the area so I’m keen to make sure that they thrive!

Closterium_lunum

Closterium lunula (400 – 663 micrometres long) from Moss Dub, showing the terminal vacuole (ringed) and (below) a close up showing rectangular crystals of barium sulphate inside the vacuole.

Reference

Brook, A. J., Fotheringham, A., Bradly, J., & Jenkins, A. (1980). Barium accumulation by desmids of the genus Closterium (Zygnemaphyceae). British Phycological Journal 15: 261-264. https://doi.org/10.1080/00071618000650251

Microscopy_at_the_Gather

Quekett Microscopy Club and British Phycological Society members getting stuck into analysis of samples from Moss Dub and the Ennerdale valley at The Gather, Ennerdale Bridge, September 2019.

Close to the edge in Wastwater …

Wastwater_190610

I’m back in the Lake District for this post, standing beside Wastwater, the most remote and least disturbed of England’s lakes and, especially obvious on a sunny day in June, the most spectacularly-situated.  I stood on the western shore looking across to the screes and, beyond to the mass of Scafell Pike, England’s highest peak, looming up in the distance.

When I was done admiring the scenery I adjusted my focus to the biology of the lake’s littoral zone and some dark brown – almost black – marks on the boulders in the littoral zone.  In contrast to the grand vista stretching away to the north, these were beyond unprepossessing and my attempts to photograph them yielded nothing worth including in this post. However, I had seen similar looking marks in Ennerdale Water and there is a photograph in “Tales from the splash zone …” that should give you some idea of what I was seeing.

Under the microscope, my expectations were confirmed.  As in Ennerdale Water, these patches were composed of Cyanobacteria – gradually tapering trichomes of Calothrix fusca and more robust trichomes of Scytonema calcareum, both encased in thick, brown sheaths which, when viewed against the granite boulders on which they lived, resulted in the dark appearance of the growths.  To the untrained eye, these barely look like lifeforms, let alone plants yet they offer an important lesson about the health of Wastwater.

Calothrix_fusca_Wastwater_June19

Calothrix cf fusca from the littoral zone of Wastwater, June 2019. Scale bar: 20 micrometres (= 1/50thof a millimetre)

Though hard to see amidst the tangle of filaments in these population, both Calothrix and Scytonema have specialised cells called “heterocysts” that are capable of capturing atmospheric nitrogen (you can see these in the photographs of Nostoc commune in “How to make an ecosystem (2)”.   Nitrogen fixation is a troublesome business for cells as they need a lot of energy to break down the strong bonds that bind the atoms in atmospheric nitrogen together.   That means that plants only invest this energy in nitrogen fixation when absolutely necessary – when the lack of nitrogen is inhibiting an opportunity to grow, for example.   The presence of these Cyanobacteria in Wastwater is, therefore, telling us that nitrogen is scarce in this lake.

The dogma until recently was that phosphorus was the nutrient that was in shortest supply in lakes, so attention has largely focussed on reducing phosphorus concentrations in order to improve lake health.   Over the last ten years, however, evidence has gradually accumulated to show that nitrogen can also be limiting under some conditions.   That, in turn, means that those responsible for the health of our freshwaters should be looking at the nitrogen, as well as the phosphorus, concentration and, I’m pleased to say, UK’s environmental regulators have now proposed nitrogen standards for lakes.   That marks an important shift in attitude as, a few years ago, DEFRA were quite hostile to any suggestion that nitrogen concentrations in freshwaters should be managed.   In this respect, the UK is definitely out step with the rest of Europe, most of whom have nitrogen as well as phosphorus standards for freshwaters.

Scytonema_crustaceum_Wastwater_June16

Scytonema cf calcareum from the littoral zone of Wastwater, June 2019. Note the single and double false branches.   Scale bar: 20 micrometres (= 1/50thof a millimetre)

Wastwater flows into the River Irt and, a few kilometres down from the outflow, I found another nitrogen-fixing Cyanobacterium, Tolypothrix tenuis.  Once again, I could not get a good photograph, but you can see images of this in an earlier post from the River Ehen in “River Ehen … again”.   Nitrogen fixing organisms, in other words, are not confined to the lakes in this region, which raises the question why the UK does not have nitrogen standards for these as well (see “This is not a nitrate standard …”).   In rivers such as the Irt and Ehen that are already in good condition, it might only take a small increase in nitrogen concentration for the ecology to change.   Whether the loss of these nitrogen-fixing organisms will be noticed is another question.

For now, I am just happy to see that nitrogen in lakes has finally made it to the regulatory agenda.  It has taken about 15 years for the science to percolate through the many layers of bureaucracy that are an inevitable part of environmental management.  Give it another decade and maybe we’ll get nitrogen standards for rivers too.

References

Maberly, S. C., King, L., Dent, M. M., Jones, R. I., & Gibson, C. E. (2002). Nutrient limitation of phytoplankton and periphyton growth in upland lakes. Freshwater Biology. https://doi.org/10.1046/j.1365-2427.2002.00962.x

Moss, B., Jeppesen, E., Søndergaard, M., Lauridsen, T. L., & Liu, Z. (2013). Nitrogen, macrophytes, shallow lakes and nutrient limitation: Resolution of a current controversy? Hydrobiologia. https://doi.org/10.1007/s10750-012-1033-0

P.S. any guesses as to which 1970s prog rock group I was listening to over the weekend?  The clue is in the title.

How to make an ecosystem (2)

Ennerdale_Apr19

My most recent visit to Ennerdale and the River Ehen almost did not happen: unexpected overnight snowfall led to my wheels spinning on the Whinlatter Pass before I retraced my steps to Braithwaite and followed roads at lower altitudes around the outskirts of the fells.   Fieldwork in the morning took place amidst intermittent snow showers but, by the afternoon, it was dry if not quite as balmy as the visit I described in “Croasdale Beck in February”.   “Unseasonable”, I was reminded, is a two-edged term.

There was little incentive to linger with my arm in the agonisingly cold water, so this post is about some algae growing on dry land that caught my eye.   Amidst the gravel in a farmyard in Ennerdale Bridge I saw some dark brown leathery growths that I recognised straight away as the Cyanobacterium Nostoc commune (see “Nosing around for blue-green algae …”).  It looks rather nondescript, even slightly unsavoury, with the naked eye but, under the microscope, the rosary-like structure of the filaments suspended within a jelly-like matrix is revealed.  The slightly larger cells with thicker walls and lighter contents are the heterocysts, responsible for fixing nitrogen from the atmosphere (fulfilling the same function as the nodules on the roots of legumes).

Nostoc_commune_Ennerdale_Apr19_#1

A patch of Nostoc communein a farmyard in Ennerdale Bridge in April 2019.   The picture frame covers about 30 centimetres. 

Nostoc_commune_x1000_Apr19_#1

Nostoc communefrom Ennerdale Bridge under the microscope.  Scale bar: 10 micrometres (= 100thof a millimetre). 

This type of coarse, well-drained gravel is a good habitat for Nostoc and, once you know what you are looking for, it is a common sight on gravel driveways, usually to the annoyance of the owners.   If there is only a small amount, the best way to control it is simply to pick up the colonies and toss them onto the compost heap.  However, once it is established, this can be a big undertaking and many people are quite happy to tolerate some of this brown gunk on their driveways.   On the other hand, it can sometimes get out of hand and the consequences of not doing anything are well illustrated by the photograph below.  The Nostoc colonies have spread but these, in turn, have created a habitat into which first mosses and later grasses can establish.

This small farmyard on the edge of the Lake District contains, in short, the first stage of an ecological succession.  We could think of a gravel driveway as a mini-desert, as the copious Cumbrian rainfall will not be retained in the surface layers, making it hard for plants to survive.   However, if a tough organism such as Nostoc is able to establish itself, then this, in turn, will trap water and make the driveway more amenable to slightly more fussy organisms such as mosses.   As the moss and Nostoc grow together so, eventually, grasses are able to establish too.  Were there to be no interruption to this process then, eventually, decades later, we might even see trees growing on this driveway.

It is hard to imagine, but just about every type of terrestrial habitat started out, aeons ago, as a bare rock surface.  Various forms of physical weathering start the process of breaking this up allowing, over time, organisms such as Nostocto get a foothold and convert the virgin surface into a mature ecosystem (you can read about another example in “How to make an ecosystem”).   It may take centuries for this to happen in the natural world, so it is particularly fortuitous to see this human-assisted succession so well developed.  At some stage, I suspect, the owner will decide that enough is enough, and rake the gravel.  Meanwhile, however, we have a rare opportunity to reflect on the role that primitive micro-organisms play in shaping even the grandest of our natural habitats.

Nostoc_commune_Ennerdale_Apr19_#2

A lawn of Nostoc, moss and grass growing on a gravel driveway in Ennerdale Bridge, April 2019.  

Reference

Miles, J. & Walton, D.W.H. (1993).  Primary Succession on Land.  Special Publication of the British Ecological Society 12, Blackwell Scientific Publications, Oxford.

More about measuring biomass …

The previous post showed how the proportions of green algae and diatoms changed as the total quantity of algae in the River Ehen waxed and waned over the course of a year.   The BenthoTorch, however, also measures “blue-green algae” and so let’s look at how this group changes in order to complete the picture.

Before starting, though, we need to consider one of the major flaws of the BenthoTorch: its algorithms purport to evaluate the quantities of three major groups of algae yet, in my posts about the River Ehen I have also talked about a fourth group, the red algae, or Rhodophyta (most recently in “The only way is up …”).  Having pointed a BenthoTorch at numerous stones with thick growths of Audouinella,we can report that Rhodophyta seem to be bundled in with the blue-green alga signal, which is no great surprise given the similarity in their pigments.  It is, however, one of a number of examples of the need to interpret any BenthoTorch results with your brain fully engaged, and not just to treat outputs at face value. Similar questions need to be asked of the Xanthophyta and Chrysophyta, though the latter tend not to be common in UK streams.

cyanos_in_Ehen

Relationship between the proportion of “blue-green algae” (Cyanobacteria and Rhodophyta) and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (c.) and Croasdale Beck (d.).  The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.  

In contrast to the green algae and diatoms, the Cyanobacteria/Rhodophyta signal shows a strong negative relationship as biomass increases though, again, there is enough scatter in this relationship to make it necessary to approach this graph with caution.  I suspect, for example, that the data points on the upper right side of the data cloud represents samples rich in Audouinella, which tends to occur in winter when biomass, generally, is much greater.   On the other hand, Croasdale Beck, in particular, has a lot of encrusting Chamaesiphon fuscus colonies which are pretty much perennial (see “a bigger splash …”) but whose relative importance in the BenthoTorch output will be greatest when the other two groups of algae are sparse.   I suspect that encrusting members of this genus are favoured by conditions that do not allow a high biomass of other algae to develop, as these will reduce the amount of light that the Chamaesiphonreceives.

Thicker biofilms in the River Ehen often have some narrow Phormidium-type filaments as well as small bundles of nitrogen-fixing Calothrix, but the overall proportion is generally low relative to the mass of diatoms and green algae that predominate.    But that is not really telling us the whole story.  I finished my previous post with a graph showing how the variation in biomass increases as the biomass increases.  The heterogeneity of stream algal communities, however, cannot be captured fully at the spatial scale at which the BenthoTorch works: there is a patchiness that is apparent to the naked eye: one of our sites has distinct mats of Phormidium autumnale towards one margin, and dense Lemaneagrowths in the fastest-flowing sections, largely attached to unmovable boulders, which makes biomass measurement very difficult. I’ve also written about distinct growths of Tolypothrix and its epiphytes (see “River Ehen … again”), another alga which forms discrete colonies at a few locations. I try to collect a random sample of stones from a site but there are constraints, including accessibility, especially when the river rises above base flow.   In the River Ehen we also have to take care not to disturb any mussels whilst removing stones.

Whilst our sampling cannot really be described as “random” I do think that there is sufficient consistency in the patterns we see for the results to be meaningful. We could spend a lot more time finessing the sampling design yet for little extra scientific gain.   I prefer to think of these measurements as one part of a complex jigsaw that is slowly revealing the interactions between the constituents of the dynamic ecosystem of the River Ehen.   The important thing is to not place too much faith in any single strand of evidence, and to have enough awareness of the broader biology of the stream to read beyond the face value indications.