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 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.

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.

Viridiplantae_organisation

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.

Conjugatophyceae_orders

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 …

References

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.

Appendix

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
Chlorophyta  
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 …
Charophyta  
Charophyceaee Life in the deep zone …
Conjugatophyceae Desmidiales: Desmid diversity

Zygnemetales: Fifty shades of green

Klebsormidiaceae The River Ehen in November

 

Talking about the weather …

September is here.  When I visited this site two months ago we were in the midst of the heatwave and the samples I collected from the Wear at Wolsingham were different to any that I have seen at this location before, dominated by small green algae (see “Summertime blues …”).   As I drove to Wolsingham this time, I could see the first signs of autumn in the trees and the temperatures are more typical of this time of year.   We have had rain, but there has not been a significant spate since April and this means that there has been nothing to scour away these unusual growths and return the river to its more typical state.

That does not mean, however, that there have been no changes in the algae on the submerged stones.  Some of these differences are apparent as soon as I pick up a stone.  Last month, there was a thin crust on the surface of the stones; that is still here but now there are short algal filaments pushing through, and the whole crust seems to be, if anything, more consolidated than in July, and I can see sand grains amidst the filaments.   Biofilms in healthy rivers at this time of year are usually thin, due to intense grazing by invertebrates, so I’m curious to know what is going on here this year.

A cobble from the River Wear at Wolsingham, showing the thick biofilm interspersed with short green filaments.   Note, too, the many sand grains embedded in the biofilm.  The bare patch at the centre was created when I pulled my finger through it to show how consolidated it had become.  The cobble is about 20 centimetres across.

Many of the organisms that I can see when I peer at a drop of my sample through my microscope are the same as those I saw back in July but there are some conspicuous differences too.   There are, for example, more desmids, some of which are, by the standards of the other algae in the sample, enormous.   We normally associate desmids with soft water, acid habitats but there are enough in this sample to suggest they are more than ephemeral visitors.   And, once I had named them, I saw that the scant ecological notes that accompanied the descriptions referred to preferences for neutral and alkaline, as well as nutrient-rich conditions.  Even if I have not seen these species here before, others have seen them in similar habitats, and that offers me some reassurance.    In addition to the desmids, there were also more coenobia of Pediastrum boryanum and Coelastrum microporum compared to the July sample.

A view of the biofilm from the River Wear at Wolsingham on 1 September 2019. 

There were also more diatoms present than in my samples from July – up from about 13 percent of the total in July to just over 40 per cent in September.   The most abundant species was Achnanthidium minutissimum, but the zig-zag chains of Diatoma vulgare were conspicuous too.  The green filaments turned out to be a species of Oedogonium, not only a different species to the one I described in my previous post but also with a different epiphyte: Cocconeis pediculus this time, rather than Achnanthidium minutissimum.   I explained the problems associated with identifying Oedogonium in the previous post but, even though I cannot name the species, I have seen this form before (robust filaments, cells 1.5 to 2 times as long as broad) and associate it with relatively nutrient-rich conditions.  That would not normally be my interpretation of the Wear at Wolsingham but this year, as I have already said, confounds our expectations.   I did not record any Cladophora in this sample but am sure that, had I mooched around for longer in the pools at the side of the main channel, I would have found some filaments of this species too.

Desmids and other green algae from the River Wear at Wolsingham, 1 September 2019.  a. Closterium cf. acerosum; b. Closteriumcf. moniliferum; c. Cosmarium cf. botrysis; d. Closterium cf. ehrenbergii; e. Coelastrum microporum; f. Pediastrum boryanum.   Scale bar: 50 micrometres (= 1/20th of a millimetre).  

It is not just the differences between months this year that I’m curious about.  I did a similar survey back in 2009 and, looking back at those data, I see that my samples from August and September in that year had a very different composition.   There was, I remember, a large spate in late July or early August, and my August sample, collected a couple of weeks later had surprised me by having a thick biofilm dominated by the small motile diatom Nitzschia archibaldii.   My hypothesis then was that the spate had washed away many of the small invertebrates that grazed on the algae, meaning that there were few left to feed on those algae that survived the storm (or which had recolonised in the aftermath)..   As the algae divided and re-divided, so they started to compete for light, handing an advantage to those that could adjust their position within the biofilm.   This dominance by motile diatoms was, in my experience of the upper Wear, as uncommon as the assemblages I’m encountering this summer, though probably for different reasons.

Other algae from the River Wear at Wolsingham, September 2018.    The upper image shows Diatoma vulgare and the lower image is Oedogonium with epiphytic Cocconeis pediculus.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

I suspect that it is the combination of high temperatures and low flows (more specifically, the absence of spates that might scour away the attached algae) that is responsible for the present state of the river.  This, along with my theory behind the explosion of Nitzschia archibaldii in August 2009, both highlight the importance of weather and climate in generating some of the variability that we see in algal communities in rivers (see “How green is my river?”).   The British have a reputation for talking about the weather.   I always scan the weather forecasts in the days leading up to a field trip, mostly to plan my attire and make sure that I will, actually, be able to wade into the river.  Perhaps I also need to spend more time thinking about what this weather will be doing to the algae I’m about to sample.

Desmid diversity …

Back in September, I wrote about a joint British Phycological Society and Quekett Microscopical Club field weekend looking at desmids in the Lake District (see “Desmid Masterclass”, “Lessons from School Knott Tarn” and “Different tarn, different desmids …”).  Dave John sent some of the samples that we collected to David Williamson, the UK’s leading expert on desmids but, at 92, too frail to join us, and he has now sent back some fine drawings illustrating the range of desmids that he encountered.

Two of the tarns (Long Moss Tarn, Kelly Hall Tarns) are already recognised as Internationally Important Plant Areas (IPAs) for desmids because of their desmid diversity and containing internationally very rare desmids (based largely on David Williamson’s records) so their diversity is not a complete surprise to us.  Nonetheless, David found a total of 129 desmid taxa in the three tarns, whilst another desmid specialist, Marien van Westen, identified almost 160 desmids in another set of samples from the same tarns.

The drawings are arranged in three plates, one for each tarn.   Desmids identified by David Williamson from the three tarns are illustrated.  The desmids have been numbered and the captions prepared by David John who is analysing the findings and comparing them with surveys dating back to the 1970s.   David Williamson has drawn the taxa at different scales to roughly balance the arrangement on the collage, and adjusted the sizes so important details are visible.   No details of the chloroplasts are given since all samples had been preserved in formalin.  A few of the desmids, particularly those that are very long, have not been included in the plates.

Desmids from Long Moss Tarn (SD 292 936), September 2017.   Long Moss Tarn is shown in the photograph at the top of this post.

Desmids from Kelly Hall Tarn (SD 289 933), September 2017.

Desmids from School Knott Tarn (SD 427 973), September 2017.

Key

1-Actinotaenium diplosporum; 2-Actinotaenium turgidum;  3-Bambusina borreri;  4-Closterium acerosum var. borgei; 5-Closterium angustatum;  6-Closterium archerianum var. pseudocynthia;  7-Closterium archerianum; 8-Closterium attenuatum;  9-Closterium baillyanum var. alpinum; 10-Closterium baillyanum; 11-Closterium closterioides; 12-Closterium costatum; 13-Closterium dianae var. arcuatum; 14-Closterium dianae var. minus;  15-Closterium didymotocum; 16-Closterium incurvum; 17-Closterium intermedium; 18-Closterium kuetzingii;  19-Closterium lunula; 20-Closterium navicula;  21- Closterium setaceum; 22-Closterium striolatum; 23-Cosmarium amoenum; 24-Cosmarium anceps; 25-Cosmarium binum; 26-Cosmarium brebissonii; 27-Cosmarium contractum;  28-Cosmarium davidsonii; 29-Cosmarium debaryi;  30-Cosmarium depressum; 31-Cosmarium formosulum; 32-Cosmarium hostensiense; 33-Cosmarium incrassatum var. schmidlei; 34-Cosmarium margaritatum; 35-Cosmarium margaritiferum; 36-Cosmarium monomazum var. polymazum;  37-Cosmarium obtusatum;  38-Cosmarium ornatum; 39-Cosmarium ovale;  40-Cosmarium pachydermum; 41-Cosmarium pachydermum var. aethiopicum; 42-Cosmarium perforatum var. skujae; 43-Cosmarium portianum; 44-Cosmarium punctulatum;  45-Cosmarium quadratum; 46-Cosmarium quadrum; 47-Cosmarium subochthodes var. majus; 48-Cosmarium subtumidum var. groenbladii;  49-Cosmarium subundulatum; 50-Cosmarium tetragonum var. ornatum ; 51-Cosmarium tetraophthalmum; 52-Cosmarium variolatum;  53-Cosmocladium tuberculatum; 54-Desmidium aptogonum; 55-Desmidium swartzii; 56-Docidium baculum; 57-Euastrum ampullaceum; 58-Euastrum ansatum;  59-Euastrum bidentatum var. speciosum; 60-Euastrum gemmatum; 61-Euastrum luetkemulleri; 62-Euastrum oblongum; 63-Euastrum pectinatum; 64-Euastrum pulchellum; 65-Euastrum verrucosum; 66-Gonatozygon aculeatum; 67-Gonatozygon brebissonii; 68-Groenbladia undulata; 69-Haplotaenium minutum;  70-Hyalotheca dissiliens;  71- Micrasterias americana var. boldtii; 72-Micrasterias compereana; 73-Micrasterias crux-melitensis; 74-Micrasterias denticulata; 75-Micrasterias furcata; 76-Micrasterias pinnatifida;  77-Micrasterias radiosa; 78-Micrasterias rotata; 79-Micrasterias thomasiana; 80-Micrasterias truncata; 81-Netrium digitus; 82-Netrium digitus var. latum; 83-Netrium interruptum;  84-Penium exiguum; 85-Penium margaritaceum; 86-Pleurotaenium coronatum var. robustum;  87-Pleurotaenium ehrenbergii; 88-Pleurotaenium truncatum; 89-Sphaerozosma filiforme; 90-Staurastrum arachne;  91-Staurastrum arctiscon; 92-Staurastrum bieneanum; 93-Staurastrum boreale var. robustum; 94-Staurastrum cristatum; 95-Staurastrum dilatatum; 96-Staurastrum inconspicuum; 97-Staurastrum kouwetsii; 98-Staurastrum lapponicum; 99-Staurastrum maamense; 100-Staurastrum polytrichum; 101-Staurastrum productum; 102-Staurastrum quadrangulare; 103-Staurastrum striolatum; 104-Staurastrum teliferum; 105-Staurastrum tetracerum; 106-Staurodesmus convergens; 107-Staurodesmus convergens var. wollei; 108-Staurodesmus cuspidatus var. curvatus; 109-Staurodesmus megacanthus; 110- Xanthidium antilopaeum; 111-Xanthidium antilopaeum var. laeve; 112-Xanthidium antilopaeum var. polymazum; 113-Xanthidium cristatum.

Taking desmids to the next dimension …

Participants at the British Phycological Society / Quekett Microscopical Club field weekend at the Freshwater Biological Association, September 2017.  Scale bar: one metre (= 1,000,000 micrometres).

A theme that has run through this blog over the years has been that what you see down a microscope is often a highly distorted view of reality and at the end of our weekend of desmid hunting, Chris Carter gave a talk that also made this point, using desmids as a case study.  In essence, we had spent much of Saturday and Sunday morning peering down microscopes at three-dimensional objects that appeared, as a result of the very shallow depth of field that is characteristic of high magnification images, two dimensional.   We were then matching these to two-dimensional representations in the Floras and identification guides that we had to hand.  Dave explained a few tricks that experts use, such as applying gentle pressure to a coverslip with a fine needle, to turn desmids in order to see them from other angles but, mostly, we were restricted to very flattened views of desmids.

Chris has tackled this problem from several directions over the years, including experiments with anaglyphs (see “Phworrrrhhh …. algal sex in 3D!“) as well as the very careful manipulation of a long, cylindrical Pleurotaenium that won the Hilda Canter-Lund prize earlier this year.   He has also produced a number of plates with desmids laid out almost as if on an engineer’s drawing board, with front, top and side views.   Several of these are on Algaebase, but one example is reproduced below.  Microscopists learn to use the fine-focus control to appreciate the depth of the objects that they are examining and Chris also shows how it can reveal the nature of surface ornamentation on different parts of the cell.  The temptation, given a series of photos such as these (excluding the side view) would be to use “stacking” software to produce a single crisp image.  This is appropriate in some situations but you are, in truth, just producing a crisp two-dimensional image rather than offering any insights into the true shape of the cell.

Staurastrum furcatum from Botswana, photographed by Chris Carter.

Another technique that can be used to generate three-dimensional images is, of course, scanning electron microscopy.  However, this is beyond the budget for anyone outside a major institution.  This has helped greatly get a better understanding of the morphology of diatoms, in particular, but the third dimension comes at a price.   Scanning electron micrographs take us to an opaque, monochrome world, purged of the vivid colours that the microscopic world usually offers us.

Chris’ pièce de résistance, however, was a three-dimensional model of a Staurastrum, produced by the 3D printing company Shapeways and loosely-based on various pictures of S. furcatum and presented to him as a 70th birthday present by his son.  The main point is to demonstrate the symmetry and gross features of a typical Staurastrum rather than to be a taxonomic blueprint. The designers were very helpful but it does hint at what is possible with modern technology.

Chris Carter’s three-dimensional model of Staurastrum.  It is about six centimetres across.   You can buy your own copy from Shapeways by following this link …

The missing ingredient in this recipe is imagination.  Or, to be more precise, the viewer’s imagination as Chris has clearly demonstrated that he is not lacking in that department.  Once you have a sense of the three-dimensional form of a Staurastrum, you be able to use that knowledge every time you look at a two-dimensional image of a desmid through a microscope.   Seeing, as Ernest Gombrich reminds us in his great book Art and Illusion, is as much about using prior experiences to interpret the raw data collected by our optic nerves as it is about the patterns of light that stimulate our retinas.   Just as a child can look at a two-dimensional image of a cat in a book and match this to the real creatures that he or she encounters, so knowing about Staurastrum’s third dimension helps us to interpret the flat shapes that we see.

At a more basic level, all identification is a matter of matching the objects we see either to schemata stored in our memory or to patterns in books.  This, in turn, helps us to understand why the microscopic world seems so strange and mysterious to those who do not study it.  It all comes down to having (or not having) the prior experiences that generate recognition.   At one level, there are gasps of astonishment as people with none of these schemata in their memories encounter the beauty of desmids for the first time.  And then there is Frans Kouwets, another speaker at the meeting , who is busy cataloguing 750 different species of one genus, Cosmarium.   And in between there are the rest of us …

Frans Kouwets explains his fascination with Cosmarium to the British Phycological Society / Quekett Microscopical Club field meeting at the Freshwater Biological Association in September 2017.