Building landscapes …

The other excursion during our weekend of microscopy at Malham Tarn described in the previous post saw us walking in the opposite direction from Tarn House, not towards Tarn Moss but following the Pennine Way south, skirting the edge of the Tarn, then heading striking along the ancient Mastiles Lane to the point where it crossed Gordale Beck.  We were on the Carboniferous Limestone here, but at a point where the water percolating through met the impervious Silurian slates and bubbled to the surface.   That water had, however, been enriched with carbon dioxide released by the microorganisms in the soil and the carbonic acid that resulted from this encounter dissolved the limestone so that the water that emerged from the numerous springs and seepages in the area was saturated with calcium carbonate.

Once exposed to the air, however, the carbon dioxide diffuses out of the water, shifting the equilibrium so that the calcium carbonate deposits as a fine layer on any available surface.  Organisms that live in these streams have to cope with at least two problems as a result:  the steady accretion of calcite around them, and the tendency for any phosphorus in the water to precipitate at the same time, thus depriving them of vital nutrients. 

Conspicuous on the bed of Gordale Beck at this point were dark hemispherical colonies of Rivularia haematites, a member of a cyanobacterium genus that has featured in this blog on several occasions (see “Blue skies and blue flowers in Upper Teesdale”).  The colonies here were amongst the largest I had seen – all easily visible to the naked eye against the pale coloured limestone boulders and cobbles in the stream.   The hemispherical colonies are really tiny stromatolites, with clear laminations visible in sections showing their growth over several years alternating between favourable and unfavourable conditions.   

Rivularia haematites in Gordale and Mastiles Beck, September 2021.   Top: colonies of Rivularia just below the water level in Gordale Beck (the largest is about 5 mm across); bottom left: a “stromatolite” of R. haematites from Mastiles Beck; bottom right: miceroscopic view of colonies (scale bar: approximately 50 micrometres (= 1/20th of a millimere)).   The photographs at the top of the post (taken by Heather Kelly) shows Janet’s Foss and the waterfall in Gordale Scar, about 1.5 and 2 km respectively downstream from where we were standing.  

These are the most obvious of the cyanobacteria of the area, but there are others hiding in plain sight – in particular fine filaments of Schizothrix calcicola and Phormidium incrustatum, which coat the bed of the stream and seepages but which also contribute to more impressive structures.   Had we made our way downstream from where we were standing we would have entered Gordale Scar, a narrow limestone valley created when the roof of an underground canyon collapsed.   At a few points Gordale Beck or its tributaries tumbles down waterfalls where the aeration of the water hastens the loss of carbon dioxide, encouraging the deposition of calcite.    These waterfalls are colonised by mosses – principally Cratoneuron commutatum – amongst which cyanobacteria also grow and on which the calcite is deposited.   Over time these mixtures of moss, cyanobacteria and lime grow into substantial “screens” of tufa.  The most impressive of these in the Malham area are found in Gordale Scar and, a little further downstream, at Janet’s Foss.

There was a debate, for a long time, about the role that mosses and algae played in the deposition of calcite in tufa.   It was thought that photosynthesis by these organisms would, by removing carbon dioxide, shift the chemical equilibrium and encourage precipitation of calcite.  Now, however, the consensus is that the mosses and algae play a physical role, providing surfaces onto which the calcite deposits, rather than actively promoting precipitation.   Whatever the nuances, the cyanobacteria are playing a role as “ecosystem engineers” (see “How to make an ecosystem” and “How to make an ecosystem (2)”).  Thousands of visitors look at Janet’s Foss or scrabble over the waterfall in Gordale Scar every year, but how many are aware that these are living structures, built, in part, by some of the most ancient organisms on our planet?  

Allan Pentecost explains the ecology of Goredale Beck and Mastiles Beck to participants on the BPS/QMC microscopy weekend at Malham Tarn Field Studies Centre, September 2021.  

Reference

Pentecost, A. (1981).  The tufa deposits of the Malham district.  Field Studies 5: 365-387.

Some other highlights from this week:

Wrote this whilst listening to:    P.J. Harvey via YouTube

Cultural highlights:   Norwegian black comedy, Ninja Baby.  Will not be to everyone’s tastes, but we enjoyed it.

Currently reading:  Three Men in a Boat by Jerome K. Jerome.

Culinary highlight:  Two contenders this week: haggis, neeps and tatties served in a whisky cream sauce in Tynholm Inn just off the A82 in Scotland and a south Indian meal at Dosa Kitchen in Jesmond, Newcastle.

Invisible worlds at Malham Tarn

Dicky Clymo’s first – or maybe second – ecology lecture to Westfield College undergraduates in the early 1980s used Malham Tarn as a case study in how ecosystems change in space and time.   Malham Tarn is one of just two natural lakes in the Yorkshire Dales (the other is Semer Water – see “Lake lakelake lake”).  The permeable limestone bedrock means that most surface water seeps away; however, Malham Tarn sits on the underlying impermeable Silurian slate and water which has drained through the limestone bubbles up in a series of springs to the north-west of the tarn, picking up calcium and other ions on the way to create hard water with a unique ecology (see “Everything is connected …”). 

Over the millenia, however, fine sediments deposited by the streams that bring the spring water to the tarn accumulated and the north-west corner of the lake became shallower to the point where it was no longer open water, but a shallow fen colonised by amphibious plants.   In parts, those colonising plants have been replaced by alder carr, allowing Dicky to demonstrate the principle of ecological succession.   However, as the north-west corner of the lake slowly turned into a mire, so the influence of the calcium-rich springs lessened whilst that of rainwater increased.   The water in parts of the mire is now extremely soft and the juxtaposition of very soft and very hard water creates a wonderfully wide range of habitats within which to search for aquatic organisms. 

John Lund, for example, estimated that “there is little doubt that a thousand [species of algae] could be found in this area” which made it an excellent location (following Windermere and Ennerdale Bridge) for the third of the British Phycological Society / Quekett Microscopical Club microscopy weekends.   This meeting was originally planned for spring 2019 but the pandemic intervened, and it is only now that it feels safe to be meeting face-to-face once again.   We spent part of the weekend looking at Malham’s famous tufa-forming streams and the other part looking at samples we collected from the softer waters Tarn Moss.   For Dave, in particular, these habitats have an irresistible pull, as rich sources of desmids, but there was a wide range of other algae present too.  

Collecting algae samples from the fen area of Tarn Moss during the BPS/QMC microscopy weekend at Malham Tarn Field Studies Centre, September 2021.  The photograph at the top of the post shows Malham Cove, 3 km to the south of Malham Tarn on a visit in 2019.

My guide for describing the algae that we found was an old paper by John Lund from the now defunct journal Field Studies.   He recorded 199 genera from Malham Tarn in this paper and it was interesting to see how our haul from Tarn Moss compared with his account.   For example, we found the cyanobacterium Anabaena to be quite common in squeezings from the pools on Tarn Moss whereas he only recorded it from plankton in the tarn itself (though it was quite abundant in some years.  His paper does not record how many visits he made or what time of year, so there may be good reasons for differences.  The section on permanent pools is very short and does not mention any algae at all by name.   Nonetheless, his records are a very useful starting point for a modern visitor to the area.   

As well as Anabaena, we also found colonies of the cyanobacterium Aphanothece microscopica in a squeezing of Utriculaira from one of the pools on Tarn Moss, as well as green algae, diatoms and dinoflagellates.  Of particular interest were small, writhing cells of Euglena mutabilis (last encountered in “A visit to Loughrigg Fell”).   This is an unusual Euglena because it does not possess a flagellum.  It is also tolerant of very acid conditions – being encountered when the pH is as low as 2.0.  Allan Pentecost made a special study of E. mutabilis from Tarn Moss, published in the journal Field Studies in 1982.  

The most abundant diatoms in the squeezings from Tarn Moss was Tabellaria flocculosa, although there were also a few chains of T. quadriseptata, distinguished by the short spines on the edge of the valve (easily visible under a light microscope but not clear in the photograph below).  More about these in The bluffer’s guide to Tabellaria …. Finally, a few dinoflagellates were observed.  Most were too fast-moving to be captured on camera, but I have photographed Cystodinium cornifax (Lund recorded this as Glenodinium, but from similar habitats to us).   The dinoflagellates are a group that have not been encountered much in this blog over the years and this is the first photograph of one from my own samplings.  That does not mean that they are rare, only that I do not look in the right places.

Algae (other than desmids) and a protozoan from Tarn Moss, Malham, September 2021. a. Anabaena sp. (lower filament with hormogonium); b. Ankistrodesmus cf. fusiformis; c. Stentor; d. Aphanothece microscopica; e. Pediastrum boryanum; f. Eudorina unicocca; g. Tabellaria flocculosa; h. Tabellaria quadriseptata; i.-k. Euglena mutabilis; l. Cystodinium cornifax.  Scale bar: approximately 50 micrometres (= 1/20th of a millimetre).

Then, of course, there were the desmids.  Squeezings from Sphagnum and Utricularia revealed rich hauls of these beautiful algae, and a few of the more common are illustrated in the figures below.   All are from Tarn Moss as most desmids prefer soft water.  However, John Lund also records Oocardium stratum, a filamentous desmid that grows in highly calcareous environments, from Gordale Beck.   We did not see it on our visit, though it has been recorded in the recent past.  

Desmids from Tarn Moss, Malham: a. Closterium ehrenbergii; b. C. acutum; c. C. kuetzingii; d. Micrasterias rotata; e. M. truncata var. bahusiensis; f. Staurastrum telierum; g. Euastrum denticulatum.  Scale bar: approximately 50 micrometres (= 1/20th of a millimetre). 

I did not have my usual microscope and camera with me on this weekend trip.  Consequently, all the images of algae in this post were taken with my iPhone via a HookUpz adapter.  Unfortunately, I did not have an eyepiece graticule on my microscope, so the scale bars are all estimates and need to be treated with caution.  

This was, I guess, another example of “scientific jazz” (see “Jammin’ in the key of algae …”) insofar as we had a theme (Malham Tarn and its environs) but no set objectives other than observing as much as possible of the algal diversity in a relatively short period of time.   Different people improvised in different ways – the desmids were popular, but one of our number used the “coverslip capture” method to look at the diatoms present on the fine sediments of Malham’s littoral zone, and others looked at the Cyanobacteria and diatoms associated with Goredale and Mastiles Becks.   The weekend was more about asking questions than finding answers.  And about drinking craft beer.  But that’s a story for another day …

More desmids from Tarn Moss, Malham, September 2021: a. Netrium digitus; b. Pleurotaenium trabacula; c. Tetmemorus brebissonii; d. Cosmarium reniforme.   Scale bar: approximately 50 micrometres (= 1/20th of a millimetre).  

References

Lund, J.W. (1961). The algae of the Malham Tarn district.  Field Studies 1: 85-119.

Pentecost, A. (1982).  The distribution of Euglena mutabilis in Sphagna, with reference to the Malham Tarn North Fen.  Field Studies 5: 591-606.

Pentecost, A. (1991).  A new and interesting site for the calcite-encrusted desmid Oocardium stratum Naeg. in the British Isles.  British Phycological Journal 26: 297-301.

Piggott, M.E. & Piggott, C.D. (1959). Stratigraphy and pollen analysis of Malham Tarn and Tarn Moss.  Field Studies 1: 84-101.

Some other highlights from this week:

Wrote this whilst listening to:    Nubiya Garcia’s Prom, available on the BBC iPlayer.

Cultural highlights:   Rewatched Educating Rita, which is as good now as it was when I saw it on first release.  And Arab Strap’s gig in Newcastle this week.

Currently reading:  The Secret Life of Bees by Sue Monk Kidd

Culinary highlight:  make-it-up-as-you-go-along gooseberry mayonnaise on top of smoked mackerel on top of a less-than-perfect minestrone soup converted into a risotto (the irony being that we followed Felicity Cloakes’ “How to cook perfect minestrone soup” recipe)

Fieldwork notes, August 2021

Fieldwork in the Lake District in late August is a test of driving skills and patience as much as of biological acuity.   We are forever tucking our car into the side of the road to let another squeeze past, before reversing into impossibly tight spaces in crowded car parks.   Then, as we make our way to the locations where we sample, our bright orange bathyscope draws us into conversations about what we are doing.   In a week or so, the schools will be back, and the families will be replaced by couples then, as autumn descends, the crowds will thin out as the weather becomes less clement.  I’m not complaining: I realise that I am privileged to work in an area that many people choose to take their holidays.   That is worth the inconveniences several times over. 

This post is a “stream of consciousness” from that trip, offering three observations from different locations that follow on from stories and themes I’ve written about over the past few months.    First stop: the River Cocker at Low Lorton, a few kilometres downstream from the point where it flows out from Crummock Water.   This is well-lit site with a shifting substratum composed largely of pebbles and small cobbles but the sight that greeted us was one of extensive growths of the green alga Ulothrix zonata.  My first reaction is surprise, because green algae are rarely prolific at this site.   However, this is late summer, the water is warm and there is abundant sunshine.  Why should I be surprised that algae are prolific?  The surprise, when I sit down and consider the matter, is that the algae are not being grazed as fast as they grow.   There’s a knee-jerk response that assumes that prolific algae in rivers equates to excess nutrients and that, certainly, should never be ruled out entirely.  However, when you are considering an otherwise healthy river, then it is important to consider not just the resources that help the alga proliferate but also the grazers and diseases that limit that proliferation.  The two should be roughly in balance such that green algae in rivers are, typically, present but not the first thing you notice about a stream.  However, this equilibrium is never perfect and the point to start worrying is not when you notice that a river bed is green, but if it is green when you come back to visit a month later (see “The wrong kind of green …”).  

Close-ups of the river bed in the Cocker at Low Lorton, August 2021.  The green alga is almost entirely Ulothrix zonata.  The photograph at the top of the post shows the sampling location, looking upstream towards the road bridge.

From the River Cocker we drove past Loweswater, then skirted the fells to descend into Ennerdale.   We drove along the length of Ennerdale Water along the forest track and then followed the River Liza, the main inflow to the lake, about a kilometre upstream to the first bridge.  I’ve written about this location before (see “Ever changing worlds …”) and subsequent observations have largely confirmed what I wrote there about the interplay between the cyanobacterium Stigonema mamillosum and green algae.   Today, the wiry dark brown/black filaments of Stigonema were almost completely overgrown by algal filaments (mostly Klebsormidium flaccidum with some Bulbochaete and others mixed in).  

A mat of Stigonema mamillosum in the River Liza almost completely overgrown by green filaments.   

My final stop in this post was a few miles further downstream.   The River Liza flows into Ennerdale Water which, in turn, feeds the River Ehen and I was curious to see what had happened at one of our regular sites on this river following the changes I wrote about in “Under the weather …”.   On our last visit, the green algae here seemed to have recovered from their chytrid-induced lurgy and were unusually prolific for the time of year (see “Under the weather …”).   The hunch I outlined earlier in the post that we should only be worried if algal growths persist was partially borne out: the Oedogonium that I had recorded in June and July was largely absent.  However, the lush grass-green flocs that I had seen in July were replaced by extensive mats of Phormidium autumnale.  This cyanobacterium has long been resident along the margins of the stream at this location but was now dominating the cobbles and small boulders in the middle of the channel.   I wish I knew what was going on here, and I’m intrigued to know how the river will change next.  Early autumn is usually the time that the red alga Auodinella hermanii proliferates.   Will the rise of P. autumnale inhibit its ability to gain footholds on the stone surfaces?  I’ll have to wait and see.  

Phormidium autumnale growing mid-channel in the River Ehen, just above Ennerdale Bridge, August 2021.

My final observation on this day of disparate observations is that observational acuity starts to wane if you spend too long in the field.   It is never a good idea to cram too many site visits into a single day and the time had come to draw the day’s fieldwork to a close.   Stripping off waders, and packing field gear away, we made our way to the Shepherd’s Arms for a restorative cup of tea and, later, a pint of Loweswater Gold: the perfect prescription for keeping those phycological antennae in tip-top condition …

Some other highlights from this week:

Wrote this whilst listening to:    Faye Webster’s Atlanta Millionaires Club

Cultural highlights:   Nothing of note this week, unfortunately

Currently reading:  Still reading The Sea is Not Made of Water: Life Between the Tides by Adam Nicholson.

Culinary highlight:   Fish finger bhorta

Cover versions …

Lots of good performances are raised by a cover of someone else’s hit, and Slime Time was no exception.  The cover version that we chose was a short film, “Moon, Mud and Microbes” by filmmaker Susi Arnott and University of Westminster scientists Jane Lewis and Dain Son, which shows the algae which live in intertidal mud in the Thames in central London.

It depicts the changes over a single tidal cycle using time-lapse photography to compress the entire cycle into six minutes.   The film opens with a view looking upstream towards Tate Modern as the moon sets over a darkened London and, as dawn breaks, the tide gradually recedes, gradually revealing, first, some old wooden pilings and then the unprepossessing brown tidal mudflats.   The location then shifts to St Saviour’s Dock, just east of Tower Bridge, where we zoom in on one section of the mudflat and the magic starts to happen: patches of green and a darker chocolate brown start to appear on the grey-brown mud.   These, we told our Slime Time audiences, are algae “commuters” who make a daily migration up through the fine sediments, in order to do their important “work” of pumping oxygen into the atmosphere.   Their upward movement is stimulated largely by light, but also by internal body clocks – something to which an audience of campers can relate: we invariably wake up as soon as daylight floods through the thin walls of our tent on the first morning under canvas but, as days go by, our bodies slowly adapt to override this stimulus.  

Two stills from Moon, Mud and Microbes showing the mud at St Saviour’s Dock, approximately 30 minutes apart in real time, showing the algae rising to the surface.   The montage at the top of the post shows the Omni Tent ten minutes before our performance, slowly filling as Rachel Williams, our compère, pulls in the punters, and finally co-presenter Thomaz Andrade gets Slime Time under way.   

The film then cuts from the mud of St Saviour’s Dock to a view of the organisms responsible for the change in colour under the microscope.   We can see large green cells of Euglena ehrenbergii and needle-shaped diatoms, Cylindrotheca gracilis with their yellow-brown chloroplasts.  Other diatoms appear in subsequent scenes, but these were the most abundant types.   The Cylindrotheca cells are about a tenth of a millimetre long, giving our audience a rough idea of the scale of what they were observing.  All these algae were moving around the screen, to the surprise of our audience who assume “plants” to be static and only “animals” to move (apart from the cheerful clever clogs in the second row who pointed out that Venus Fly Traps also move).   We didn’t want to get into the deep technical arguments about whether algae were, in fact, plants (see “Identity crisis”) or into the details of how they move.   Simply revealing an aspect of natural history hitherto hidden from our audience was enough.  

Euglena ehrenbergii and Cylindrotheca gracilis: the two most common algae in the microscopic samples shown in Moon, Mud and Microbes.  The Cylindrotheca cells are about 100 micrometres (= 1/10th of a millimetre) long.

From looking at specimens using a light microscope the film then moves on to show some stills taken using a Scanning Electron Microscope, allowing details of individual pores (no more than a micrometre – a millionth of a metre – across).   The image below shows one of the diatoms we saw: Hydrosera triquetra.  This is a species that was formerly regarded as a tropical species, but which was found in the Thames in 1971 and which is now common as a golden-brown zone on the lower part of river walls between Greenwich and Putney.   There are also a number of records from other north European estuaries.  

A cell of Hydrosera triquetra from the St Saviour’s Dock mud sample.  Note the smaller diatom growing as an epiphyte on the side of the cell.   Hydrosera triquetra cells are typically about a 10th of a millimetre in diameter.  A still from Moon, Mud and Microbes.  

For the last 90 seconds or so the film cuts back to views of the mud, showing the algae “commuters” gradually disappearing back into the mud before the incoming tide covers them all with water again.  The wooden pilings which we saw emerge at the start of the film now disappear back underwater and the light slowly fades at the end of the day.   The final shot is a view looking downstream from the South Bank near the National Theatre towards Blackfriar’s Bridge and St Paul’s Cathedral.   The tide is, again, high and the moon is rising.   And so the cycle begins again … 

Reference

Coste, M. & Ector, L. (2000).   Diatomées invasives exotiques ou rares en France: principales observations effectuées au cours des dernières décennies.   Systematics and Geography of Plants 70: 373-400.

Tittley, I. (2014).  Non-native marine algae in southeastern England. Bulletin of the Porcupine Marine Natural History Society 1: 28-32.

Some other highlights from this week:

Wrote this whilst listening to:    Soar, the collaboration between Welsh harpist Caitlan Finch and Senegalese kora player Seckou Keita.   The answer to the question: “what would Bach’s Goldberg Variations sound like if played on traditional West African instruments.   And Nubya Garcia’s first album 5ive.   Audio methadone to help me through post-Green Man cold turkey.   

Cultural highlights:  New BBC series Vigil which is, in essence, a Golden Era crime mystery but set on a nuclear submarine rather than in a country house.  

Currently reading:  The Sea is Not Made of Water: Life Between the Tides by Adam Nicholson.

Culinary highlight:  Lamb henry, a half shoulder of lamb slow cooked until it is meltingly tender and falling off the bone.  Served with a rich gravy and mash in the Shepherd’s Arms, Ennerdale Bridge, our regular fieldwork home-from-home.   Washed down with a pint of Loweswater Gold.  

Jammin’ in the key of algae …

It’s official: I have now performed at a major music festival.  In its 20-year history Green Man has seen countless guitar solos, plenty of saxophone and keyboard breaks and even, yea gods, a few drum solos.  On Friday and Saturday, for the first time, it experienced its first microscope solo.  In the spirit of improvised jazz, I went out on stage with a few themes in mind but from that point on what happened was down to what I saw through the microscope eyepiece (and which the audience saw on a screen behind me) and not to any pre-prepared script.  

Some context: I’ve been to the Green Man festival in the Brecon Beacons a few times in the past as a punter, and there is always a thriving science zone, Einstein’s Garden.  Last time I was here, in 2019, I noticed some unprepossessing flocs of algae floating in an artificial pond close to the impressive Mountain Stage (shown above) and thought: “they would look really good projected on to a big screen”.  And so the idea of “Slime Time” was born.   The second key component of Slime Time is a six-minute film of algae growing on exposed mudflats in the Thames in central London came when I met filmmaker Susi Arnott, but that will be the subject of a future post and I’ll focus on the improvised microscope solos in this one.

We pitched Slime Time to Green Man in early 2020 and they liked it enough to put us on the bill for the festival.   Shortly afterwards a certain pandemic kicked off and Green Man 2020 was cancelled, along with every other cultural event.   For a long while we thought that the 2021 festival wouldn’t go ahead either but, in mid-July, the Welsh Government finally gave the go-ahead and we all had about a month to get everything together ready for the gates opening ot the public on 19 August.  

The pond which had first inspired me to pitch the idea had, however, changed in the two years since I first saw it.  Gone were the floating flocs of algae and, in their place was a dense mat of duckweed.  Underneath this, however, I found some fine filaments, rough rather than slimy to the touch, which turned out to be blanket weed (Cladophora glomerata) when I got back to the tent and peered at it through my field microscope.  This is not the most exciting filamentous alga to show an audience but, fortunately, I had stuffed a small bottle of Spirogyra from Cassop Pond (see “Promising young algae …”) into my cool box just before leaving.

The artificial pond on the Green Man site, showing the dense cover of duckweed (photo: Sophie Perry)

A festival setlist, however, needs more than two tunes so I needed to do some more exploration around the Green Man site before hitting the stage.   First stop was the Monmouthshire and Brecon Canal which runs along the edge of the Usk Valley and is a ten minute walk from one of the festival’s gates.   That, however, was a disappointment.  The propellers from a steady stream of canal boats stir up the fine bottom sediments to create a turbid brown soup within which no respectable algae are likely to survive.   So, after a pleasant walk along the towpath, fuelled by some welsh cakes sold to me in aid of a cancer charity by two young girls and their grandmother, I headed back, still needing a banger to bring the show to a close.  

The Monmouthshire and Brecon Canal near the Green Man Festival site, August 2021.

The River Usk, which runs along the northern edge of the festival site, provided the inspiration for my closing number.   This is a river of high conservation value for several reasons, including the presence of the Twaite Shad (Alosa fallax).  Pete, the security guard watching the exit to the Green Man site as I left, told me that he had seen a large brown trout resting in a backwater close to the bridge, as well as spotting a salmon leaping.  As we chatted, there was the briefest of flashes of iridescent blue as a pair of kingfishers flew low along the water and my journey down to the river’s edge also disturbed a dipper.   These, I felt, were all good signs, reflecting Natural Resources Wales’ (NRW) assessment of the ecology here as “good status”.  

The River Usk looking upstream towards the entrance to the Glenusk estate, August 2021

Down at the water’s edge, however, the omens did not look so good, with stones smothered by a thick film of loosely-attached brown flocs which disintegrated to the touch.   Under the microscope these revealed themselves to be mostly made of the chain-forming diatom Melosira varians, along with Diatoma vulgaris(abundant in rehearsals with the field microscope, less common during my live set …) and a chain of Fragilaria(abundant during the live set, not apparent when photographing the diatoms on my return).   Overall, the impression that these algae gave me was that NRW’s overall assessment of “moderate status” is about right, reflecting concerns about the elevated concentrations of nutrients present in the river. To be fair to NRW, rivers never look at their best in late summer (see “Summertime Blues …”).   A combination of low flows, warm water and long hours of bright sunshine create ideal conditions for algae to grow and to outpace the invertebrates that feed upon them.  

As with a guitar solo, there comes a point when you pass from embellishing the main theme into self-indulgence and, with an audience composed largely of primary-aged children and their parents, I felt that my improvisations around the theme of the River Usk had reached a natural conclusion.   And, in the time-honoured festival fashion, it was time to say “that’s all we’ve got time for, you’ve been a great audience, enjoy the rest of the festival …” before throwing my microscope against an amplifier and leaving the stage to an overwhelming howl of feedback.    In my dreams …

The margin of the River Usk at Glenusk (beside the Green Man festival site) in August 2021 showing the thick film of Melosira-dominated biofilm smothering the rocks.   
River Usk diatoms: photographs taken on the same sample as I used for the live show, but stored in my tent for three days before I had a chance to examine it again. Consequently the chloroplasts of some diatoms are past their best.   a., b.: Ulnaria ulna; c. Diatoma vulgaris; d., e.: Didymosphenia geminata; f. Cymbella cf. neolanceolata; g. Achnanthidium minutissimum; h. Navicula tripunctata; i., j. Navicula sp (possibly N. cryptocephala, but N. gregaria was also present in the sample); k. Placoneissp., l. Cymatopleura solea; m. Melosira varians; n. Cocconeis pediculus growing on a filament ofOedogonium.   Scale bar: 20 micrometres (= 1/50th of a millimetre). We did not see Didymosphenia geminataCymbella neolanceolataCymatopleura solea or Oedogonium/Cocconeis pediculus during either of the live shows, so these are included here as “studio overdubs”.

Note: I was asked, after our show, for some guidance on how to buy a microscope.  If you are interested in exploring the microscopic world for yourself, this post, “Getting started with microscopy” offers some hints on buying your first microscope.

Some other highlights from this week:

Wrote this whilst listening to:    the Far Out Stage at Green Man which was about 200 metres from our tent as the crow flies.  The first draft of this post was written longhand into my notebook whilst Crack Cloud and Porridge Radio were on the stage on Sunday afternoon.  Other highlights of the weekend include Wet Leg (opening the festival on Thursday afternoon), Nubiya Garcia’s excellent jazz, Thundercat, Laura Marling’s LUMP, Richard Dawson, Big Joanie, interplay between harp and kora from Caitlan Finch and Seckou Keita,  and headline sets from Caribou and Mogwai (whose soundcheck provided a backdrop to our Saturday show).  The overwhelming memory of Green Man 2021 will be the joy on musician’s faces as they played to live audiences again; in many cases this was their first show since the start of the pandemic.

Cultural highlights:  Green Man (see above)

Currently reading:  Pie Fidelity by Pete Brown: a paean to British Food.   Bought at Book-ish, the excellent Crickhowell independent bookshop that operates a pop-up shop on the festival site.

Culinary highlight:  Green Man is also a street food and beer festival.   From the many stalls on offer, the highlight (by a small margin, because all were very good) was Taste Tibet, finalists in the streetfood category of the BBC Food Awards.   Of the beers on offer, Stouty McStout Face, from Mad Dog Brewery, Porthcawl, is worth the price for the name alone, but is also a very decent pint.   

Duckweed delights …

I’m hopping around in both time and space in recent posts.  The previous one was based on a visit to the Lake District in June whilst this one crosses the Pennines to report on a visit to Cassop Pond in May.   There is always a lag when studying diatoms, because of the preparation steps involved, but it has been longer than normal this time due to the pressure of other work.   In The Diatoms of Cassop Pond, I wrote that I had found 98 different species in samples collected in January, February and March.  Having looked at samples from April and May, this list has now been extended to 134 species but this is about more than just “twitching” because I’m also intrigued by how habitats around the lake margin differ in the diatoms they host.  

One of the most distinctive features of Cassop Pond are the extensive flocs of the liverwort Riccia natans, and I’ve noted the predilection of nitrogen-fixing diatoms (principally Epithemia adnata) for this substratum in previous posts (see “Working their passage …”).   One part of the margin, however, is dominated by duckweed (Lemna minor).   There is always some duckweed mixed in with the Riccia flocs, but it is usually a minor component.  How different, I wondered, were the diatoms when I looked at an almost pure growth of duckweed?  

Monoraphid diatoms from Cassop Pond, April and May 2021.   a., b. Cocconeis euglypta; c.,d. C. lineata; e. C. pseudolineata; f.g. Cocconeis euglypta/lineata/pseudolineata raphe valves; h.-l. Lemnicola hungarica (two raphe and three rapheless valves).   Scale bar: 10 micrometres (= 1/100th of a millimetre).

I had a hunch that I knew the answer, because I wrote about the epiphytes of duckweed in a previous post (“The green mantle of the standing pond …”), describing work in a Norfolk pond that had identified two species as being particularly characteristic epiphytes: Lemnicola hungarica and Sellaphora saugeressii (formerly S. seminulum).   I had found both of these in other samples from the pond, but they were more abundant on the Lemna than elsewhere.  L. hungarica accounted for five percent of the total number of valves, whilst S. saugeressii accounted for three percent.   By contrast, the Lemna samples had much less Epithemia (a single valve, compared to ten percent in the sample from Riccia) whilst the assemblages on the two plants were both dominated by Cocconeis spp.   The difference in representation of Epithemia is intriguing, based on what we know about its ecology, as it suggests that the Lemna assemblages are less nitrogen-limited than the assemblages on Riccia.   My hunch is that the most Lemna-rich area of the pond is a heavily-shaded inlet and, with the light needed for photosynthesis in such short supply, the plants grow more slowly and, as a result, the demand for nitrogen is lower.

One other species that I found for the first time in the May sample was Adlafia bryophila, a small diatom whose habitat is described as “locally frequent on intermittently wet bryophytes, aerophilous”.  In Cassop Pond it seems to show an opposite tendency: avoiding the bryophyte (Riccia fluitans) and inhabiting a fully aquatic habitat.   A good lesson in the need to treat ecological notes in the taxonomic literature with a healthy dose of scepticism.   

Biraphid bisymmetrical diatoms from Cassop Pond, April and May 2021.  a. Brachysira neoexilis; b. Navicula cryptotenella; c. N. cf. reichardtiana; d. Cavinula cocconeiformis; e. Adlafia bryophila; f.-h. Sellaphora saugeressii.   Scale bar: 10 micrometres (= 1/100th of a millimetre).
Heteropolar and dorsiventral taxa from Cassop Pond, April and May 2021.  a. Gomphonema capitatum; b. G. exilissimum; c. G. calcifugum; d.,e. Rhoicosphenia abbreviata (raphe and girdle views); f. Reimeria sinuata; g. Encyonema neogracile.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

There’s a lot more going on in Cassop Vale than just the changes I’ve written about in the pond itself.  You can follow changes in the higher plants in Heather’s blog [https://heatherkelly.blog/2021/08/04/cassop-vale-july-2021/] but our two approaches still miss out great swathes of the biodiversity.  There is a lot more that could be written about the bryophytes, for example, though that is way outside both of our comfort zones.  And we haven’t even started thinking about the animal life.   Nonetheless, we can both agree that the little we have discovered only serves to remind us both about how much more there is to learn, and how difficult it is to make generalisations even about a habitat that we know reasonably well.

Bacillariales and Surriellales from Cassop Pond, April and May 2021.  a. Nitzschia acicularis; b. N. dravillensis; c. N. paleacea; d. N. inconspicua; e. Tryblionella debilis; f. Surirella angusta.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Some other highlights from this week:

Wrote this whilst listening to:    As The Love Continues, new album by Mogwai, headliners at Green Man next weekend.  Getting back into the festival vibe …

Cultural highlights:  Shiva Baby, a comedy set in a Jewish community in New York

Currently reading:  Culture Warloads: My Journey into the Dark Web of White Supremacy by Talia Lavin which, by macabre coincidence, includes a chapter on the world of incels.

Culinary highlight: Sunday lunch at a local restaurant.  Good food but, more importantly, the first time the whole family had eaten a meal together since April 2019.

Under the weather …

One of the curiosities of the algal communities of rivers is how much they can change in relatively short periods of space and time, even when major drivers of change, such as season and pollution, can be discounted.   In the River Ehen, for example, there are significant and predictable changes in the quantities of algae recorded over the course of the year, but also, less predictably, changes in the composition of the often-conspicuous green flocs between the lake and Ennerdale Bridge.   The floc in the photograph below, for example, looked very much like the flocs I had seen a few kilometres upstream, but close examination showed that their compositions were entirely different.   And, to add another level of complication, the Oedogonium I saw in July was different in microscopic appearance to that which I saw in June.

Last year, I pondered the reasons behind these observations, speculating that some of the unpredictable changes we see in space and time may be due to pathogens (see “The stream eats itself …”) and, a couple of months back, I found some filamentous algae from streams in the Lake District that were infested with chytrids.  I look at these streams often enough that I know that the algae here are not typically infested to this extent, so it lends some circumstantial support to the possibility that these are driving some of the changes that we see in these streams.   

Oedogonium filaments infested with chytrid sporangia in the River Ehen, June 2021.  Scale bar: 20 micrometres (= 1/50th of a millimetre).  The photograph at the top of the post shows the River Ehen near the outflow from Ennerdale Water in July 2021

The population of Oedogonium that I mentioned in the first paragraph was heavily infested by chytrid sporangia during my June visit.   Quite a few of the filaments were in an unhealthy state, with many epiphytic bacteria apparent even with a light microscope.   When I came back in July, however, there were lush growths of filamentous algae which turned out, on microscopic examination, to be Oedogonium, but the cells were broader, less elongate (length:breadth mostly < 2) and with fewer cap cells.   Moreover, the cells looked healthier, with no chytrid sporangia and few or no bacterial filaments visible.   I would wager that this is a different population, possibly resistant to the earlier chytrid, possibly even a different species.   It is difficult to identify Oedogonium in the vegetative state, and it is not clear how much of the visible variation between June and July populations is expected within a single species.  We can’t rule out the possibility that this is a case of the much talked about “herd immunity” spreading out from a few resistant strains which, eventually, dominate the community. 

Oedogonium filaments from the River Ehen just above Ennerdale Bridge in June (a.) and July (b., c.).   Note the abundant bacteria and generally unhealthy condition of the June filament compared to the July filaments.  Scale bar: 20 micrometres (= 1/50th of a millimetre).
A monospecific growth of Oedogonium in the River Ehen, just upstream from Ennerdale Bridge, photographed in July 2021.  A patch of the cyanobacterium Phormidium autumnale is just visible in the foreground. The picture frame is about 30 cm across.

About five kilometres away, also during my June visits, I found a population of Draparnaldia glomerata in Croasdale Beck which also had an infestation of chytrids.  Croasdale Beck is a wilder, more torrential stream than the River Ehen and the harsh hydrological regime means that green algae are rarely as conspicuous as they are in Ehen.  A long period of dry, warm weather had probably encouraged the algae to grow but the patches of algae, in turn, fostered an epidemic of chytrids.  When we came back in July, the Draparnaldia had disappeared, replaced by a population of a different filamentous alga, Microspora amoena.  

Top picture: patches of Draparnaldia glomerata in Croasdale Beck, Cumbria, June 2021.  Lower pictures: chytrid sporangia attached to Draparnaldia filaments.  Scale bar for lower pictures: 20 micrometres (= 1/50th of a millimetre).  

I’m left scratching my head about the importance of chytrids in changing the composition of filamentous algal assemblages.  My June and July 2021 visits to the Lake District would suggest that they are likely agents of change, but I’ve also seen shifts in composition between visits when there are no obvious chytrid infestations.  And the literature does not help us much either: a quick search on Google Scholar reveals no recent ecological studies on chytrids on river algae.   There is a tradition of looking at the importance of chytrids in determining phytoplankton dynamics but very little on their role in stream ecosystems, so I am left speculating.  It seems like a productive area for research not just to satisfy the rather nerdy curiosity of people like myself, but also because these pathogens are one means by which the scant resources of streams can pass from one organism to another.   Nature is sometimes red in tooth and claw but, we are increasingly realising, it catches whatever bug is doing the rounds and throws a sicky.  Just like us.   

Some other highlights from this week:

Wrote this whilst listening to:   Waltz for Debby, a 1961 recording by the Bill Evans Trio.  

Cultural highlights:  Host, a 2020 horror film set within a Zoom call.  The first film that I’ve seen that was directly inspired by the pandemic.

Currently reading:  The Wind in the Willows, by Kenneth Grahame.  

Culinary highlight: our son is back after three years in China, so all meals over the last week have had a celebratory feel, regardless of what’s actually being eaten.

The view from the north-west shore of Buttermere with Fleetwith Pike and Haystacks in the background, June 2021.

Change of tenants …

July seems to be over almost as soon as it had begun, and I had to combine a brief visit with a therapeutic cycle ride in order to keep to my promised schedule of monthly trips throughout the year.   Temperatures had dropped after a month of very warm dry weather and there was even a smear of rain in the air as I made my way down the hill towards the lake early one morning wondering what I would see.

There is always a “macro” and a “micro” aspect to the changes – the arrival and disappearance of the Spirogyra flocs being one example (see: “Promising young algae …”) and this time was no exception.  Even from a distance, I could see the surface dotted with green-brown flocs of Riccia fluitans that were certainly there last month (see “Cassop Pond in June”) but certainly not as extensive.  The second obvious change was the presence of the aquatic plant Hippurus vulgaris, Mare’s Tail, in the shallow margins of the pond.   The photograph below also shows the prolific Riccia fluitans flocs growing around its base.

Hippurus vulgaris, Mare’s Tail, growing at the margins of Cassop Pond, with prolific Riccia fluitansflocs at the base.   You can see the Riccia flocs in open water in the picture of the pond at the top of the post.

The microscopic world had changed too, most conspicuously by the proliferation of cyanobacteria – blue-green algae.   There was one form with heterocysts (specialist cells for nitrogen fixation) which I called “Anabaena cf. variabilis” on first encounter, and another without, Pseudanabaena cf. limnetica.   Life in the proximity of Riccia fluitans had changed too: in June I wrote about the presence of Cladophora filaments within the flocs, albeit not always in a healthy state.   In July these filaments had largely disappeared but filaments that I described as “Anabaena” when I encountered them free-floating now seemed to be attached to the Riccia fronds.   I am being circumspect about names because, although there are certainly species of Anabaena that live attached to surfaces rather than in suspension, I am not ruling out the possibility that these actually represent a form of Nostoc.   What I can be sure about is that these were nowhere near as prolific last month as they were in July.

My plate below also includes a cell of Nitzschia acicularis which glided into my field of view whilst I was photographing the Anabaena.   This is a diatom that can live both as plankton and on and around surfaces.  However, the clumps of Anabaena-Nostoc proved very difficult to photograph and a description will have to suffice for now.

Cyanobacteria and a diatom from Cassop Pond, July 2021.   a., b.: Anabaena cf variabilis; c., d.: Pseudanabaena cf. limnetica; e. Nitzschia acicularis.   Scale bar: 10 micrometres (= 1/100th of a millimetre).  

The presence of Anabaena should not be a great surprise: throughout the year I have been observing diatoms that are capable of nitrogen fixation in the pond; now I see the other major algal group that is capable of nitrogen fixation proliferating.   The warm weather and bright sunshine will have given a boost to many of the plants, and those nutrients that were already scarce will have been in even greater demand.   The supermarket shelves, to use a currently relevant metaphor, have some gaps and the algae have resorted to their equivalent of “panic buying”.  In this case, they are paying well over the normal price (in terms of their energy reserves) for the nitrogen compounds that are essential ingredients of the proteins they need to grow.   

I’ve mentioned before that the nitrogen-fixing diatoms on and around Riccia fluitans are probably paying some “rent” to the liverwort (see: “Working their passage …”) and the same may well be true for the Anabaena/Nostoc that were attached to the Riccia fronds.   There are some references to associations between Nostoc and the genus Riccia, mostly in older literature.  Surprisingly for me, one of the references was in a paper by my mentor Brian Whitton, who observed colonies of Nostoc intermingled amongst fronds of Riccia in terrestrial habitats on the Indian Ocean island of Aldabra.   There is no evidence that the relationship between cyanobacteria and Riccia and other liverworts is as developed as that between them and the aquatic fern Azolla (see “Escape to Southwold …”).  One older paper that I found described Nostoc as a “space parasite”, colonizing cavities and intercellular spaces in the host organism.   A modern interpretation would probably be of mutualism, with both partners gaining and neither losing substantially from the arrangement.  

All this confirms the prediction I made in Cassop Pond in June – that Cladophora might struggle to survive in Cassop Pond as the summer progresses.   But having seen the microscopic changes that have happened so far in these unprepossessing handfuls of wet liverwort, I am beginning to be intrigued by what I might see on my next visit …

References

Duckett, J.G., Prasad, A.K.S.K., Davies, D.A. & Walker, S. (1977). A cytological analysis of the Nostoc-bryophyte relationship.   New Phytologist 79: 349-362.

Nathanielsz, C.P. & Staff, I..A. (1975).  On the occurrence of intracellular blue-green algae in cortical cells of apogeotropic roots of Macrozamia communis L. Johnson.  Annals of Botany 39: 363-368. 

Whitton B.A. (1971).   Freshwater and terrestrial algae of Aldabra.  Philosophical Transactions of the Royal Society Series B 260: 249-255.

Some other highlights from this week:

Wrote this whilst listening to:   Robert Plant and the Band of Joy.  

Cultural highlights:  First night of the Proms, on BBC iPlayer.  In particular, Sibelius’s Second Symphony

Currently reading:  Hannah Arendt’s Eichmann in Jerusalem: a Report on the Banality of Evil.   Gruelling.

Culinary highlight: a lateral tasting of Ardberg whiskies (with the vicar), booty from recent trip to Islay.

Swimming in a sea of ignorance …

The American quantum physicist John Archibald Wheeler once said: “We live on an island of knowledge surrounded by a sea of ignorance.  As our island of knowledge grows, so does the shore of our ignorance”.   He was talking metaphorically but, last week, I stood on the shore of a sea loch in Argyll with a very literal interpretation of Wheeler’s words churning through my mind.   Most of us gaze at the surface of the sea with very little sense of what lies beneath.  What we do know comes mostly from natural history documentaries which, in turn, tend to focus on the extremes of the biological world, whether in terms of bizarreness, anthropomorphism, violence or beauty.    We may paddle in rock pools and get a sense from what is exposed at low tide of a fecund underwater world, but rarely get – or take – the opportunity to enter into that world ourselves.  

In many ways, that encapsulates my creed for this blog: finding the extraordinary that lurks right under our noses without us noticing.   But most of my posts are about the freshwater world and, when it comes to marine life, my island of knowledge is small, and the surrounding sea of ignorance is vast.   However, last week I had an opportunity to rectify that and, after squeezing into a wet suit and donning a mask, snorkel and flippers, I slipped into Loch Caolisport, on the west coast of the Kintyre peninsula in Argyll.   “Slipped” is, perhaps, disingenuous: it was more of an undignified crawl, due to the awkwardness that flippers confer on the hitherto straightforward act of walking.  As soon as I was a few metres from land and able to push off and float in the water, however, I became part of this new world.   

The first impression is of the extraordinary quantity and variety of the seaweeds on the seabed.  We are used to seeing a narrow fringe of exposed seaweeds, floppy and flattened on rocky shores when the tide is out; we are less used to seeing them buoyant and supported by the water.  But seaweeds, like many amphibious organisms, are as elegant in one medium as they are awkward in another.  Think how ungainly a seal is when it hauls itself on land, yet how sleek they are when seen swimming underwater.   The kelps were in their natural habitat whilst I was the ungainly intruder.  

Laminaria digitata dominating the kelp beds in the shallow waters off Loch Caolisport, Argyll, Scotland, July 2021.

Brown seaweeds clearly dominated the assemblages but many of the growths of sea oak (Halidrys siliquosa) were covered with tufts of a red alga Ceramium.   I saw crusts of the red alga Hildenbrandia (a relative is common in rivers: see “More about red algae”) and there were a few tufts of green algae too. The wracks, the seaweeds I most associate with rocky shores, were, by contrast, noticeable by their absence.  They are mostly species of the intertidal zone and the areas that I was swimming through were permanently submerged.   Further away – too far for a timid first-time snorkeller to contemplate – there were beds of sea grass that others in our party explored.

Sea oak, Halidrys siliquosa with epiphytic Ceramium in the shallow waters off Loch Caolisport, Argyll, Scotland, July 2021.

Rising above the other algae were graceful fronds of another brown alga, Sargassum muticum.  Some of these were almost two metres tall, with a central stem and many laterals, each divided several times to give a fern-like appearance.  Closer inspection revealed the presence of many small air bladders (“aerocysts”) which make the whole plant buoyant.  These bladders are, in a way, equivalent to tree trunk as both share a primary purpose of raising the plant above the other vegetation to capture as much light as possible.  

The curiosity of Sargassum muticum, however, is that it shouldn’t be here at all.   It is an invasive species, originally from Japan, that has spread around the world, possibly as the Pacific Oyster was introduced for cultivation.   The first record of S. muticum in the UK was in 1973, but it is now widespread.   However, the first time snorkeller may be forgiven for not knowing this: for them it is one more constituent of these magical underwater gardens.   You don’t know that it should not be there, unless someone tells you this.   You cannot depend entirely on sensations to make judgements about the environment; you need information too.  More particularly, in this case, you need evidence that it was once absent.  I recall a similar circumstance, walking with a friend on a riverbank near our home in Durham.   She had grown up seeing riverbanks bordered by dense stands of Himalayan Balsam (Impatians glandulifera) and had no idea that these, too, were recent arrivals, only becoming widespread in the last 30 years or so.   I had experienced the change; she was dependent on other people’s testimony.   Some older marine biologists must remember these waters before Sargassum muticum intruded but, for most, it has become just one more fixture in the underwater panoramas that greet the intrepid explorer of Argyll’s fecund fringe.

Sargassum muticum in shallow waters off Loch Caolisport, Argyll, Scotland 2021.
A detail of the fronds of Sargassum muticum in Loch Caolisport showing aerocysts.   Photograph by Heather Kelly.

Note

The picture at the top of this post shows Claggain Bay on the west coast of Islay, not Loch Caolisport.  It is hard to take good photographs of water bodies from a low standpoint and there was a rocky promontory at Claggain Bay that afforded a fine view across, whereas the immediate surroundings of our location beside Loch Caolisport had relatively low relief.  

Thanks, too, to Paul Brazier for gently correcting some of my original identifications.

Some other highlights from this week:

Wrote this whilst listening to:   Amy Winehouse.  The 10th anniversary of her death.  

Cultural highlights:  Rewatched Quentin Tarantino’s Once Upon a Time in Hollywood (see below)

Currently reading:  Quentin Tarantino’s novelisation of Once Upon A Time in Hollywood.  Cleverly written in a faux-pulp fiction style, it actually serves as a “Director’s Cut” for the film, adding in some extra background details about the characters.

Culinary highlight: Islay single malt whisky (see above).

The circle of life …

I’m a long way from my regular hunting grounds for this post.  The photograph above shows ponds at King’s Dike Nature Reserve on the site of a former brick works near Peterborough.   It’s some way from Chris Carter’s regular haunts too, but a contact passed on some material which he examined under his microscope, and found a population of Cymbella neocistula, including a perfectly-aligned auxospore.   Over the many years that I have been writing this blog, I don’t think that I have ever talked in detail about the life-cycle of diatoms and this sample presents not just an opportunity to do this, but also to draw your attention to a wonderful new book on diatoms.  

The picture below shows a living cell (left) and five cleaned “valves” (i.e. half the silica cell wall) in order of increasing size.   If images of almost any other plant or animal was presented in this way, you would probably assume that the youngest (i.e. smallest) were on the left, and the oldest (i.e. largest) were on the right.  In the case of diatoms, however, the opposite is true.  Diatoms are living embodiments of The Curious Case of Benjamin Button, with young cells being larger than the older ones.  In the case of diatoms, gradual reduction in size with successive generations is a consequence of how diatom cells divide.   Think of a diatom cell as a Petri dish with two overlapping halves, one slightly larger than the other.   When the cell divides both the upper and lower valves become the upper valves of the two daughter cells.  This means you have one cell which is the same size as the parent, and one which is slightly smaller    Repeat this several times, and the average size of the population will inevitably decrease.  Obviously, this sequence generating ever-decreasing cell size cannot proceed for ever so at some point, sexual reproduction is triggered and, freed temporarily from the constraint of a rigid cell wall, a larger cell can form.  Details of the internal “trigger” are hazy, but I like to think of it as analogous to the implications for humans of tight y-fronts

A live cell (left) and five cleaned valves of Cymbella cf neocistula from pond at King’s Dike Nature Reserve.  Photographs: Chris Carter.   10 micrometres = 1/100th of a millimetre.  The picture at the top of the post shows the ponds at King’s Dike, taken by Jonathan Graham.  

In order to place this into context, I have included a diagram from a new book, The Amazing World of Diatomsby Masahiku Idei, Shinya Sato and David Mann, which is packed full of (mostly) electron micrograph images showing the range of forms of diatoms.   The picture shows the life cycle of pennate diatoms, and runs anti-clockwise.   At the bottom you can see an “auxospore”, the end result of two gametes coming together to form a zygote.   Chris noticed an auxospore of Cymbella neocistula during his investigations and took photographs at four different focal planes to show the structure. At the top left of the next plate the focus is on one of the two parent cells then, zigzagging right and then left, you see the large auxospore underneath and, finally, bottom right, there is the lower of the two parent cells.  

The life-cycle of a pennate diatom: Fig. 6 from The Amazing World of Diatoms by Masahiku Idei, Shinya Sato and David Mann
A “stack” of images showing an auxospore of Cymbella neocistula sandwiched between two parent cells.   Photographs: Chris Carter.   10 micrometres = 1/100th of a millimetre.  

How long does all this take?   There is some evidence that many diatoms in temperate rivers have a two-year life cycle (see “Diminishing with age …” and “Diatoms and the space-time continuum”) but there is still much that we don’t know.   The study of algae and other “lower” plants was once referred to as “cryptogamic botany” with the word “cryptogam” meaning “hidden reproduction”.   Sex in diatoms is certainly not as in-your-face as that of larger orchids but it is there and happening under our very noses.  I’ve mentioned elsewhere that “seeing” is something that we do with pre-conditioned brains, as much as with our eyes (most recently in “The bluffer’s guide to Tabellaria“), and spotting the sexual stages of diatoms is a good case of this.  It is not helped, too, by the tradition of focussing attention on “cleaned” (i.e. dead) valves rather than living cells.   Perhaps it is less a case of “cryptogamic botany” and more to do with diatomists behaving as if they are Victorian maiden aunts of the botanical world, refusing to acknowledge the truth of what is taking place right in front of their noses.

Taxonomic note:

I’ve called the diatom featured in this post “Cymbella cf neocistula” because some aspects (number of stigmata, shape of central area) of some valves do not correspond entirely with the description.  There may be more than one species present, or it may be that the literature does not fully capture the scale of variation possible within populations.   

Reference

The Amazing World of Diatoms by Masahiku Idei, Shinya Sato and David Mann is available from Amazon.  You can buy the printed version from https://amzn.to/3wMtjtt but it is also possible to buy a Kindle version from other Amazon sites.

Some other highlights from this week:

Wrote this whilst listening to:   nightlife in Bowmore.   Not exactly a throbbing metropolitan hub of Bacchanalian pleasure, but the little that is happening does seem to be taking place right outside our bedroom window.

Cultural highlights:  We’re on the Isle of Islay in the Inner Hebrides, so the highlights for me have to be the distilleries.   Bowmore is about 200 m away, and Bruichladdich is visible on the other side of Loch Indaal.   Kilochoman deserves special mention for a very fine oat milk flat white whilst Ardberg loses points for an underpowered Americano (I understand other liquid refreshment is available at both locations).

Currently reading:  In a Lonely Place by Dorothy B. Hughes, 1940s crime noir in a Raymond Chandleresque setting and style which, I am sure, must have inspired Patricia Highsmith’s dark Ripley novels.

Culinary highlight: Islay single malt whisky (see above).