Structural engineering with diatoms …

I’ve been looking at shallow, calcareous lakes in two very different locations over recent weeks: on the Shetland Islands north of Scotland and in Greece.   Climatically, they are different worlds but there are surprising similarities in the diatoms that I find in the two habitats: many species are common to both but, even when the species are not identical, genera that are not widespread in other habitats are well represented in these lakes in both the Shetlands and Greece.   There must be something about these lakes that makes them attractive to a few genera over almost all other possible habitats despite the differences in climate.   

One of the genera that falls into this category is Epithemia, which I also find in my local calcareous pond (see “Working their passage …”); another is Rhopalodia, a relative of Epithemia and a third is Mastogloia, the subject of this post.   Mastogloia has a very unusual structure.   If you focus carefully on the top of the valve, you see striae and a raphe; if you then adjust the focus very slightly a series of chambers (“partecta”) will come into view, arranged in a row along each side of the valve.  They look a little like a row of cabins along the two sides of a boat.

Mastogloia (probably M. lacustris) from Limni Trichonida in Greece, December 2020.   Seven focal planes of the same frustule, seen in girdle view showing how the organisation of the partecta on girdle bands on either side of the cell.   The image at the top of the post shows Mastogloia species from the Shetland Islands: a., b., c., d., g.: M. baltica; e., f.: M. dansei; h., i. M. elliptica; j. M. dansei.  g. – k. from Carter & Bailey Watts (1980).  Scale bars: 10 micrometres (= 100th of a millimetre).

When seen from the side, rather than from above, we see that Mastogloia cells are, typically, rather deep so, pushing our nautical metaphor just a little further, they are rather ungainly.   Although they have raphes and are, in theory, capable of movement, these are not going to be found darting around like species of Nitzschiaand Navicula, constantly adjusting their position in relation to light and other resources.  Instead, Mastogloia have a very different set of priorities, with the partecta playing a key role in enabling these.

We see the partecta as empty chambers because we usually look at diatoms after they have been treated with oxidising agents that remove all organic matter.  When viewed live, however, Mastogloia are often seen surrounded by extracellular strands, capsules and tubes and it is assumed that these are secreted from pores which are, in effect, the “portholes” associated with the partecta.   The empty “cabins” we see under the microscope are, in fact, busy little slime factories.  A lot of different extracellular structures have been described, particularly from the many marine representatives of this genus, and most seem to be designed to keep the cell in one place, rather than to help it adjust position. 

For the freshwater species, calcareous habitats offer some particular challenges to organisms: the porous nature of the rock means that there is often a high risk of drying out, high calcium carbonate concentrations lead to the precipitation of calcite.  In the process, phosphorus is also removed from the water, trapped in the calcite crystals.  Evelyn Glaiser and colleagues suggested that this combination of characteristics favoured organisms that produced a lot of extracellular polysaccharides.  Firstly, the strands act to bind inorganic particles and the microbial life into dense aggregations and slows rates of desiccation.  Second, this dense moist matrix will provide not just organic matter but also microbes that can break it down to recycle nutrients for the diatoms and other organisms to use.   Third – not mentioned by Evelyn Glaiser and colleagues but touched upon in “A journey to the headwaters of the River Coquet” and other posts – the algae themselves may contribute to this recycling via enzymes present in their extracellular polysaccharides. 

A schematic view of a Mastogloia cell with polysaccharide strands emerging from pores and tangled around mineral particles. 

Mastogloia, in other words, is not just part of the structure of biofilms in in shallow calcareous lakes and ponds, it actively creates these.   In the Florida Everglades it is a keystone species, around which thick microbial mats form and within which, in turn, other species, some unique to these ecosystems, are found.   It does this through diverting energy and resources to produce extracellular polysaccharides and because the scant nutrients mean that more competitive algae are kept out.   However, if this delicate balance is disrupted and nutrients become more widely available, then Mastogloia is out-competed, the mats lose the structural integrity that Mastogloiaimparts, and the biodiversity associated with them disappears.   A classic ecological cascade: for want of a nail, the shoe is lost and so on …

References

Carter, J.R. & Bailey-Watts, A. (1981).  A taxonomic study of diatoms from standing waters in Shetland.  Nova Hedwigia 33: 513-629.

Glaiser, E., La Hée, J.M., Tobias, F.A.C. & Wachnicka, A.H. (2016).  Mastogloia smithii var. lacustris Grun.: a structural engineer of calcareous mats in karstic subtropical wetlands.  Proceedings of the Academy of Natural Sciences of Philadelphia 160: 99-112.

Hain, M.K., Winsborough, B.M., Davis, J.S. & Golubic, S. (1993).   Extracellular structures produced by marine species of Mastogloia.  Diatom Research 8: 73-88. 

Some other highlights from this week:

Wrote this whilst listening to: The Go-Betweens, an Australian indie band from the early 1980s.  There has been press coverage of a new book about one of their members and this reminded me of the one occasion I saw them whilst a student in London in 1982 or 1983.   I remember that the venue was full of goths hoping to see another band on the bill, The Dancing Did, who did not show up for some reason.  I found their sole album, And Did Those Feet, on Spotify too: 

Cultural highlights:   Watched the new film Ammonite about Mary Anning.   It plays fast and loose with history (see “It all started here …”) but is well-acted.  Charlotte Murchison, the character played by Saoirse Ronan, lived for a while in Barnard Castle, about 30 kilometres from Durham.

Currently reading:  Persepolis by Marjane Satrapi, autobiographical graphic novel about growing up in Iran during and after the 1979 revolution.

Culinary highlight:  Normandy-style galettes followed by a chocolate and ginger cheesecake.

The Natural History of Upper Weardale

A very short post this week, having been knocked for six by my first dose of the Covid vaccine.  I’m going to use it to publicise The Natural History of Upper Weardale, a new book published by Durham Wildlife Trust that provides an accessible insight into the geological, geomorphological, climatic, ecological and human influences that have shaped the dale.  I contributed to the chapter on freshwater life and Heather wrote (and illustrated) the chapter on the plants of Weardale.  You can buy it (and its companion volume, The Natural History of Upper Teesdale) from the DWT website.

At some point in the not-too-distant future, you’ll also be able to access the book via the Living Uplands website.  The Upper Teesdale book is already here, with the individual chapters all downloadable as pdfs.  Weardale and Teesdale both have their own individual characters and putting the chapters of the two books alongside each other helps to understand the reasons behind this.   No-one would readily associate the northern Pennines with volcanoes, but it is two igneous rocks that are largely responsible for the differences between the two dales.   Whin Sill outcrops much more in Teesdale and is responsible for the dramatic waterfalls at High Force and Cauldron Snout which have no counterparts in Weardale, whilst the “Weardale Granite” was responsible for the mineral veins and associated mining activity which, though present in both dales, is more widespread and obvious in Weardale.    

David Attenborough said: “no-one will protect what they don’t care about; and no one will care about what they have not experienced”, and Buddha said: “true love is born from understanding”.  I hope this book helps people to understand Weardale better and, with this understanding, to have better experiences in Weardale.  That will give us a firm foundation for conserving and protecting this beautiful landscape.  

Some other highlights from this week:

Wrote this whilst listening to:  Lana Del Rey’s new album Chemtrails Over the Country Club.  And some vintage Mahavishnu Orchestra with John McLaughlin.

Cultural highlights:  Unusual low-key Canadian film Mouthpiece and, whilst crashed out with post-vaccination blues, the 2018 film about a dysfunctional all female punk rock band, Her Smell

Currently reading:  Matt Haig’s Midnight Library, having finished Penelope Fitzgerald’s The Bookshop.  More time than usual for reading this week. 

Culinary highlight:  Persian New Year feast of tahdig served with cauliflower roasted with harissa and tahini.

Tangled up under blue skies …

We were in the Lake District at the turn of the month to collect some of our regular samples and measurements and our reward for some very cold water and slightly higher river levels than was ideal was a day of cloudless, still weather with some inviting vistas of the peaks.   That invitation will, however, have to wait, as we have work to do and, having dragged our eyes away from the distant view of Great Gable, we peered at the algae in the littoral zone of Wastwater and then made our way to the two sites on the River Irt, which flows out from the lake.

I wrote about one of these sites after our visit in December (see “As old as the hills …”) and was intrigued to see how the situation that I described there had changed over the intervening two months.   The cyanobacterial growths that I described back in December were, if anything, more prolific now, and whilst I was puzzling over their identity then, I am now almost certain (having shown the material to Brian Whitton) that it is Tolypothrix distorta var. penicillatus.  Last time, I saw enough double false branches to suspect a species of Scytonema; this time, the branches were almost all single and the general habit of the plant also pointed to T. distorta var. penicillatus.   

These growths were still associated with an acrocarpous moss (probably Racomitrium aciculare), presumably gaining some support and protection from stems and leaves.  The key difference this time, however, was that the Tolypothrix itself seemed to have been colonised by other organisms.  In addition to some very small cyanobacterial cells (guessing Chamaesiphon – see “More from the River Ehen”), there were also a few longer filaments (likely Heteroleibleinii – see “River Ehen … again”) as well as a few diatoms (mostly Fragilaria gracilis).  Those earlier posts also highlight the relationship between Tolypothrix and aquatic mosses and were written at approximately the same time of year.  

Tolypothrix distorta var. penicillatus at Cinderdale Bridge in the River Irt, March 2021.  Scale bar: 10 micrometres (= 1/100th of a millimetre).   The photograph at the top of the post shows the view along Wastwater towards Great Gable.

I’ve tried to capture this relationship in a painting, with Tolypothrix filaments entangled around Racomitrium aciculare leaves (note the characteristic wavy outline of the cells) .  The false branches typically have a heterocyst (cell responsible for nitrogen fixation) at the base, and then run almost parallel to the main filament for some time. The epiphytes grow on the thick colourless sheath.  Compare the images in this post with those taken in December to see just how the abundance of epiphytes has changed in two months.    

This is the second post in a row in which I’ve mused about the relationship between bryophytes and algae.   When I was writing about Riccia fluitans and Epithemia, I speculated that the liverwort may gain some nitrogen from the relationship and the same possibility must exist for Tolypothrix and Racomitrium.   In both cases, we are probably looking at an arrangement that is too loose for the term “symbiosis” to be appropriate but general proximity of organisms in environments where resources are scarce must lead to mutual benefits.  I would go further and suggest that patchy distributions of algae is a common property of nutrient-poor streams, and these patches are often composed of more than one species.  So it is fair to speculate that they may gain both physical support and opportunities to share nutritional and other benefits too.   It would, alas, also be very difficult to prove.

A depiction of Tolypothrix distorta var. penicillatus growing amidst filaments of the moss Racomitrium aciculare in the River Irt, Cumbria, March 2021.  

Once we had packed away our samples from the Irt our fieldwork was finished.  The sky was, however, still blue, the wind still absent, and the fells tempting.  Our only constraint was the limited amount of daylight left to us.   Rather than attempt anything too ambitious, we drove to the Honister Pass, between Buttermere and Borrowdale, parked at the slate mine at the top and headed up the steep fell to the south.  Our car had done half the climbing for us and we were on Grey Knott and striding through the rough grassland towards Brendreth within an hour of parking.  The views down the Ennerdale and Buttermere valleys were quite spectacular, with the lakes shimmering in their respective valleys. Forgive me if I end the post with a couple of memories of those very special views.  

Some other highlights from this week:

Wrote this whilst listening to:  Omega, debut album by jazz saxophonist Immanuel Wilkins.  And Julio Iglesias singing Begin the Beguine at the funeral of an elderly aunt.

Cultural highlights:  My Donkey, My Lover and I: a French comedy set in the Cévennes.  

Currently reading:  Lightseekers, by Femi Kayode, a crime novel set on a Nigerian university campus.

Culinary highlight:  definitely not the food I encountered on the journey to and from the funeral I mentioned above.   I am now definitely inoculated against fast food chain burgers for several months at least. 

The Liza Valley and Ennerdale Water from near the summit of Brandreth, March 2021.  
Buttermere and Crummock Water taken from between Grey Knott and Brandreth.  March 2021. 

Working their passage …

My February visit to Cassop Pond was a warmer experience than my January visit, though copious rain meant that the footpaths we followed to reach the reserve were extremely muddy.  I’ve been gazumped in my endeavour to bring you the phycological riches we encountered on this trip by Heather, who has already mentioned the two species of Trentepohlia that we noticed growing on the wall of an abandoned quarry in her post about the visit.  However, I still managed to find plenty to amuse me in the pond itself. 

On first glance, the pond still does not look very prepossessing.  The ice that had covered the pond during our January visit had gone, but there were still few signs of life above the water level.  It is only when we look below the surface that we find evidence of activity.   Standing at the edge of a marshy area, churned up by the herd of highland cattle who roam the reserve, I can see greenish flocs which, from a distance, they look like filamentous algae.  Up close, however, they are a tangle of narrow, branched strap-like fronds.   This is Riccia fluitans, one of just two free-floating liverworts found in Britain (the other one, Ricciocarpus natans, is also found here, but that’s a story for another day).   In my photograph, you’ll also see a few leaves of Lemna minor, duckweed.  That will become way more prolific later in the year.  Today, Riccia fluitans is the star of the show.

Flocs of the liverwort Riccia fluitans floating just below the surface in Cassop Pond, February 2021.
A close-up of the floc of Riccia fluitans in Cassop Pond.  Each frond is about a millimetre wide.

When we move in even closer and peer at Riccia fluitans through a microscope, the reason for its buoyancy becomes clear: there are a number of floatation chambers.  This, in turn, creates the impression that the cells that contain chloroplasts are forming sinuous bands along the thallus.   I had never thought of chloroplasts as being particularly heavy, but many aquatic plants are parsimonious in the way that they deploy their photosynthetic arsenal.  We saw this in the moss Sphagnum (see “Back to the bog”) and also, last year, in Potamogeton polygonifolius (see “The dark side of the leaf …”)

The thallus of Riccia fluitans viewed under a microscope, showing the mix of green photosynthetic cells and empty cells which help it to remain close to the surface. The thallus is about half a millimetre across.

One other characteristic that Riccia fluitans shares with Potamogeton polygonifolius (and, for that matter, Lemna minor: see “Cassop”) is that it carries passengers.   Cranking up the magnification on my microscope, I can just make out the outlines of several cells of the diatom Epithemia.  I’ll write more about the diatoms from Croft Kettle at a later date but Epithemia is an interesting find here because it is a diatom that is capable of nitrogen fixation (see “More about Croft Kettle”).  I’ve recorded Epithemia from Cassop Ponds in the past, but never noticed its affinity for Riccia fluitans before.   Rooted plants can draw on nitrogen in the sediments to meet their needs; however, free-floating plants depend on what can be acquired from the water.  We also know now that many aquatic organisms are quite leaky so an alga that has a trick like nitrogen fixation up its sleeve may be serving as a food bank for the host organism.   It might just be that the buoyancy that Riccia fluitans provides is a quid pro quo for these nutrients.   This looks like being a mutually beneficial relationship: this small corner of nature being less “red in tooth and claw” than “you scratch my back, I’ll scratch yours” …

Epithemia species just visible on the lower surface of Riccia fluitans from Cassop Pond.  The two cells at the centre of the image are each about a tenth of a millimetre long. 

Some other highlights from this week:

Wrote this whilst listening to: Kin Sonic by Congolese musicians Jupiter & Okwess.   

Cultural highlights:  The Martian, which is basically about growing potatoes in inhospitable environments.  “Fear my botany powers, Mars!”

Currently reading:  My Last Supper: One Meal, a Lifetime in the Making by Jay Rayner.  

Culinary highlight: Persian kababs in a meal box from Berenjak’s restaurant in London.