Nostoc

by

Landscape architects …

As my teaching in Galway straddled a weekend, we took advantage of the location and snuck off to the Burren for a couple of days where, fortuitously, the sun shone and our waterproofs stayed packed in the bottoms of our rucksacks.  The Burren is an area of limestone geology mostly in County Clare in the west of Ireland, noted for its botanical diversity.  Some of these plants (such as spring gentian – Gentiana verna) we already knew from Upper Teesdale but the Burren is much larger and the mild Atlantic climate allows some unique plant assemblages to thrive. 

Of particular interest are the extensive areas of limestone pavement, whose grikes are crowded with the wild flowers for which the area is famous.  This is a classic feature of Karst landscapes influenced by glaciers.   In Britain, the best-known site is Malham Tarn in north Yorkshire, though the pavements themselves did not make it into my most recent series of posts from that region (see “Building landscapes …”). But I’m in Ireland now, looking out at limestone pavement on a scale that dwarfs anything Yorkshire has to offer and I’ve also found a way of weaving this landscape feature into a blog primarily concerned with algae… 

The surface of the limestone pavement is pitted with solution hollows – depressions typically 30 cm or more in diameter that probably formed at points where rainwater collected, starting a slow process of chemical and physical erosion.   The mild acidity of rainwater gradually dissolves the limestone, but achingly slowly – probably no more than five millimetres of limestone is removed every 100 years – so the hollows that I photographed for this post were several thousand years in the making.    

A solution hollow (“kamenitza”) on limestone pavement near Mullaghmore, June 2022.   The photograph at the top of the post shows limestone pavement on the hillside above the Caher valley in the Burren, with the hills of Connemara in the far distance.   

There were unprepossessing dark-brown lumps at the bottom of several of these which could easily have passed for some type of animal droppings.   Close-inspection, however, showed these to have a gelatinous quality rather than the fibrous nature we might expect if these were droppings.   These are colonies of the cyanobacterium Nostoc commune, a frequent subject on this blog.   The last time I wrote about these in detail I explained how neglected patches of Nostoc on the gravel roadway of a Cumbrian farm were the pioneer stage of plant succession (see “How to make an ecosystem (2)”) and we are seeing exactly the same process happening here on the Burren.   First, the tough, resilient colonies of Nostoc arrive, capable of surviving both the high radiation experienced on the light-coloured limestone and the long periods of desiccation.   Nostoc, moreover, has the ability to fix atmospheric nitrogen, effectively acting as a natural “fertilizer”.  Slowly, over time, the Nostoc traps windblown soil and creates a “soil” within which mosses and higher plants can take root.   The early arrivals are likely to be grasses and sedges, but we saw a wide range of plants, including orchids, growing in solution hollows.

Close-ups of the solution hollow from the previous photograph, showing Nostoc commune growing in the bottom.

We mostly live in places where the local rock has already been weathered into soils, or where other natural processes have dumped weathered rock from elsewhere.  So we rarely stop to consider where soil comes from.   It is only in a few places such as a limestone pavement that we can see the primordial stages of soil – and, therefore, habitat – creation taking place under our noses.   Even here, the process is very slow but, nonetheless, we can stand on the Burren and reflect that at one point in the past, everywhere was bare rock, and if it were not for unphotogenic brown blobs that might pass for animal droppings, it would never have been converted into soil at all.

Nostoc commune from solution hollows on limestone pavement in the Burren, May 2022, at two different magnifications.  Scale bar: 10 micrometres (= 1/100th of a millimetre).  

References 

Cabot, D. & Goodwilie, R. (2018).  The Burren.  New Naturalist, HarperCollins, London. 

Doddy, P. & Roden, C.M. (2014).  The nature of the black deposit occurring in solution hollows on the limestone pavement of the Burren, Co. Clare.   Biology and Environment: Proceedings of the Royal Irish Academy 114B: 71-77.

Doddy, P. & Roden, C.M. (2018).  The fertile rock: productivity and erosion in limestone solution hollows of the Burren, Co. Clare.  Biology and Environment: Proceedings of the Royal Irish Academy 118B: 1-12.

Some other highlights from this week:

Wrote this whilst listening to: musical methadone to ease the cold turkey arising from a week and a half in Ireland.  In particular, Glen Hansard’s band The Frames and The Corrs.  What’s their connection? (answer at the bottom)

Currently reading: David Cabot and Roger Goodwillie’s New Naturalist volume on The Burren.

Cultural highlight: while I was in Ireland, I started watching the adaptation of Sally Rooney’s Conversations with Friends on the RTE Player.

Culinary highlight:  Dinner at The Cliffs of Moher hotel at Liscannor featuring a baked fillet of John Dory and a rather good fondant potato followed by a splendid sunset.

Answer: Glen Hansard and The Corrs all had roles in the 1991 film The Commitments.

by

Microscopic mysteries in Cassop Pond …

I’ve been following the composition of algae in and around flocs of Riccia fluitans all year and have now tried to follow-up my picture of the algae in Pond Politics, in which I showed the liverwort and duckweed with attendant diatoms from samples collected in April, with one showing the situation in late summer.   In the meantime, we have seen Cladophora glomerata come and go (see: “Cassop Pond in June”) followed by the arrival of Nostoc.  In my post Change of Tenants I found filaments apparently free-floating and very similar ones attached to the fronds of Riccia fluitans.   I had wondered if the free-floating filaments were Anabaena sp. but now think that they are more likely to be Nostoc, mostly living in and around the Riccia fronds but detached by the various manipulations required to get a sample of the Riccia floc from the pond, back home and then under my microscope.    

We can get a better idea of the relationship between these filaments and the liverwort from some photographs that Chris Carter took for me.   Previous encounters with Nostoc in this blog have been with terrestrial species that form well-defined firm colonies, readily visible with the naked eye (see “How to make an ecosystem (2)”).   This is different: the colonies are, at best, ill-defined and on a scale not easily seen without magnification.   Based on the descriptions in the Freshwater Algal Flora of the British Isles, I think that these belong to Nostoc coeruleum, but Brian Whitton (who wrote the account) urges against placing too much confidence on differentiations made on the basis of often vague descriptions in the literature.   The habitat notes for N. coeruleum do, however, match the conditions in Cassop Pond: “In standing fresh or slightly brackish water, among submerged plants, free-floating or loose on bottom sediments …” 

Nostoc cf coeruleum on fronds of Riccia fluitans from Cassop Pond, Co. Durham, July 2021.  Photographs: Chris Carter.

Based on these photographs, it looks like Nostoc forms a layer on the surface of the Riccia leaves, gaining proximity to light in return for sharing the nitrogen it fixes from the atmosphere.   Curiously, I did not see Nostoc growing amongst the Lemna fronds, which suggests some host-specificity in the relationship.  Whether it is correct to describe this as a “symbiosis” rather than a mutually-beneficial interaction is not clear and would take rather more work.   That I did not see Nostoc until the middle of the year suggests that this may just be an opportunistic association rather than a long-term relationship.   

My picture tries to capture some of what is happening in these flocs.  Because the flocs grow in and amongst Phragmites, the left hand side of the picture has the stem of Phragmites along with some diatoms attached (Gomphonema, Epithemia and Achnanthidium).  On the right-hand side, by comparison, there are fronds of Riccia fluitans and associated Nostoc coeruleum.   There are also some diatoms – Cocconeis and Rhoicosphenia – growing on the Riccia fronds.   Finally, there are some floating leaves of Lemna minor, each with a hanging root.  Once again, there are diatom epiphytes – Cocconeis on the underside of the leaves and Fragilaria on the roots.  Finally, I have included some flagellated cells of the euglenophyte Trachelomonas, which are frequent amongst the Riccia fronds at this time of year (see also: “Puzzling puddles on the Pennine Way“).  It is quite a “busy” image but still, with eleven species represented, it is a highly simplified version of what is actually present in the margins of Cassop Pond.  It is also a snapshot – a moment in time – whereas the real assemblages will be forever shifting with the vagaries of wind and local disturbance, as well as changing in response to the seasons. 

Some other highlights from this week:

Wrote this whilst listening to:    Lambchop’s 1996 album How I quit smoking.  

Cultural highlights:  The Green Knight, new film adaptation of the Arthurian legend Sir Garwain and the Green Knight.

Currently reading:  Between the Assassinations by Arvinda Adiga, a. novel about a fictional town in India in the 1980s. 

Culinary highlight:  The bar is set fairly high hereabouts and I usually forget the basics, such as fresh sourdough bread with jam made from fruit from the allotment.  Not by me, I hasten to add.   

by

How to make an ecosystem (2)

Ennerdale_Apr19

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

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

Nostoc_commune_Ennerdale_Apr19_#1

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

Nostoc_commune_x1000_Apr19_#1

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

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

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

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

Nostoc_commune_Ennerdale_Apr19_#2

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

Reference

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

by

Nosing around for blue-green algae …

One of the ironies of teaching a course on algal identification in the Lake District is that we actually take the students out of the Lake District on the first field trip in order to introduce them to the enormous variety of Cyanobacteria (blue-green algae).   This is because the southern part of the Lake District, where the FBA is located, is situated on the Silurian Slates, which means that the streams, lakes and tarns have fairly soft water.   Cyanobacteria, on the other hand, tend to be most abundant and diverse in hard water, so we drive about 40 minutes south and east from Windermere to a limestone escarpment called Whitbarrow, where there are a number of calcareous flushes and springs that are ideal for our purposes.

We always visit the floor of an abandoned quarry in this area which has several such flushes. The quarry owners had systematically removed the limestone until they had reached the Silurian Slate underneath. This, in turn, formed an impermeable layer that intercepted any water that had percolated through the limestone. The quarry floor was, typically, slippery with calcium-rich water that had seeped out from the surrounding limestone, as well as the mucilage that the algae produced.   There were also, dotted around, several unprepossessing brown objects that, to the untrained eye, could easily be mistaken for the droppings of a small animal or bird (a Peregrine falcon was circling overhead during our visit). Once the students have the courage to pick these up, they see that they are composed of a firm jelly-like substance that is, we persuade them, actually an alga and should, therefore, be dropped into one of their specimen tubes to take back for closer investigation.Nostoc_Whitbarrow_May2014

Animal, vegetable or mineral?   Colonies of Nostoc commune on the floor of Whitbarrow Quarry, May 2014.

Once back in the FBA’s laboratory (complete with panoramic views of Windermere), we can dissect out small pieces of the jelly-like material and squash it onto a microscope slide.   What they see when they peer down their microscopes is a plethora of chains of bead-like cells of a Cyanobacterium called Nostoc commune.   Most of the cells have contents that have a granular appearance, with a background of bluish-green photosynthetic pigments.   A few of the cells, however, are rounder and clearer: these are the “heterocysts”, cells that are especially adapted to “fix” atmospheric nitrogen and so help the organism survive in nutrient-poor habitats.   The jelly-like matrix slows the rate at which water evaporates from the colonies, with the outer layers drying to form a tough, leathery skin around the colony.

Nostoc_commune_stack_#1

Nostoc commune from Whitbarrow Quarry under the microscope.   The cells are approximately 5 micrometres (1/200th of a millimetre) across.<br

There is a fascinating short paper by Malcolm Potts on the origin of the name “Nostoc”.   Because Nostoc colonies often appeared very quickly following heavy rain (because the dried colonies absorb water quickly and swell in size), there was a belief in Medieval times that Nostoc fell from the sky.   A German mystic and alchemist, Parselus, was the first to use the name “Nostoc”, claiming that it was “…excrement blown from the nostrils of some rheumatick planet.   The name, indeed, is strongly suggestive of both the Old English word Nosthryl and the German term nasenloch both, as Potts politely explains, “…that part of the human anatomy intimately associated with extracellular polysaccharide.”

Reference>

Potts, M. (1997). “Etymology of the Genus Name Nostoc (Cyanobacteria)” (pdf). International Journal of Systematic Bacteriology 47 (2): 584. doi:10.1099/00207713-47-2-584