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).

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

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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.

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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.

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.

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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