Pleasures in my own backyard


One of the delights of my part of County Durham is the range of natural history that is available without the need to travel great distances.  That, indeed, has been the theme of this blog right from the start (see “Cassop”) and today’s post continues the theme of nature on my doorstep, with a visit to a local nature reserve within walking distance of my house.  Like Cassop Pond, it is at the foot of the Magnesian Limestone escarpment and, at this time of year, the grassland is rich with Northern marsh and Common spotted orchids.   It is, of course, the ponds that draw my attention: they are rich in aquatic plants including, once I start to look closely, beds of the alga Chara, which I’ve written about before (most recently in “Everything is connected …”).  And then, once my eyes are adjusted to looking at natural history at this more intimate scale, I can see that the stones on the bottom of the pond are covered with tiny snails (probably Hydrobiidae) with shells coiled in the shape of an ice-cream cornet.  Freshwater snails crawl across submerged surfaces rasping off attached algae with their tough radula so I started to wonder what snails in this particular pond might be feeding upon.


Submerged stone from the pond at Crowtrees Nature Reserve, County Durham, covered in Hydrobiidae snails (left: the stone is about 10 cm across) and (right) a stone removed from the bottom of the pond showing the marl-covered part that was exposed and the marl-free part that was buried in the sediment. 

Viewed from just above the water, the surface of the stone looked as if it could be an algal film but, when I picked it up, the stone did not have the yielding texture that I associate with such films, but was a hard, mineral-rich marl.  More intriguingly, it was only present on the exposed surfaces, possibly, I suspect, due to the subtle interactions between chemistry and biology that I wrote about in “Everything is connected …”.

The calcite crystals make it hard to get a good view of the material under the microscope, but I managed to see a number of diatoms, mostly Gomphonema pumilum, or a relative, but also a good number of tiny, slightly asymmetric cells of a species of Encyonopsis, a genus that was, until recently, included in Cymbella, and which is usually a good indication that the water is about as untainted by human influences as it is possible to get.   It is, however, hard to get a really clear view of these under the microscope as they were scooting around.   With valves that are barely more than a hundredth of a millimetre long, I really needed to use an oil immersion objective to see them clearly, but the calcite crystals on the slide made it almost impossible to get a clear view of the live cells.  Not surprisingly, most of what we know comes from studies of carefully-cleaned preparations of the empty frustules.   Encyonopsis shares with Tyrannosaurus Rex the distinction of being an organism better known dead than alive.   It is rather ironic, given that healthy populations are living so close to my house, but that’s very often the case with diatoms.

There was one other abundant alga living amidst the rock (and, indeed, probably the major food source of the snails), but I am having some problems giving it a name, so a full account of that one will have to wait until another day.


Diatoms at Crowtrees Nature Reserve, July 2016: a.-d.: Gomphonema (possibly G. pumilum) in girdle and valve views; e.-g.: Encyonopsis sp.   Scale bar: 10 micrometres (= 1/100th of a millimetre).


You must be taking the piss?


You can find algae growing in the most unlikely places.   Look at the photograph above.   To the left you can just see The Swan and Three Cygnets, a popular pub beside Elvet Bridge in Durham.  The arches of Elvet Bridge extend onto the bank of the rivers, and one sits just beside the pub (hidden, in this photograph, by a large tree).  Under this arch, growing on the wall in a patch from approximately 30 centimetres above the ground, there is a lush patch of green algae.

Think about it: pub; arch nearby that is sheltered from the elements and from the gaze of passers-by; patch of algae from slightly below waist height downwards.  The technical term amongst ecologists for algae that like such habitats is “nitrophilous”: an alga that thrives in environments where the nitrogen concentrations is elevated.   Do I need to spell it out?  Pub, beer, sheltered archway, waist height …

I do not usually lurk around such unsavoury environments, but I was looking for a particular genus of green algae called Prasiola, which is known to favour habitats such as these.  Indeed, I have in front of me a paper from a learned journal that states “… consistent dampness and nitrogenous availability from animal wastes is likely to be the primary determining factor” for those trying to understand the distribution of this genus.


Left: the arch of Elvet Bridge with the Swan and Three Cygnets pub to the left; right: a patch of Rhizoclonium cf riparium on the north side of the arch.  Top image shows the Elvet Bridge with Durham Cathedral and Castle in the background.

To the naked eye, these patches of green algal growths on the damp, shaded arches, confirmed my pre-conceptions, so I was a little surprised when I put a small piece pulled from one of these under my microscope and saw unbranched filaments of cells, instead of the flat sheets that are characteristic of Prasiola.  The combination of unbranched filaments with a single net-like chloroplast indicates that this belongs to the genus Rhizoclonium, and Chris Carter, when he examined the material, suggests it is R. riparium, a common species of brackish and marine environments, including habitats such as salt-marshes where it will be periodically exposed to the atmosphere.  These patches of Rhizoclonium are like mini-vertical saltmarshes, subject to occasional immersion in squirts of high conductivity liquid, which will then be retained within the tangle of filaments.  Gradual evaporation from these patches (limited, due to the shaded microhabitat within the arch) will make the residual liquid yet more concentrated, and offering a selective advantage to an organism adapted to coping with salinity fluctuations in estuarine environments.


Rhizoclonium cf riparium from a damp arch of Elvet Bridge, Durham, April 2016 (photograph: Chris Carter)

Fabio Rindi and Mike Guiry made a particular study of algae associated with subaerial environments a few years ago but, curiously, their papers include no records of Rhizoclonium.   By contrast, I have so little experience of these habitats that I cannot say whether this was just a lucky (or unlucky – I was looking for Prasiola, remember) coincidence.  However, it does serve as an intriguing reminder that algae grow in some very unlikely places.  The problem is not that algae can’t grow on urine-soaked walls; it is that not many of us are interested in examining the natural history of such unsavoury habitats.


Rindi, F. & Guiry, M.D. (2003).  Composition and distribution of subaerial algal assemblages in Galway City, western Ireland.   Cryptogamie, Algologie 24: 245-267.

Rindi, F. & Guiry, M.D. (2004).  Composition and spatial variability of terrestrial algal assemblages occurring at the bases of urban walls in Europe.   Phycologia 43: 225-235.

Rindi, F., Guiry, M.D., Barbiero, R.P. & Cinelli, F. (1999).  The marine and terrestrial Prasiolales (Chlorophyta) of Galway City, Ireland: a morphological and ecological study.   Journal of Phycology 35: 469-482.