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.