Fade to grey …

Priestclose_pond_July19

Prudhoe is a small town in Northumberland whose most famous inhabitant doesn’t exist*.   I came here to have a look at a pond in Priestclose Wood, a nature reserve operated by Northumberland Wildlife Trust which hit the local headlines recently for a suspected pollution incident.  You can see the scum on the surface in the photograph above and it does have an oily appearance, so anyone might be forgiven for calling the Environment Agency and asking them what was going on.

The query worked its way through the Environment Agency and ended up in my in-box in the middle of last week, with a specimen falling onto my doormat a few days later.   Having had a good look at it through my microscope, I drove out to the pond on a damp afternoon to take a look myself.   It is just a small pond, perhaps 30 metres across, set amidst the oak, birch and rowan-dominated woodland, which means that much of the lake is in almost permanent shade and, perhaps more important for the development of surface films, sheltered from the wind.  The surface film was just as I had been led to expect, despite the efforts of folk from the wildlife trust tried to disperse it last week in case the newts which lived here were threatened.   It was greyish-brown in colour, and covered the entire surface.  When I stirred it with a twig, it broke up, quickly closing up again as the water settled.   I then skimmed a sample bottle across the surface layer and harvested a yellow-brown suspension which I brought home for a closer look.

Priestclose_scum_July19

The Chromulina scum on the surface of the pond in Priestclose Wood: the left hand image was taken after stirring the surface with a stick to break up the oily layer; the right hand image shows the golden-brown algae that I scooped from the surface.  The picture at the top of the post shows the pond with its covering of algae.

The fresh sample was dominated by large numbers of tiny cells darting around.  Closer observation showed this to be oval, with a single yellow-brown cup-shaped chloroplast and what looked like one flagellum.   Over time, however, these cells slowed down, became rounder and started to aggregate in groups.   These, rather than the motile cells, proved rather easier to photograph.   I suspected that we were looking at a Chrysophyte, and Dave John later confirmed it to be Chromulina ferrea (the chloroplasts lack a pyrenoid, otherwise it would be C. aerophila).   If that is the case then there will be a second, much smaller flagellum too, but which is much harder to see with the light microscope.

Both of these species were described by John Lund in 1942 from ponds in Richmond Park whilst he was a PhD student at Queen Mary College London.   They are “neustonic”, meaning that they are adapted to live at the air-water interface, which also explains why they form the surface film that we saw in the pond at Priestclose Wood.   John Lund gives a detailed description of just how the behaviour of the alga leads to the formation of these films.   However, apart from John Lund’s original observation, the only other record in the British Freshwater Algal Flora is from a pond near Orpington in Kent, close to Dave John’s house.   Such is the nature of phycological records: it is not necessarily the algae that are rare so much as the people who notice them.

Chromulina_ferrea_Prudhoe_July19

Chromulina cf ferrea from the pond in Priestclose Wood, Prudhoe, Northumberland, July 2019.   The left hand image shows a clump of sessile cells, photographed at 400x magnification; the right hand images show sessile cells at x1000 using brightfield (upper) and phase contrast (lower) illumination.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 

The local paper comments that the pond usually has a covering of duckweed at this year and blames the algae for killing this off.  The reality may be more complicated: duckweed (Lemna minor) can appear and disappear rapidly in a pond without any obvious cause (see “The green mantle of the standing pond …”) so it is equally possible that the duckweed disappeared for an unrelated reason (a virus, perhaps?) and this created an opening into which the Chromulina was able to expand.   We’ll probably never know the truth.   Maybe the duckweed will be back next year; maybe not.

Looking back at earlier posts, I see that the only other time a chrysophyte was the subject, I ended bemoaning circumstances where the these alga were both a “natural” part of the habitat’s biota whilst, at the same time, lacking in aesthetic appeal (see “A brief excursion to Norway”).   The same situation seems to apply here: an otherwise attractive woodland pond now covered with a greyish film which is, as far as I can tell, a “natural” phenomenon.  It is a shame if these are the only times that the lay-public encounter the chrysophytes as some of them are very beautiful under the microscope.   But, at the same time, the is no law that says nature has to be pretty.  Maybe it is our preconceptions that sometimes need adjusting …

* Ruth Archer, from the BBC Radio 4 series The Archers

References

John, D.M., Whitton, B.A. & Brook, A.J. (2011).  The Freshwater Algal Flora of the British Isles.  2ndedition.  Cambridge University Press, London.

Lund, J.W.G. (1942). Contributions to our knowledge of British Chrysophyceae.  New Phytologist41: 274-292.