The little tarn of horrors …

In addition to desmids, we found several other algae in the samples collected from Cogra Moss.  One of these consisted of colonies of cells in mucilaginous masses attached to floating mats of vegetation (which looked like terrestrial grasses).  We decided that these were probably Chrysocapsa epiphytica, the second representative of the Chrysophyta I’ve described in this blog this year (see also “Fade to grey …”).  As is the case for Chromulina, much of what we know about Chrysocapsa epiphytica is down to the patient work of John Lund who first described this species back in 1949.

Chrysocapsa_epiphytica

Colonies of Chrysocapsa epiphytica growing on submerged vegetation at Cogra Moss, Cumbria, September 2019.  Cells are 7.5 – 15 micrometres long and 7.5 – 12 micrometres wide. 

He described the various mucilaginous lobes as “reminiscent of the …. human brain”.  The spherical, oval or ovoid cells form a layer, two to four cells deep, at the surface of the colony.   The cells have the typical yellow-brown colour of chrysophytes and, though it is hard to see the chloroplasts in this photograph, John Lund says that there are usually two, sometimes four, in mature cells.

Its presence in a soft-water lake probably means that it is a species that relies on dissolved carbon dioxide rather than bicarbonate as its raw material for phytosynthesis (see “Concentrating on carbon …” for some background on this).   We know, from laboratory studies, that most chrysophytes rely exclusively on carbon dioxide, and lack the capacity to use bicarbonate.  This confines them to water where the pH is low enough to ensure a supply of carbon dioxide (the chemistry behind this is explained in “Buffers for duffers”. It may also explain why Chromulina lives in surface films rather than submerged in the pond (the locations where we’ve it found are unlikely to have sufficiently low pH).

One extra twist to the story is that many chrysophytes are “mixotrophic”, meaning that they can switch between using photosynthesis as a means of getting the carbon they need to grow from inorganic sources, and “feeding” on other organisms.  Our Chrysocapsa epiphytica, in other words,  has parked itself beside a convenient supermarket of pre-packaged carbon in the form of decaying vegetation and associated bacteria which it then ingests by a process known as “phagotrophy”.

Phagotrophy is, in fact, a very ancient characteristic, insofar as the very first eukaryotic cells were the result of Cyanobacteria-type cells being ingested by larger heterotrophic cells and being retained as on-board “energy farms” rather than digested and treated as one-off vegetarian dinners.   However, the shift to a permanent role for chloroplasts within a eukaryotic cell involved a lot of rewiring of intercellular machinery, and effectively “switching off” the intercellular mechanisms involved in phagotrophy.   Retaining the ability to “feed” on bacteria alongside a capacity for photosynthesis is the cellular equivalent of a hybrid car: there is a lot more to cram under the bonnet.  Flexibility, in other words, comes at a cost.

On the other hand, phagotrophy does not just result in extra carbon for the Chrysocapsa cells in Cogra Moss.   In an oligotrophic tarn such as this, the extra nutrients that are obtained when the bacteria are absorbed will also be a useful boost.   Once again, though, you can see that, in environments where nutrients are more plentiful, the cost to the cell of maintaining the equipment required for phagotrophy outweighs the benefits.

I’m sure that a close inspection of the land around Cogra Moss would have revealed insectivorous plants such as Drosera(sundew) and we also recorded Utricularia minor, an aquatic insectivorous plant, in another tarn we visited whilst desmid-hunting (see “Lessons from School Knott Tarn”).  Chrysocapsa is, in many senses, a microscopic equivalent of these carnivorous plants.   OK, so it has a taste for bacteria rather than flesh but, somewhere out there, there must be a sub-editor in search of a headline …

References

Lund, J.W.G. (1949). New or rare British Chrysophyceae. 1.  New Phytologist48: 453-460.

Maberly, S. C., Ball, L. A., Raven, J. A., & Sültemeyer, D. (2009). Inorganic carbon acquisition by chrysophytes. Journal of Phycology 45: 1052-1061. https://doi.org/10.1111/j.1529-8817.2009.00734.x

Raven, J. A. (1997). Phagotrophy in phototrophs. Limnology and Oceanography 42: 198-205. https://doi.org/10.4319/lo.1997.42.1.0198

Saxby-Rouen, K. J., Leadbeater, B. S. C., & Reynolds, C. S. (1997). The growth response of Synura petersenii(Synurophyceae) to photon flux density, temperature, and pH. Phycologia 26: 233-243. https://doi.org/10.2216/i0031-8884-36-3-233.1

Saxby-Rouen, K. J., Leadbeater, B. S. C., & Reynolds, C. S. (1998). The relationship between the growth of Synura petersenii (Synurophyceae) and components of the dissolved inorganic carbon system. Phycologia 37: 467-477.  https://doi.org/10.2216/i0031-8884-37-6-467.1

Terrado, R., Pasulka, A. L., Lie, A. A. Y., Orphan, V. J., Heidelberg, K. B., & Caron, D. A. (2017). Autotrophic and heterotrophic acquisition of carbon and nitrogen by a mixotrophic chrysophyte established through stable isotope analysis. ISME Journal. https://doi.org/10.1038/ismej.2017.68

 

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.