Friends from the local Environment Agency office got in touch this week with a query about a red colouration in the water in a pond in a park in North Shields, which they suspected to be an algal growth of some kind. Their preliminary observations were sufficiently intriguing that I downed tools on Friday afternoon and headed out to take a closer look.
Two views of the red bloom in Chirton Pond, August 2016 (photos: Tina Flinn)
The photographs show just why passers-by were concerned: large blood-red areas of water, clearly visible from the banks. There were also, apparently, records of dead fish and ducks although there were ducks and moorhens swimming and feeding, apparently unaffected, by the time of my visit. A single droplet of this water, viewed under the microscope, was teeming with tiny fast-moving red cells. However, naming the organism responsible proved to be more problematic. I started with a short-list of algae that live suspended in lakes and ponds, with one of the most likely candidates being Euglena sanguinea. I wrote about Euglena last year (see “A visit to Loughrigg Fell”): E. sanguinea has, as the name suggests, a blood-red, rather than green colour, and is very widespread in ponds and other standing water bodies. However, a very quick look down the microscope ruled this out on two counts: the cells in my sample were too small (rarely longer than 12 micrometres, except cells close to division, as in the illustration below) and they also lacked the characteristic shape-changing cell wall (“pellicle”) of this genus. Close observation also suggested that these cells were prokaryotic (i.e. lacking any differentiation into internal organelles) rather than eukaryotes. So it was back to the drawing board …
Chromatium cf okenii from Chirton Pond, North Shields, August 2016. Left: a sample of water from one of the blooms; right: two individual cells. The arrow points to a flagellum on the upper cell, but note that there is also a very thin Cyanobacterium filament alongside the cell. Scale bar: 10 micrometres (= 1/100th of a millimetre).
Most of the cells were moving too quickly to allow their details to be seen or photographed clearly, and my photographs were of the small number of sessile cells, each of which had a purplish-red interior, along with a number of refractive inclusions. I was, in fact, looking not at an alga at all, but at a purple sulphur bacterium, probably of the genus Chromatium. The purple colour arises from a mixture of bacterial chlorophyll and carotenoids and, despite the similarity in appearance, they do not contain the phycobilin pigments that are characteristic of Cyanobacteria (blue-green algae). We are, in other words, looking at a completely different evolutionary lineage than anything I’ve written about in this blog previously.
Whereas photosynthesis, as practised by the green plants all around us, uses carbon dioxide and water and produces sugar and oxygen, the purple sulphur bacteria use carbon dioxide and hydrogen sulphide, and they do not produce oxygen. Instead, they produce sulphur as a waste product and, as this is a potentially-toxic compound, it is “wrapped up” in a protein layer to create the refractive inclusions that I could see inside the cells. Another feature of the photosynthesis practised by purple sulphur bacteria is that it can only take place in anaerobic conditions and this offers one possible clue to the reports of dead ducks and fish.
I did not take time to look at the sediments at the bottom of Chirton Pond but can take a guess that they are a dark oozy mass, largely composed of the decaying remains of the submerged plants that filled the pond. The bugs that are slowly digesting this submerged “compost” will suck the oxygen from the sediment faster than replacement oxygen can diffuse in from the overlying water. This, in turn, will encourage the formation of the hydrogen sulphide that Chromatium needs as its feedstock. The warm weather we are enjoying will encourage the water plants to photosynthesise and produce oxygen by day but, by night, these plants, along with all the other organisms in the pond, will be using all of this oxygen for respiration. Even during daylight, there will be areas of the pond where the demand for oxygen will exceed the supply that the plants can pump out and which can diffuse in from the air above the pond. Together, this will create the anerobic, sulphide-rich conditions that Chromatium needs.
Bearing in mind the shortage of oxygen in the pond and remembering that sulphide is, itself, toxic, the anecdotal deaths of ducks and fish is likely to be due to the same set of factors that led to the Chromatium bloom, rather than a consequence of the bloom itself. I did find a few reports of toxins produced by species Chromatium but not enough to make me think that this is a widespread problem. There are easier explanations for the problems at hand and, probably, fairly easy solutions were the problem to persist. Some judicious weeding and, possibly, dredging would reduce the amount of organic matter in the bottom of a pond and a nice ornamental fountain would keep the water well oxygenated. Then, of course, we could always wish for a cool damp August which would stop those pesky plants and bugs using up all that oxygen with their reckless respiration …
Note and references
Thanks to Dave John for pointing me in the direction of Chromatium when I was first trying to name this organism
Hurlbert, R.E. & Hurlbert, I.M. (1977). Biological and physicochemical properties of the lipopolysaccharide of Chromatium vinosum. Infection and Immunity 16: 983-994.
Nicholson, G.L. & Schmidt, G.L. (1971). Structure of the Chromatium sulfur particle and its protein membrane. Journal of Bacteriology 105: 1142-1148.