My post about John Snow pointed out that he made the link between contaminated water and disease without actually knowing what we now know about germs. In effect, Snow had made an inference based on the association between cases of cholera and the closest pump but correlation, as we tell our students, is not the same as causation. Elsewhere in London and beyond, others were desperately searching to identify the culprit itself.
Various theories had been put forward, dividing roughly into those suggesting a chemical origin and those suggesting a biological cause. One of the proponents of the latter was a doctor called Arthur Hill Hassall who looked down his microscope at samples he had collected from the reservoirs which supplied London’s drinking water and thought he had found the answer. The drawings he published in The Lancet show water teeming with algae and if this sounds preposterous, remember that this was still 20 years before Pasteur and Koch discovered bacteria, a group of organisms far too small to be seen with the microscopes available to Hassall.
Hassall published the first authoritive guide to the freshwater algae of Britain and described several new species including a diatom called Asterionella formosa which is very common in the plankton of lakes in the spring. The Latin name translates as “beautiful little star” and finding it in a sample always brings a wry smile to my face, as I recall the walk-on part this and other algae played in the story of the struggle to unravel the causes of cholera.
Asterionella formosa collected from Dannemarche Reservoir in Jersey in June 2013 by Dave John. The scale bar indicates 10 micrometres (1/100th of a millimetre).
The individuals photographed here come from a reservoir in Jersey. I spend a day each year teaching on an algal identification course based in Durham. It relies on water samples brought along by the tutors and participants which means that there is always a rich assortment of material from all over the country to examine. I was checking this sample before the class when I noticed the beaded appearance of the Asterionella. Under higher magnification, these “beads” resolved into yet tinier organisms, unicellular fungi called “chytrids” which had infected the alga.
Just as cholera was able to spread rapidly through the densely-populated regions of London in the nineteenth century, so chytrids thrive when Asterionella is most abundant. It is a reminder that diseases and infections are a natural feature of animal and plant populations, not just human scourges and, indeed, are an entirely natural way of regulating population numbers:
Little bugs have littler bugs
upon their backs to bite ‘em.
And littler ones have littler ones,
and so on, ad infinitum …