More about tastes and odours in water supplies

22 February 2012

Anabaena solitaria, showing a heterocyst and an akinete (Photo: Bill Harding)

Algae-produced tastes and odours in drinking water have been much in the news in the Western Cape during the past few days.  In this particular instance we are talking about the compound geosmin, produced by blue-green algal (cyanobacteria) species such as Anabaena solitaria –  the photo above shows a portion of an algal filament sampled from a dam near Paarl, north-east of Cape Town, this week.

As far as we know, geosmin in itself is harmless – and some ‘Wikipedia’ experts – those who can apparently morph seamlessly through scientific disciplines outside of their experiential comfort zones, will hastily tell you this.  So what?  It’s about the same as saying cobra urine is harmless.  It may be, but what’s at the other end certainly is not.  They will also go to great pains to repeat mythology that the compound is only released when the algal cells are dying.  This is altogether true, untrue and irrelevant.  Geosmin abounds in waters with actively-growing Anabaena populations where the hormogonia – the short bits of filaments that break off and form new ones, are being produced en masse [Wikipedia needs some updating on this particular topic but it does help to validate the credibility of some ‘experts’ who trot it out verbatim].  Dying cyanobacterial cells also release toxins, by the way.

The actual, relevant, point is that the presence of geosmin tells you that blue-green algae are present in the water, i.e. there could be cyanotoxins present.  It also points to the fact that something, often pollution, is supporting excessive growth of blue-green algae.

So, to boldly state that geosmin is not a health problem is being disingenuous.  This statement only holds water if it is qualified by something along the lines of:  “we are aware of the risks associated with the presence of cyanobacteria in water supplies and we have tested the water for the known toxins produced by this particular species and found that…or we have not but we have posted warnings… etc etc”.

For those interested, the photo above shows the filament cells (some actively dividing into two), an akinete (the large, dense, granular cell) and a heterocyst (the translucent cell).  Akinetes are resting cells, akin to spores, while the heterocyst provides a nitrogen-fixing function.

An interesting fact:  the first appearance of geosmin in the Theewaterskloof Dam became apparent when sheep in the Riviersonderend Valley refused to drink the water.  I travelled out to meet the farmers, tracked the problem back to the dam, and the rest is history (readers of Droplets will recall an earlier post when I first visited Theewaterskloof in the late 1980s, and was assured by DWA officials that I had no clue what I was talking about and that blue-green algae would never grow in western Cape dams).  More history.


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