Thursday, 7 December 2017

Green beer and Spirulina

A brewer on the shores of Lake Erie has produced a green beer, flavoured with kiwi fruit and with green tea added as colouring [1]. This unusual drink has received a favourable response from drinkers who like its fruity taste, but the brewers made the green beer to highlight a problem with their water supply that originates from Lake Erie. Unfortunately, the lake has recently suffered blooms of cyanobacteria (previously known as blue-green algae) that can be seen in satellite images (see below, from NOAA). Given that they are very primitive unicellular organisms, the numbers of individuals, and colonies, in these blooms are astonishing.

The cyanobacteria are fertilised by nutrients, especially phosphate, that are added to increase the growth of grass, or crops, and which run-off into the rivers and are then carried to the lake. Phosphate is a limiting nutrient in most fresh waters, so its addition causes the cyanobacteria to grow and multiply rapidly. Their numbers cannot be controlled by planktonic animals and cyanobacteria exude sticky polymers that provide a defence - the polymers also allow attachment of cells to form colonies, and enable some cells to propel themselves within the water column. As long as nutrients are available, blooms result and these inhibit the efficiency of drinking water treatment plants and, to add to the woe, some cyanobacteria produce toxins that are poisonous to humans and may be lethal to our pets.

So, are all cyanobacteria harmful to human activities? The answer is no, for without cyanobacteria and the evolution of their capacity to convert carbon dioxide and water into sugars using light energy – the process of photosynthesis – we would have no green plants and very little oxygen in the atmosphere. Indeed, we wouldn’t be here if it wasn’t for cyanobacterial evolution. The ancestors of single-celled algae ingested cyanobacteria and, by an unknown mechanism, some of these survived and became incorporated with the algal cell as chloroplasts. From single-celled algae came multicellular organisms and the complex aquatic, and terrestrial, plants that we know today.

Cyanobacteria also have a more obvious value to us, as some species are cultured in lagoons to produce intentional blooms that are then harvested. The collected mass is dried and compressed into cakes, pellets and powders that are marketed as Spirulina, being valued both as a health food and dietary supplement. There are many recipes that feature the cultured cyanobacteria [2,3] and Spirulina is even finding its way into “fine dining”, as viewers of the BBC’s Masterchef: The Professionals know. Of course, it is important to avoid harmful cyanobacteria when selecting those to be cultured for human consumption and I wouldn’t recommend harvesting the blooms from Lake Erie.

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