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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