I recently read a fascinating newspaper article on cyanobacteria, primitive micro-organisms which were amongst the first to appear on the planet a staggering 2500 million years ago. The cyanobacteria in question were from Antarctica, one of the most inhospitable environments on earth. As I read about these interesting organisms my subconscious mind tried to tell me something. After another beer, the fugitive thought allowed itself to be captured. I'd seen colonies of cyanobacteria growing in all sorts of extreme environments. The problem stemmed from not knowing they were cyanobacteria. I guess I can be forgiven for this because the name cyanobacteria is just the most recent in a short succession. When I first encountered these organisms they were known as the blue-green algae or the cyanophyta. The name problem stemmed from the fact that they had little in common with algae and large amounts in common with the bacteria. This relationship is recognized in their new name.
Cyanobacteria grow wherever there is water; many "algal" slimes are formed by them. A quick and simple test will separate a cyanobacterial slime from one generated by genuine algae (for those totally fascinated by this subject). In general, the cyanobacteria have a distinct "soapy" feel while the green algae do not.
Not all cyanobacteria seem so humble. In late 1987 I was lucky enough to visit Shark Bay in Western Australia. As well as the excitement of communing with wild dolphins at Monkey Mia, I was able to visit Hamelin pool where stromatolites are widespread. Stromatolites are large pincushion-shaped colonies of cyanobacteria which are able to secrete lime. These brain coral-like growths represent the most ancient structures produced by life. Two thousand million years ago they dominated the seas and all reefs were formed by stromatolites rather than that recent upstart, coral.
Two billion years ago the stromatolites reached their peak diversity and in quiet reefs they grew to amazing sizes. Individual cones could be tens of meters high, even up to 100 m.. This is an enormous contrast to the relatively humble mats that the cyanophyton usually espouse today.
Hamelin Pool, Shark Bay. Photo by Paddy Ryan
The world initially inhabited by the stromatolites and dominated by the cyanobacteria was very different from that of today. Not only were the organisms primitive, but they also lacked diversity. Even the atmosphere was different. If we could magically be transported back to that distant past we would soon die. The atmosphere lacked oxygen. The cyanobacteria were able to produce oxygen from the breakdown of water during photosynthesis. Over hundreds of millions of years they changed our atmosphere into the oxygen rich air we boast today. This in itself brought with it massive change. As the oxygen reacted with sunlight in the upper atmosphere it produced ozone. The earth's ozone layer came as the direct result of cyanobacterial activity. This in turn cut down the fierce ultraviolet radiation and cleared the way for the evolution of more complex forms of life. The cyanobacteria signed themselves into relative oblivion although they seem to thrive in extreme environments where more advanced life has trouble.
The stromatolites survive in Shark Bay and one or two other places because of the climatic conditions. Hamelin Pool is partially blocked from the rest of Shark Bay by a sand bar and rapid evaporation ensures that the water is extremely salty. So salty in fact that apart from a few fish and a couple of crabs I saw no other living creatures. Freed from predation, the stromatolites continue their 2000 million year old lineage. As I stepped from one ancient stone to another it was a trifle sobering to realize that everything around me owed its very existence to this primordial life form.
In the Antarctic, a research team has been investigating the cyanobacteria. Here too, the blue-greens face extremes, more so in than the Hamelin Pool lay-abouts which, in comparison, lead positively luxurious lives. Temperatures in the water can range from 28 degrees C to -8 degrees C. During the course of a winter the salinity can increase four-fold. In summer, water may dry out completely leaving a dehydrated cyanobacterial mat behind. Blown by the wind, these can end up in freshwater or hypersaline water eight times saltier than the sea. Faced with these levels of salinity, their metabolic activities cease almost immediately but they bounce back when placed in freshwater. So far all species tested have survived exposure to salt concentrations five times that of seawater. Their hardiness is almost beyond belief.
Also of interest is their response to extreme cold. They change the composition of their cell membranes by replacing a proportion of the saturated fatty acids with unsaturated ones. Like margarine versus butter in the fridge, this keeps the cell membrane soft and enable it to keep functioning. When temperatures rise the polyunsaturated fatty acids are replaced with saturated fatty acids thus maintaining the membrane's integrity.
Amazing stuff! They may be primitive holdouts from an unimaginably distant past but cyanobacteria may still have a few surprises in store for we ephemeral upstarts. For more stromatolites photos click here.
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