Cyanobacteria, the Oxygen Bringers

Cyanobacteria were allegedly some of the first microbes that existed on earth.  Evidence for their existence can be found in stromatolites, layered masses of They inhabited the seas and survived through photosynthesis, using sunlight, water, and carbon dioxide to produce oxygen and glucose.  Initially this oxygen reacted with iron in the water and formed the deposits that are today known as iron banded formations.   After the rust was used, however, oxygen had nowhere to go but up.  It started filling the atmosphere over time.  And so began the oxygen revolution. 

Oxygen was just as much of a toxin as it was a bringer of new life – it just depended upon who you were.  Microbes that could not tolerate oxygen, called obligate anerobes, either died off or retreated to anaerobic habitats.  These unusual locations, such as deep sea thermal vents, are being explored today and mined for new types of microbial life.  However, the abundance of oxygen also set the stage for the the diversification of life on earth.   Microbes that could survive the increased amounts of oxygen often made use of it to become more efficient.  Some of them were swallowed up by other living things and came to live inside them as mitochondria, producing oxygen for their hosts.  Eventually multicellular oxygen breathers emerged, and over time a vast array of oxygen breathing life populated both land and water.  Rarely, however, did these new life forms think about cyanobacteria - the microbes that made their existence possible.    

Microbial Mats

A month ago when I thought of microbial mats, the only image that popped into my head were the domed structures in Shark Bay, Australia… layers of living microbial communities that grow, flourish, and trap sediment.   In the “Living in the Microbial World” workshop at Woods Hole, we had the opportunity to venture out to the Sippowisset marsh in Woods Hole and take samples of microbial mats.  Using a scooper, we took a sample of the mat (it looked like a brownie) and put it in a plastic bag.  The mat samples had definite layering to them: the top green layer had some cyanobacteria and the pink layer had some purple bacteria.  We took the mats back to the lab and looked at them under a light microscope.  The main challenge there was separating the bacteria from the sediments.   Under the scope I could see the stringy cyanos on the top layer and the purple bacteria on the bottom layer.  Brad gave us some insight as to how the bacteria in these mats metabolized sulfur and nitrogen.  By sticking an oxygen electrode into the microbial mat, we found that oxygen levels changed with the depth of the electrode.  They were different with each layer.  We also found that when the mat was deprived of light, the microbes’ photosynthesis shut down… as did the oxygen production.  As oxygen was rapidly consumed, oxygen levels dropped dramatically.   In later posts I will write abou the interactinos between theese microbial communities.

Commensals vs. Pathogens

As I mentioned earlier, the best known (and best studied) microbes are the pathogens – those that cause illness in the host organisms they invade.  But let us not forget about our unsung heroes, the commensals.  These microbes are (usually) more friendly to their host, using a microscopic region of the host’s body as their habitat.  And make no mistake –  we are covered in them.  You have about 10 times as many commensals living on you as you do body cells.   Commensals are also the first line of defense against pathogenic microbes.   If a few pathogens try to find a parking space on your body (and they will!) fear not – because your commensals have already taken up residence, filling up most of the available niches and habitats.   Pathogens usually don’t last long because they have trouble wedging their way into the competition.   Now sometimes pathogens can get past this first line of defense and start finding a means to flourish and reproduce within your system.  That’s when you get sick and your immune system becomes active, with specialized cells such as T-lymphocytes and macrophages leading the charge. 

Commensals and their hosts can usually live together in harmony – a type of symbiosis where both host and microbial agent benefit.  But this level of mutualism is a delicate balance.   How does a relationship between a commensal and its host become adversarial?   This question will be explored in an upcoming post – stay tuned.