Oxygenation of the atmosphere – or why we love bacteria, part I

One of the critical steps in the development of life as we know it today was the evolution of cyanobacteria (also called blue-green algae).  These single-celled organisms are responsible for adding oxygen to the atmosphere, and paving the way for the higher life forms that we have today.

They were not the first autotrophs (organisms that generate their food directly from sunlight, or inorganic chemicals, as opposed to heterotrophs (like us), who get their carbohydrates second-hand by consuming other organisms),  one view is that organisms similar to purple sulfur bacteria existed first, and it is the transition from these to cyanobacteria that today’s post is about.

Purple sulfur bacteria are only able to grow under anaerobic conditions (that is, conditions without oxygen present), because the synthesis of pigments in the cell is suppressed if oxygen is present. The early atmosphere of the earth was not oxygenated as it is today (as evidenced by sulfur isotopes in rocks over 2.45 billion years old, among other indicators), although there is some debate about the exact timing of the first atmospheric oxygen (NB: This does not mean I am saying “Science does not know what happened”….the time period in discussion is a few million years, and the discussions are about whether any atmospheric oxygen was immediately reacted with iron or other chemicals in the oceans).

Ok, so about these cute little purple sulfur bacteria, they are photosynthetic, so this means they use sunlight to obtain their carbohydrates.  Unlike plants today which use chlorophyll (the reason they have a green colour, but more on that later), these purple bacteria use a compound called bacteriochlorophyll, which uses different wavelengths of light, and does not produce oxygen as a waste product – This is kind of handy, since, as we saw in the previous paragraph, sulfur bacteria do not function so well in the presence of oxygen.

So, I have just mentioned photosynthesis, which is the method that plants use to obtain sugars from sunlight.  The equation for this in sulfur bacteria is as follows (Simplified version).

CO2 + 2H2S → CH2O + H2O + 2S, (or carbon dioxide + 2 lots of hydrogen sulfide is converted using light to carbohydrate + water + 2 sulfur molecules.)

Hydrogen sulfide is the rotten egg smell, and is emitted by volcanos.  The early earth was very geologically active, and volcanic activity was much higher than it is today, providing plenty of hydrogen sulfur for these bacteria to use.

This equation is important, because it was the discovery of this method of photosynthesis which demonstrated that it is water, and not carbon dioxide which is used for producing oxygen.  Prior to that, it was thought that the carbon dioxide was split to produce the waste product of oxygen.

So, back on the early earth, there were these bacteria pumping out sulfur, and using up the CO2 and H2S from volcanos.  Because they were purple, they used light in a different wavelength to the  plants of today, which reflect back the green portion of the incoming light, and thus we see them as various shades of green.  These purple sulfur bacteria reflected light back at both the higher end of the spectrum (red) and the lower end (blue), giving them their colour.

Absorption of light for purple and green chlorophyll pigments.

(Image from http://www.sidthomas.net/SenEssence/Genes/chlinsen.htm )

So, now we have oceans filled with reddish purple bacteria! I always try imagining how our planet would have looked back then, not so much a pale blue dot with green patches, but more likely a vivid colour like we see today when we get blooms of purple sulfur bacteria in places like Yellowstone park and other oxygen low, sulfur rich hot springs.

The next development was for some bacteria to evolve to use the light that was not being used by these organisms.

As you can see from the image, the purple bacteria would have used the light at in the centre of the spectrum, leaving light at the ends available for use, so any organisms which evolved to utilise this would have had a competitive advantage. If you want to know more about the evolution of cyanobacteria, without getting overly heavy on technical stuff, there are two systems within plants which do the conversion of light, these are called Photosystem I and Photosystem II. Green sulfur bacteria have Photosystem I only, and purple bacteria have Photosystem II only, so it is currently thought that the evolution of an organism which had both, either by endosymbiosis, or by sexual reproduction (yes, bacteria get it on too!) led to the cyanobacteria. (This is not my main area of interest, so please feel free to correct me if I got that wrong).

Ok, so now we have bacteria that reflect light in the green region, and use it from the ends of the spectrum, and have two photosystems…so what?

Well, it so happens that chlorophyll (the pigment involved), when using electrons to generate energy, gets electrons by splitting water, instead of hydrogen sulfide.  (This happens in photosystem II, in case you were wondering) So, now we have a new equation for photosynthesis, which is more familiar:

CO2 + 2H2O → CH2O + H2O + O2

By using water (which was in abundance), and producing oxygen (which the sulfur bacteria could not tolerate), the cyanobacteria gained a large evolutionary advantage.  In addition, due to the release of oxygen into the atmosphere, ozone was formed by the reaction of sunlight with oxygen to form O3

Ozone is what is primarily responsible for filtering out the UV from the sun, so once atmospheric oxygen, and ozone were in place, the stage was set for the long evolutionary road which would eventually lead to me sitting here typing, and you sitting somewhere else in the world reading these words.