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Friday, February 14, 2014

Cyanobacteria, the ancient algae that could

I did two seemingly unremarkable things today. I walked by a local algae covered pond and I breathed the air while doing so. But are those things so unremarkable? Perhaps not. In fact the very act of respiration that I performed while doing that walk is only possible because of the ancient ancestors of the algae that covered the pond.

Earth was not always as we know it now. Of course this is not news to anyone familiar with dinosaurs. But while giant animals roaming the landscape might seem like an Earth that is pretty different from what we know the very ancient Earth was a place we'd never even recognize.

Ancient Earth in the time of the Archean (2.5 - 4 billion years ago) was a planet so different from what we know now—with no life at all on land, green oceans instead of blue ones, a sky that would have been tinted red all day long and the Moon would have been much larger in the sky—that it would seem like an alien planet if you were able to time travel back to it. It'd also be deadly as the atmosphere itself would have quickly suffocated you.

It would have suffocated you because there would have been no oxygen available to breathe.

And why is it that there was no oxygen? How is it that the same planet once lacked one of the most important elements to our metabolism?

Well, don't worry, it isn't that oxygen was suddenly gifted to the planet by alien planetary engineers, a wizard or trickster gods. The oxygen was always there. It was just locked up in various chemical bonds that prevented free oxygen O2, from being available. And free oxygen is what is needed for aerobic respiration in animals.

So the oxygen was there, but not available. How is that the case? Well this is because of the atomic structure of oxygen itself. It is primarily because of the number of electrons that oxygen has and the behavior of the valence shell.

What is a valence shell? Well the valence shell is the outermost electron shell which is a layer surrounding the atomic nucleus where the electrons of the atom reside. There are different layers of electron shells at differing altitudes (so to speak) above the nucleus depending on how many electrons that particular atom contains. The lowest level electron shell will contain a maximum of 2 electrons, the next layer will hold up to 8 electrons, the one above that will hold up to 18. This continues on and on but for the purposes of this article we only need be concerned with the first two electron shells.

The outermost electron shell—the Valence Shell—determines the chemical properties of the element and is thus very important. As I previously explained the shell has a maximum number of electrons it can hold. But the total number of electrons in the element includes electrons that inhabit a lower orbit. So an element, like oxygen, with 8 electrons has two electrons that will preferentially inhabit a lower orbit leaving the outer shell with six electrons and two of them will be seeking a partner.

To understand this you must realize that the electrons in the valence shell will partner up and to kind of visualize this you need to first see this diagram of how oxygen looks with its electrons arranged into their shells based on their own preferences.

This means Oxygen has two electrons on the outer shell that will be seeking new electrons to pair up with. Oxygen, therefore, will be seeking other elements looking to give up electrons either by sharing them (known as a covalent bond) or gifting them entirely and becoming positive ions.

Oxygen will do either. You see elements that are close to having a full valence shell but are just short will be highly reactive. As are elements that have a valence shell that only contain one or two electrons. To see this in action just expose an alkali metal (which has only one electron in its valence shell) to concentrated oxygen (which voraciously seeks out the electrons of other elements) and watch how it causes an exothermic reaction that is literally explosive.

Yeah that was pure sodium being dropped into water. The desire of oxygen to react with the alkali metal is so fierce that it rips itself from its covalent bond with hydrogen and absorbs the extra electrons that each sodium atom wants to get rid of.

So what does this have to do with why earth had no breathable oxygen? Well oxygen is so reactive that it almost always finds elements such as the ones previously described and bonds with them. Gifting us with a planet full of things like water (oxygen + hydrogen), Carbon Dioxide (carbon + oxygen) or rust (iron + oxygen).

Free oxygen just wasn't around because all of it was locked up in those existing chemical compounds.

And that was when the ancient ancestor to pond scum came along: cyanobacteria.

Cyanobacteria actually represented a huge step forward in evolution. Prior to the advent of cyanobacteria all previously existing life forms utilized a primitive form of photosynthesis that used H2S to donate electrons for the photosynthetic process instead of water. This meant these organisms only produced sulfur as a byproduct of their photosynthesis.

And then a mutant came along, cyanobacteria, that did use water to donate electrons and the byproduct of its photosynthesis was free oxygen!

Now it wasn't all free sailing after that. The ancient Earth was a place very unused to the presence of free oxygen. The first thing that happened was that all the dissolved iron in the oceans reacted with the new abundance of free oxygen turning into rust. This left the oceans relatively iron free, and blue, for the first time in the planet's history. After that oxygen accumulated in the atmosphere. That is until the level of oxygen rose so high it collapsed the ecosystem which was still dominated by the ancient bacteria that used anaerobic photosynthesis. This resulted in a series of extinction events called The Oxygen Catastrophe.

But, eventually the ecosystem stabilized. Cyanobacteria gave rise to new mutants that eventually gave us plant life. Eventually the rise of respirating animals would come along that would use free oxygen to synthesize Adenosine triphosphate  from glucose the main energy driver in the cells of animals.

So there you go. Breathing oxygen; more remarkable given our planet's history than you might have thought. And something that would be impossible if not for ancient blue-green algae 3.5 billion years ago.

You'll never look at pond scum the same way again.

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