Friday, September 18, 2009

Oxygen: Destroyer

Oxygen gas is an anomoly. Earth's atmosphere should not be 21% molecular oxygen because oxygen is a very greedy element. It wants electrons badly and would much rather take those electrons by binding with elements on Earth's surface, then hanging around with other oxygen atoms in the atmosphere. As long as these elements are out there oxygen will "oxidize" them, which is what happens when iron rusts or something catches fire.

So why is there always 21% oxygen in the atmosphere? No other planet in our solar system has free oxygen in it's atmosphere (even though it's the third most abundant element in the universe). But no other planet in our solar system has liquid water and no other planet in our solar system has life. That's where the connection lies. It all hinges on a metabolic process called photosynthesis that was invented about two billion years ago by the bacterial ancestor of cyanobacteria or "bluegreen algae".

Lynn Margulis and Dorion Sagan, in their book, What is Life?, call photosynthesis: "the most important metabolic innovation in the history of the planet." Why? Because with photosynthesis life could derive energy from freely available sunlight for the first time.

The first bacteria probably lived off hydrogen sulphide bubbling up from volcanic vents under the sea. They derived their energy from catalyzing the hydrogen bonds, combining hydrogen with carbon to make simple sugars. Eventually, different kinds of bacteria evolved that were able to catalyze various non-organic substances such as iron and sulphur, then other bacteria evolved that could derive still more energy by fermenting the waste products of previous kinds of bacteria. Note that all early bacteria were "anaerobic", that is they did not use oxygen in any metabolic process.

There's a pattern here. First, an organism develops a way of getting energy from one substance. Then it grows and multiplies until it exhausts that resource. A crisis ensues which leads to the evolution of new life that is able to utilize new forms of energy, then those resources are exhausted and a new crisis ensues. Out of these series of life-crises bacteria came to invent every single metabolic process utilized by living things. Margulis and Sagan's point is that the development of photosynthesis temporarily bypassed this boom and bust process. By utilizing the energy from sunlight the ancestor of cyanobacteria was able to metabolize enough energy to split water molecules into hydrogen and oxygen, obtaining hydrogen from a virtually limitless source - the oceans - something that no bacteria before it was able to do.

But photosynthesis wasn't just a brilliant solution for bacteria, it rewrote the rule book for every single new life-form that has evolved since that time 2.5 billion years ago. Because, other than bacteria, all forms of life are either photosynthesizers themselves or they survive by eating photosynthesizers or by eating animals that eat photosynthesizers. That's what we mean by "the food chain".

Let's go back to that connection between oxygen in the atmosphere, water, and life. Because they had just liberated themselves from energy scarcity, the new kids on the block - bluegreen bacteria - were able to grow and multiply and grow and grow and grow until they had covered Earth's surface, wherever there was moisture. We're talking all the oceans and most of the surface rocks. We're talking a vast global empire of bluegreen slime.

But in the process of splitting water into hydrogen and oxygen, bluegreen bacteria gave off oxygen gas as a waste product. Thus the second type of crisis: pollution. Life multiplies up to it's limit, but in this case, because we are talking about sunlight and water as raw materials there was no imminent shortage. But there was a problem with pollution. Oxygen was created as a waste product and oxygen was toxic to bacteria in those times. In fact if sufficient atmospheric oxygen had been around at the time that life originated it wouldn't have originated. The oxygen would have burned up all the organic molecules before they had a chance to react with each other. Like I said, oxygen is greedy.

At first the oxygen pollution wasn't much of a problem, because oxygen was reacting with all the surface rocks, so it didn't stay long in the atmosphere. There are red bands of oxidized iron and other oxidized minerals in the layers of ancient rocks that date back to that time about two billion years ago. And, in fact, that's the strata of rocks that supplies us with iron ore to this day. From the rock record of oxidized iron we can read back the time it took for oxygen to get a foothold in the atmosphere - four hundred million years. But once everything was oxidized, all the oxygen that was produced by photosynthetic bacteria stayed in the atmosphere, creating a vast die-off of anaerobic bacteria.

Nowadays anaerobic bacteria are far less dominant, existing only in earth, mud, stagnant water, and in the guts of animals, all places where they are safe from the ravages of free oxygen.

This global event, when oxygen increased from one millionth to one fifth of the atmosphere was, according to Margulis and Sagan, in their book: MicroCosmos, " far the greatest pollution crisis the earth has ever endured." Unfortunately, there is no fossil record of this event, as bacteria, not having bones or hard shells have left very little in the way of fossils.

Since animals evolved, one and a half billion years ago, there have been five major extinction events recorded in the fossil background. These were global catastrophes, where from fifty to ninety-five percent of all species on Earth were wiped out. The most famous of these was the Jurassic-Cretaceous event sixty-five million years ago, believed to be caused by a giant meteorite striking the Earth, leading to the extinction of the dinosaurs and paving the way for warm-blooded species like birds and mammals.

We are now, as we speak, involved in the sixth (or seventh, if you include "the Oxygen Holocaust") global extinction event, an event which is predicted to lead to the extinction of more than fifty percent of all species in our lifetime. This one is due to carbon dioxide pollution and habitat destruction caused by our clever utilization of a previously unused energy source - hydrocarbons. What goes around comes around, as they say. Whether we will survive it or not, is an open question. Stay tuned.

No comments:

Post a Comment