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Aug072012

Origin of breathing - bacteria and oxygen

Lab Rat

The Origin of Breathing - How bacteria learnt to use oxygen

July 29, 2012

The first signs of life on earth appeared about 4.5 Ga (1 Ga is an American billion, ie. 109 years) ago. It’s not yet completely certain exactly how this life arose; hot volcanic mineral springs have been suggested, as have the more traditional lightning-struck primordial soups and (rather wonderfully) radioactive beaches. At any rate something happened which lead to a little membrane-bound ball with internal nucleic acids which, crucially, could replicate…

I've never been convinced by the lightening explanation myself, but it would have been a wonderfully dramatic moment!

And then it was all over really, bar the evolution.

The atmosphere back then was very different, little oxygen and an abundance of carbon dioxide with plenty of methane being released into the atmosphere once the first life forms started eking out an entropy-defying existence. In order to get energy to power cellular processes you need to set up redox pathways, which involve cycles of electron donors and acceptors. The main electron donors around at the time were H2, H2S and CH4 and the main acceptor probably nitrogenous. Water, the electron donor used for photosynthesis, was around in abundance, but none of the little proto-life-blobs quite had the energy required to split it (or the physical proteins required back then either) so it mostly stayed unused.

Carbon dioxide levels went down, methane levels went up and the planet warmed up a little due to global warming. Things stayed like that for a billion years or so (1 Ga) and then something quite special happened, something that would have mindblowingly devastating affects on the life surrounding it.

Photosynthesis. The process by which carbon dioxide is fixed into usable sugars by the splitting of a water molecule. The process of photosynthesis produces oxygen, which is highly dangerous for cells; it can screw up the internal redox potential, create dangerous free-radicals and precipitate ions out into soluble forms. This means that from the point of view of every other organism the newly-evolved photosynthetic blobs were floating around spewing toxic gas into the atmosphere.

Floating green blobs of DEATH. Image from wikimedia commons.

The arrival of this new resource (oxygen) lead to a change in the way organisms respired as well. Up until what is sometimes called the Great Oxidation Event most respiration was anoxic, probably similar to anaerobic respiration, or fermentation,  in anaerobic bacteria around today. This process, while enough t0 keep life going, is around sixteen times less efficient than aerobic respiration. The proto-bacteria that managed to use the oxygen would therefore have gained a major energy boost.

... more



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