All I need is the air that I breathe

Posted by Rich Talcott
on Thursday, November 30, 2006

"All I need is the air that I breathe," to quote from a top-10 Hollies song from 1974. (Full disclosure: Love was also a required commodity for songwriters Albert Hammond and Mike Hazlewood.) But how did that air - or at least the oxygen vital to our existence -get into the atmosphere? It's a question that has troubled scientists for decades.

At one level, it's pretty straightforward. More than 2 billion years ago, organisms suddenly developed the capacity to release molecular oxygen into the atmosphere through photosynthesis. Scientists think cyanobacteria, which are still around as blue-green algae and the chloroplasts in green planets, were responsible. Certainly, cyanobacteria evolved the ability to convert water, carbon dioxide, and sunlight into oxygen and sugar.

The problem for scientists stems from a nasty "catch 22" - creating oxygen naturally produces the hydroxyl radical, which destroys DNA. Although cyanobacteria have enzymes to protect them, how could natural selection force the evolution of poison-protecting enzymes when there was no previous need for them?

A group of researchers led by Danie Liang of Caltech and the Research Center for Environmental Changes think they've found a way. The key involves producing small amounts of oxygen - enough to drive natural selection to create protective strategies, but not enough to kill the organisms. The researchers suspect a combination of ultraviolet light and glacial ice did the trick.

When ultraviolet light, in plentiful supply at Earth's surface before oxygen production led to the stratospheric ozone layer, strikes water vapor, the high-energy radiation converts some of the water vapor into hydrogen peroxide and hydrogen gas. Normally, hydrogen peroxide reacts fairly quickly with other molecules, but it freezes at a temperature just 1° Celsius below water's freezing temperature. So, during periods of intense glaciation, small amounts of hydrogen peroxide get trapped in the ice. (This process still occurs during springtime in Antarctica, when the ozone hole grows biggest.)

As the glaciers flow down into the ocean, the ice melts and hydrogen peroxide trickles directly into the seawater. There, another chemical reaction converts the hydrogen peroxide back to water and oxygen. The trickle would allow oxygen-protecting enzymes to evolve without flooding organisms with lethal levels of oxygen.

Evidence abounds that a "Snowball Earth" event - when glaciers extended on the continents all the way to the equator - occurred between 2.2 and 2.3 billion years ago. It seems ironic that green plants may owe their very existence to a time when only ice covered the planet. It's a simple yet elegant solution to one of advanced life's mysteries. Now, if only scientists could unravel the mystery of love.

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