Electrons, Life and the Evolution of the Oxygen Cycle on Earth
Paul G. Falkowski, October 9, 2007
Peeling away billions of years of the Earth’s history, Paul Falkowski reveals how our watery and rocky world underwent a massive transformation to become oxygen-rich and biologically diverse. He elucidates the complex geochemical and geophysical processes underlying the “Story of O” – how oxygen made its appearance on the planet.
While scientists know that photosynthesis is responsible for the air we breathe, “We don’t understand how the reaction fundamentally works,” says Falkowski. “It’s one of the most enigmatic electron transfer reactions in biology.” Just splitting water “doesn’t give you free oxygen on the planet. “ Yet somehow, on an almost unimaginable timescale, organisms with the help of the sun have been producing atmospheric oxygen, and that oxygen is in equilibrium with other gases.
Falkowski’s explanation for this alchemy involves the Wilson Cycle, where silica-rich rocks thick with organic matter get pushed up from the ocean onto land. This process, which probably first occurred three billion years ago, enabled the evolution of oxygenic photosynthesis, says Falkowski. By means of “mass independent fractionation of sulfur isotopes,” the creation of ozone, and nitrogen fixing, the Earth witnessed “a great oxidation event” around 2.3 billion years ago, triggering a shift from anaerobic to an aerobic environment in the oceans.
Suddenly (geologically speaking), the conditions were ripe for life. In Falkowski’s words, this was the “big flip.” The right ocean chemistries encouraged the emergence of primitive biological cycles. Ocean-based bacteria and other simple life forms developed the photosynthetic machinery for feeding themselves, generating oxygen and recycling nitrogen and phosphorus. In the last 200 million years, oxygen production climbed, leading to a proliferation of life forms. The rise of large placental mammals can be seen as the “evolutionary consequence of the rise of oxygen,” says Falkowski.
But the delicately balanced metabolic processes on which life depends are now moving out of equilibrium, he says. “In the last 150 years, humans have extracted huge amounts of buried organic matter and consumed it at unprecedented rates in recent geological memory. The result is a change in temperature of the Earth and atmospheric concentrations of CO2.” The glaciers are going. Falkowski sees a faint hope for the planet if humans can figure out how to scale up such biochemical reactions as hydrogen generation from the splitting of water or creation of fuels from cellulose.
Source: MIT World