Finally, in some prokaryotes, electron transport systems efficient enough to expel more H+ than necessary to regulate pH evolved.
These cells could use the inward gradient of H+ to reverse the H+ pump, which now generated ATP instead of consuming it.
Such anaerobic respiration persists in some present-day prokaryotes.
Photosynthesis probably evolved very early in prokaryotic history.
The metabolism of early versions of photosynthesis did not split water and liberate oxygen.
Some living prokaryotes display such nonoxygenic photosynthesis.
The only living photosynthetic prokaryotes that generate O2 are cyanobacteria.
Most atmospheric oxygen is of biological origin, from the water-splitting step of photosynthesis.
When oxygenic photosynthesis first evolved, the free oxygen it produced likely dissolved in the surrounding water until the seas and lakes became saturated with O2.
Additional O2 then reacted with dissolved iron to form the precipitate iron oxide.
These marine sediments were the source of banded iron formations, red layers of rock containing iron oxide that are a valuable source of iron ore today.
About 2.7 billion years ago, oxygen began accumulating in the atmosphere and terrestrial rocks with oxidized iron formed.
While oxygen accumulation was gradual between 2.7 and 2.2 billion years ago, it shot up to 10% of current values shortly afterward.
This oxygen revolution had an enormous impact on life.
In its free molecular and ionized forms and in compounds such as hydrogen peroxide, oxygen attacks chemical bonds, inhibits enzymes, and damages cells.
The increase in atmospheric oxygen likely doomed many prokaryote groups.
Some species survived in habitats that remained anaerobic, where their descendents survive as obligate anaerobes.
Other species evolved mechanisms to use O2 in cellular respiration, which uses oxygen to help harvest the energy stored in organic molecules.
Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 26-11