Ancient Purple and Green Bacteria

Why is the state of the seas important?

(*) Today’s oceans are oxygenated all the way to the seafloor.

(*) Marine animal life depends on this oxygen. Plant life and much microbial life depends on current conditions too.

(*) The oceans could not become oxygenated until after the atmosphere, which appears to have seen its first traces of oxygen 2.2-2.3 billion years ago.

(*) If the oceans had became oxygenated at about the same time, then animal and plant life could have evolved in the oceans for at least the last 2 billion years.

(*) If there was a major delay in the oceans becoming oxygenated, then there was much less time available for marine life to evolve.

(*) The oxygenation of the oceans was delayed by buffering from massive amounts of reduced iron and sulfur.

(*) Basalt, erupting at mid-ocean ridges reacts with seawater to produce a steady supply of ferrous iron and other reduced species. This would ‘mop up’most oxygen being produced by photosynthesis at the surface.

(*) Also, there was little sulfate in the oceans before the atmosphere became oxygenated.

(*) Once the atmosphere contained some oxygen, sulfate could be produced by the weathering of rocks and carried to the oceans by rivers.

(*) Sulfate-reducing bacteria in the oceans would then have converted this sulfate to sulfide (ierotten egg gas), making the oceans toxic and preventing most animal and plant life from establishing a marine habitat.

Research conclusions include:

(*) Independent evidence that late Paleoproterozoicocean had low oxygen and sulfate concentrations,
(because few algae and high activity of methanotrophs)

(*) and high sulfide concentrations…
(because purple and green sulfur bacteria were present)

(*) …that extended high into the water column, up to where sunlight could penetrate.
(because the purple and green sulfur bacteria need sunlight to live)

(*) First molecular evidence for a complex Paleoproterozoicmicrobial ecosystem.

(*) Implication: modern complex life (animals and plants) could not have begun evolving in the oceans until ~0.6 billion years ago.