Free oxygen can only exist when there is no active molecule to bind it. This accounts for the extended period in which there was only single celled anaerobic bacteria in the oceans--there was an incredible amount of atmospheric iron that had to be fixed as iron oxide by the waste product of the bacteria, before oxygen could accumulate.
This resulted in a huge geological band of rust in much of the world. And when the iron was finally fixed, the percentage of oxygen in the atmosphere multiplied. And once there was enough oxygen in the atmosphere, oxygen levels in the oceans increased so much that bacteria had to evolve to live around it.
Now, this took place long before the events discussed in the article, but it took a vast amount of time.
A couple questions. How could there be free iron in the atmsphere? What form would/could it have existed in? Where did the oxygen come from to bind with the iron?
My understanding is that the majority of the oxygen that exists in the atmosphere today has it's source in the oceans. Were the bacteria in the anaerobic conditions of the oceans of the time capable of producing it; were they capable of photosynthesis? If that was the case, the oceans would have been oxygenated before the atmosphere. How could the atmosphere been oxygenated first?