[gundog]

Apparently Diskagna housed a photosymbiont responsible for some of the early oxygenation

Don’t plants require oxygen to metabolize in the absence of light? The net result is more oxygen than carbon dioxide, but they still must metabolize at night.

[alexander_busek]

Don’t plants require oxygen to metabolize in the absence of light?

Is that a question, or a claim? Because... Wikipedia is your friend!

The net result is more oxygen than carbon dioxide, but they still must metabolize at night.

If given a chance, plants would consume all of the oxygen they produce (during daylight, by means of photosynthesis).

It's because, on the whole, they don't get that chance - because they die first - that they yield a net plus of oxygen in the atmosphere. However, if heterotrophic organisms like bacteria, molds, insects, or human beings can get to them - i.e., if they aren't effectively isolated from the biosphere by being buried and locked into geological strata before they can rot, etc. - then all of that oxygen will be consumed by the organisms that rot or otherwise consume them.

Regards,