Serious curiosity
Is it possible to genetically modify photo-plankton to produce a higher levels of oxygen?
If so, is it possible to also cultivate said plankton to be more bacteria and virus resistant as compared to more common plankton?
If all of that is possible, given the above mentioned circumstances; will the new plankton culture be able to have a significant impact on oxygen levels on earth? Possibly bringing them to pre-Pleistocene levels?
Finally, given all of the aforementioned circumstances; even if in millions of years, can a modified version of photo plankton possibly cause the return of megafauna through a raised oxygen production?

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