[Psycho_Bunny]

They hypothesize that melting glaciers increased the amount of nutrients in the ocean and led to a proliferation of single-celled organisms that liberated oxygen through photosynthesis.

It seems just as likely that the temperature increase that melted the glaciers allowed for an extended range of ocean-going, oxygen producing microorganisms.  

Or there was a combination of both.

[metmom]

Isn't that why you're supposed to refrigerate food, to slow down bacterial growth? Of course the warmer water would encourage bacterial growth. Nutrient content also wouldn't be quite so critical if the single celled organisms were able to perform photosyntehsis.