I had a nightmare where we bred some super plankton to capture CO2 and it quickly colonized and killed the entire ocean and we doomed the world. On the bright side, if this were possible, plankton would have already done it right?
Yup. I'm sure they tried at some point and got beaten into equilibrium by other slimy things that were also trying to proliferate.
Phytoplankton produce about half the oxygen produced on the planet.
So, unlikely.