Corals ASSIMILATE the most CO2 - They use it for their structures. Tourism is what is killing the corals. Corals like it tough all the sewage of the hotels in tourist areas like the Carribean is going straight into the water - this brings too many nutrients into the water and the corals die from the algaes that this produces blocking the sunlight etc.

Plants although they create O2 - the plants respire O2 at night-time ie they breathe O2 also.

The o2 producing winner are the algaes at the poles. Without them you die... the oceans die everything dies, which is why North korea has had the ability to kill us all since 1975 with its dipped missiles.

regards
jules