He could have been clearer there, too. I'm not sure he pinned it exclusively on acidification. Certainly some phytoplankton seem to do alright with lower pH levels. But generally if you heat up the ocean it's going to tend to stratify and stop circulating and all parts of the system suffer then. Phytoplankton do, in fact, account for a very large portion of the oxygen that is produced by photosynthesis--not surprising given that about three quarters of the earth's surface is covered by water.
The main thing, though, is that with 20% of the atmosphere oxygen, it would take a long time (like thousands of years), even with no new atmospheric O2 being produced by photosynthesis and even with all the hydrocarbons on the planet being oxydized, for that to make a significant dent in the overall oxygen levels, as far as I understand it. But really, we should even be thinking of messing around with anything that could significantly reduce O2 levels in the atmosphere over ANY time frame.