Even in the lowest warming scenario examined, 1.5°C, recovery of the North Atlantic Phytoplankton Bloom did not occur within 5 centuries under zero emissions, nor did it occur within one century under negative emission

They are saying that the plankton will be 84% gone by 2100 if emissions continue, and if the warming passes 2 degrees it will be be a tipping point that is irreversible in human timescales.

Plankton are the base of the marine food web and they are responsible for half of the planetary photosynthesis. Plankton consume vast amounts of CO2 and produce about half of the atmosphere’s oxygen.