Unless the beasties all the way up the food chain exhale all the CO_2 that the phytoplankton converted into sugar, there would still be a net removal of CO_2 from the atmosphere. (Not that it really matters, since excess CO_2 is a trailing indicator of global temperature, as we now see clearly these last ten years.)
This has been demonstrated beyond doubt. How come we are spending trillions now to "control Carbon and CO2?" We are sacrificing the world's economy and common sense to pseudo-science.