An experiment about ocean feeding, as a way of cutting carbon dioxide from the atmosphere, is at the same time both failed and successful. (In the movie sense. In the real science sense, any significant result is a success, the only failure would be an ambiguous result or an obviously tampered with result.)
The experiment suggests its a failure, because the resulting bloom did not sink to the ocean. Instead, it was eaten by small prawns. The carbon will not be locked away at the ocean bottom. Instead, it will become food, and the food of food, and generally enrich the life of the ocean. Arguably this is a big success, because this solves both the global warming and fish stock depletion problems in a single stroke. All our carbon problems will be turned into fish, which we promptly eat.
Now, if you wanted the carbon to sink, like this group wanted, apparently what you need is hydrosilicic acid, common in polar waters and rare in the tropics. This material is essential for diatomes to build their cell walls. And unfortunately, chemical production of silcic acids are not carbon neutral, so back to the drawing board.
Unless you can figure out some way to reshape the ocean's currents.....