It still seems like the only possibly scalable solution. A hypothetical genetically engineered plant being dumped from ships in an ocean dead zone / desert, where it will do photosynthesis and drop pellets of heavier than water (nondegradeable) biopolymer to the ocean floor. No need to dump the fertilizer itself if the plant can store enough when it’s bred in a floating bioreactor. It only needs to produce polymers.

Only in the vast ocean deserts could we use exponential growth of plants and the abundant solar energy and space.