I apologize because I don’t have a source in mind, but my recollection from studying this in grad school (which was admittedly about a decade ago) was that sequestration was one of the hardest parts of this. Creating a bloom of algae was feasible, but even if we ignore a lot of other ecosystem management complications that others have pointed out, there wasn’t a reliable mechanism to convert a bloom of algae into a long-term carbon store.

I could be mistaken here. I’m open-minded towards this kind of geoengineering. But I’m also very skeptical that if this could work, it could do so at a rate that would enable us to continue burning fossil fuels at scale, and there is a strong base of support for this technology among people with that attitude.