The numbers in the black circles denote individual processes in the order they were discussed. 1. Photosynthetic carbon fixation by Sargassum consumes CO2 dissolved in seawater. 2. Sargassum provides habitat for epibiontic encrusting organisms to calcify, thereby generating CO2 through complicated feedbacks in the seawater carbonate system. 3. Nutrients taken up by Sargassum become unavailable for phytoplankton thereby reducing the natural carbon sequestration by phytoplankton. 4. CO2 deficient seawater (driven by photosynthesis) can be subducted below the surface layer before it fully equilibrates with atmospheric CO2. 5. Sargassum biomass can be harvested and transported to shore and used for Bioenergy with Carbon Capture and Storage (BECCS) causing CO2 discounts. 6. Alternatively, Sargassum could be deposited on the seafloor but a large fraction of the respired Sargassum carbon would come back to surface on centennial-millenial timescales. 7. Sargassum increases albedo at the sea surface but could also induce complicated indirect albedo effects of unknown sign and magnitude.