Fig. 4. Summary of feedbacks discussed in this study.

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.