Underestimated carbon sequestration effects of seaweed farming and carbon sequestration capability of sunken wild macroalgae in coastal oceans

Under climate change scenarios, seaweed (also called macroalgae) have attracted wide attention due to their significant carbon sequestration capabilities through various pathways, such as the burial of macroalgal debris or particulate organic carbon in local sediments, export to the deep ocean, and contribution of recalcitrant DOC (RDOC) to seawater. Since farmed macroalgae are harvested and removed from the ocean after maturity for various uses such as food or materials, their carbon sequestration primarily occurs during the macroalgal growth period. We revealed that the total RDOC contributed by kelp over its growth cycle equals the carbon content of the harvested kelp biomass, and a considerable portion of kelp-derived RDOC molecules can be transported over long distances and reach the deep sea. In addition, deep-sea macroalgae sinking for carbon sequestration is gaining global attention recently as a potential carbon dioxide removal (CDR) strategy. However, this strategy has faced criticism for limited understanding of its actual carbon sequestration effects and environmental impacts, as well as the ethical concerns. While, in the Yellow Sea, millions of tons of macroalg Ulva prolifera sink to seafloor annually following green tides, yet their fate and carbon sequestration capacity remain unclear. Our simulated two-year degradation of U. prolifera revealed that approximately 38% of the carbon in sunken macroalgal biomass was not respired back into CO₂, indicating significant potential for long-term carbon retention, despite its ethical and environmental impacts.