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 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, to enhance the carbon sequestration capacity of macroalgae, the strategy of sinking cultivated macroalgae into the deep sea, i.e., deep-sea macroalgae sinking, has been proposed. However, this strategy has faced criticism for being ahead of science due to the limited understanding of its actual carbon sequestration effects and environmental impacts, as well as the ethical concerns. As a counterpart to this, the natural event of huge amounts of macroalgal (Ulva prolifera) biomass sinking to the seafloor occurred annually following green tides in the Yellow Sea since 2017. We investigated the ultimate fate of these sunken biomass carbon of U. prolifera, especially the amount and forms of macroalgae-derived carbon that can be retained in the ocean after long-term degradation, which is crucial to understanding their actual carbon sequestration capacity and offers insightful perspectives on the efficacy of deep-sea macroalgae sinking as a CDR strategy.
 

seaweed farming, macroalgal bloom, carbon sequestration, recalcitrant organic carbon, biocarbonate