Dissolved Iron from Steel Slag with Its Chelating Agent Promotes Seaweed Growth

Blue carbon ecosystems are crucial for carbon sequestration on a global scale. However, it is unclear how we could promote and maximize carbon sequestration. Here, we demonstrate that providing an iron source to seaweed fostered its growth through increased photosynthetic efficiency and transformed the carbon into a biomass. Firstly, we revealed that the mixture of the steel slag and DTPA eluted iron dramatically in seawater. Next, we applied the eluate of the slag-DTPA pellet to the seaweed. The results for the eluate treatment showed a 25.8% increase in the photosynthetic pigment level and a 44.9% increase in the seaweed weight. Furthermore, we confirmed no elution of potential toxic substances from the steel slag and DTPA pellet. Finally, we applied the pellet at a depth of 15 m near seaweeds and observed a 52.0% increase of carbon weight in the pellet treated group, while the non-treated group showed only a 10.3% increase for five months. This study indicated that steel slag-DTPA pellet treatment induced seaweed growth and efficiently transformed its carbon into a seaweed biomass. Thus, steel slag and its chelating agent may contribute to the promotion of sea forestation and a subsequent increase in carbon sequestration known as blue carbon.