Ocean Alkalinity Enhancement (OAE) is a geoengineering technique aimed at increasing oceanic carbon sequestration by dispersing alkaline minerals in the ocean. It is a promising negative emission strategy to combat the escalating climate crisis, but the interactions between OAE materials and phytoplankton remain poorly understood. This study aimed to examine the interactions of several potential OAE materials, including dolomite, olivine, limestone, and steel slag, a byproduct of the steel industry, with two common marine phytoplankton species, the diatom Thalassiosira weissflagii and the dinoflagellate Amphidinium cartera. We investigated the responses of phytoplankton, changes in seawater carbonate chemistry, and the characteristics of mineral particles. 

Our data suggest that steel slag significantly enhances seawater carbon capture and increases trace metal availability for phytoplankton at concentrations ranging from 10 mg/L to 1000 mg/L. While phytoplankton exhibited neutral responses to dolomite, olivine, and limestone, steel slag stimulated their growth and photosynthetic performance in trace metal-deplete conditions. Additionally, phytoplankton enhanced the dissolution of steel slag by actively taking up the necessary elements. Notably, the presence of steel slag might inhibit bacterial growth, thereby reducing their carbon decomposition activity. However, in environments rich in trace metals, 100 mg/L steel slag showed toxicity to both phytoplankton species, whereas 10 mg/L did not negatively impact them and still enhanced carbon capture in seawater. The synergistic interactions between steel slag and phytoplankton suggest a potential for increased carbon sequestration efficiency, highlighting steel slag as a promising OAE material when managed properly.