Abstract:

Enhanced weathering (EW) is a geoengineering technique designed to accelerate the natural weathering process. By spreading alkaline minerals over land and ocean, this technique aims to enhance ocean alkalinity and carbon sequestration. While EW is considered as a promising strategy for marine carbon dioxide removal (marine CDR), its impacts on phytoplankton activities remain poorly evaluated. In this study, we investigated the interactions between several potential materials for EW and two phytoplankton species, the diatom Thalassiosira weissflagii and the dinoflagellate Amphidinium cartera. We conducted culture experiments with varying concentrations of minerals, and investigated the responses of phytoplankton, changes of seawater carbonate chemistry, and the characteristics of the mineral particles. Our data demonstrated that, compared to the minerals including olivine, limestone, and dolomite, the steel slag, a by-product from steel production industry, can better enhance seawater alkalinity and increase trace metal availability for phytoplankton, promoting their growth, photosynthesis, and hence, carbon fixation. Additionally, it can inhibit bacterial growth, and therefore, decreased their carbon decomposition activity. Besides, phytoplankton can promote minerals dissolution through active element absorption. Under trace metal-depleted conditions, both species benefited from steel slag addition. However, when trace metals were sufficient, steel slag became toxic for diatom T. weissflagii at 100 mg/L, while the harmful algal blooms-associated species A. cartera still benefits from the slag at the same concentration. Our results provide new insights into the EW application, suggesting that steel slag has a good potential to work as a material for marine CDR under proper control. 

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