Enhanced weathering is a geoengineering technique that accelerates the natural weathering process to enhance ocean alkalinity and increase carbon sink by spreading alkaline minerals over land and ocean. While it is considered as a promising strategy for marine carbon dioxide removal (marine CDR), its impacts on phytoplankton activities remain poorly evaluated. Here, we investigated the interactions between phytoplankton and steel slag as a potential material for enhanced weathering. We measured the growth rates and photosynthetic activities of a diatom Thalassiosira weissflagii and a dinoflagellate Amphidinium cartera in response to the addition of different concentrations of the materials as a replacement of trace metal solutions. Our preliminary results demonstrate that steel slags can supplement the trace metal requirements of both species, but the two species have different sensitivities to the addition levels. Compared to the negative control groups without trace metal addition, both species under steel slag treatment increased their growth rates significantly. The diatom increased the growth rates more significantly with 1000mg/L steel slag compared to the 10mg/L and 100mg/L steel slag treatment groups, while the dinoflagellate had similar growth rates under the three steel slag concentrations. For both species, the photosynthetic activities of groups with steel slag addition are comparable with the positive control group cultured in f/2 medium, indicating that steel slag addition can compensate for trace metal deficiency. Moreover, by investigating the mineral particles before and after the culture experiments via scanning electron microscope and X-ray photoelectron spectroscopy (XPS), our data demonstrate that the microalgae could selectively enhance the releasing of bioessential elements from the minerals, including iron, zinc, and silica. Our results provide solid evidence that steel slag could provide trace metals to phytoplankton under proper control and thus has a good potential to work as a material for marine CDR, and their mineral dissolution processes in seawater are significantly impacted by microbial activities of different species.

Enhanced weathering,Steel slag,Diatom,Dinoflagellate,Photosynthesis