Metal bioaccumulation from diatoms to Antarctic krill under ocean alkalinity enhancement with steel slag

Abstract Ocean alkalinity enhancement (OAE) is an emerging carbon dioxide removal (CDR) strategy that utilizes alkaline materials to increase alkalinity in seawater and associated CO2 storage as bicarbonate. Steel slag is a potential alkaline source material for OAE due to its widespread availability and comparatively high efficiency to enable CDR. However, steel slag also contains trace metals. To assess the risks of metals related to slag-based OAE on marine food chains, we conducted a 14-day experiment culturing the diatom Chaetoceros neogracilis and Antarctic krill Euphausia superba under an OAE scenario (+ 30 µmol kg⁻¹ alkalinity). We used a two-factorial design, where slag-treated and control diatoms were fed to krill cultured in slag-treated and control seawater, respectively. Slag-treated medium (enriched with 0.004 g L−1 slag powder for 48 hours after which the medium was 0.2 µm-filtered) had elevated dissolved Al and Mn concentrations. Diatoms cultured in slag-treated medium showed elevated intracellular Mn and lower total particulate Cr compared to the control, though growth and photosynthetic efficiency were unaffected. Krill metal content was not affected by the OAE- treatments, but varied significantly among body parts, with the digestive gland and stomach showing higher concentrations than the muscle. Krill faecal pellets had the highest metal: carbon ratios, indicating efficient metal excretion. Bioaccumulation factors of Al and Mn in krill were low, suggesting limited metal accumulation from seawater. Biomagnification factors for Al and Mn were consistently below 1, suggesting a low potential for additional uptake of metals from diatoms. Our results indicate that steel slag leachate at the level and timescales applied here may have a limited effect on trophic transfer of some key metals from diatoms to Antarctic krill.