Ecotoxicological impacts of ocean alkalinity enhancement (OAE) on the European flat oyster Ostrea edulis: Insights from tissue metabolomics and metal analysis.

Ocean alkalinity enhancement (OAE) is an emerging negative emission technology that increases seawater carbon sequestration capacity while potentially mitigating ocean acidification. However, environmental impacts of OAE on marine invertebrates remain poorly understood, hindering the environmental risk assessment needed ahead of large-scale implementation. This study provides a chronic evaluation of two different OAE approaches on the cellular/tissue level of juvenile Ostrea edulis, a key species for marine ecosystem restoration. Oysters were exposed for 63 days in an indoor tank system to dissolved alkalinity enhancement (NaOH and CaCl2 · 2 H2O, representing alkalinity addition without feedstock-derived metals) versus olivine-based coastal enhanced silicate weathering (CESW), both targeting total alkalinity (TA) levels of +250 and + 500 μmol kg-1. Trace metal accumulation was assessed via ICP-OES, and untargeted metabolomic profiling of 27 polar metabolites was performed in gill and mantle tissues using 1H-NMR spectroscopy. Nickel accumulated dose-dependently across all tissues in oysters exposed to olivine treatments (3-10 μg g-1 dry weight), while cobalt and chromium showed no treatment-related accumulation. Metabolite responses were small and largely treatment-specific. Dissolved alkalinity was associated with an increased energy demand for acid-base homeostasis, with decreased valine concentrations and elevated N-acetylcysteine at high ΔTA. Olivine treatments showed elevated succinate and glutamate at high ΔTA, possibly indicating a metal-related onset of mitochondrial disturbance and oxidative stress. As alkalinity treatments were represented by single tanks, these findings identify a potential environmental risk of olivine-based CESW which should be quantified in replicated experiments and could be extended to other marine species.