Alkaline mineral dissolution can impair embryonic development in the Pacific oyster ( <i>Magallana gigas</i> ), raising caution for ocean alkalinity enhancement

Abstract Ocean alkalinity enhancement (OAE) is a promising marine carbon dioxide removal (mCDR) approach aimed at mitigating climate change by increasing oceanic CO2 uptake. However, its potential biological effects remain poorly investigated. Here, we examined the impact of a wide range of alkalinity levels—from ambient (∼2300 µmol kg⁻¹) to 11 000 µmol kg⁻¹—achieved through the dissolution of olivine, limestone, ground oyster shell, and sodium carbonate on the embryonic development of Pacific oysters (Magallana gigas), a key species in marine ecosystems and aquaculture to defining safe operating limits for various OAE deployment scenarios. We found that relatively modest levels of olivine dissolution, within the ranges feasible for OAE (ca. +600 µmol kg⁻¹ of total alkalinity), can impair embryonic development of oysters, likely due to the release of trace metals such as nickel. Limestone showed no developmental effects while oyster shell displayed moderate and variable negative effects. Sodium carbonate had no adverse impacts. Our study shows that the careful selection of alkaline materials is key to minimise ecological risks in OAE, emphasising the need to assess biological impacts for sustainable mCDR deployment.