Preliminary evaluation of ferrous sulphate toxicity on larvae of the sea urchin Paracentrotus lividus: implications for ocean iron fertilization.

Ocean iron fertilization (OIF) has emerged as a potential marine carbon dioxide removal strategy, involving the addition of ferrous sulphate (FeSO₄) to iron-limited waters to stimulate phytoplankton blooms and enhance carbon sequestration. However, the ecotoxicological impacts on non-target organisms remain poorly characterised. We evaluated FeSO₄ toxicity using embryo-larval bioassays with the sea urchin Paracentrotus lividus, a standard model for marine ecotoxicology. FeSO₄ demonstrated an EC₅₀ of 60.81 μg/L (95% CI: 53.29-68.33 μg/L), compared to 17.90 μg/L for copper used as positive control. Despite lower overall toxicity, FeSO₄ induced substantial malformations (54.3% at tested concentrations) with a distinct damage profile characterised predominantly by skeletal abnormalities (68% of malformations), particularly cross-linking of apex spicules (56%), whilst copper primarily caused developmental arrests (58%). The comparable toxic potency between FeSO₄ and copper raises significant concerns for large-scale OIF operations, particularly when considered alongside other ecological risks including oxygen depletion from remineralisation of algal biomass. Although theoretical dilution calculations suggest average iron concentrations below toxic thresholds under ideal mixing conditions, oceanographic stratification phenomena, particularly seasonal thermoclines and diurnal heating cycles, could create localised high-concentration zones. Our findings emphasize that OIF environmental risk assessments must consider near-field exposures and stratification dynamics rather than relying solely on post-mixing average concentrations, and should incorporate comprehensive zooplankton monitoring alongside deployment protocol optimisation to minimise ecological impacts on these critical trophic intermediaries.