Reviews and syntheses: Experimental evidence of phytoplankton responses to Ocean Alkalinity Enhancement

Abstract. Ocean alkalinity enhancement (OAE) has emerged as a promising carbon dioxide removal strategy aimed at increasing seawater alkalinity and enhancing long-term oceanic carbon uptake. However, concerns remain regarding the potential ecological impacts of large-scale alkalinity manipulation on marine planktonic communities, particularly phytoplankton, which play a central role in marine biogeochemical cycles and primary production. This review synthesizes the current experimental evidence on phytoplankton responses to different OAE approaches, including hydroxide-based, mineral-based, and bicarbonate-based treatments. We compare early conceptual predictions with recent laboratory, microcosm, mesocosm, and modelling studies, highlighting how the field has evolved from theoretical risk assessment toward increasingly mechanistic and experimentally grounded investigations. Current evidence generally indicates that phytoplankton communities exhibit a relatively high tolerance to moderate CO₂-equilibrated OAE scenarios, with limited effects on biomass, productivity, and community composition. More pronounced physiological and ecological responses are observed under unequilibrated or mineral dissolution treatments, where rapid pH shifts, trace metal release, and nutrient interactions may alter species-specific performance and community dynamics. Responses vary substantially across taxonomic groups and environmental contexts, reflecting the importance of carbonate chemistry, nutrient availability, and trace metal sensitivity in shaping OAE outcomes. Overall, existing studies suggest that phytoplankton responses to OAE are more nuanced and context-dependent than initially hypothesized, although substantial uncertainties remain regarding long-term ecosystem restructuring and large-scale biogeochemical feedback.