Ecological feasibility of ocean alkalinity enhancement: Physiological and metabolic responses of Isochrysis galbana.

Ocean alkalinity enhancement (OAE) has emerged as a promising marine negative emission technology for climate change mitigation, yet its ecological safety, particularly regarding key primary producers, remains incompletely understood. Isochrysis galbana is a widely distributed nanophytoplankton that serves as a valuable feed in aquaculture. This study aimed to systematically investigate the physiological and metabolic responses of I. galbana to simulated ocean alkalization under gradient OAE scenarios (Δ total alkalinity ≈ +600 and +1200 µmol kg⁻¹). A 16-day experiment was conducted to verify the established alkalization regime and to evaluate the subsequent changes in algal growth, antioxidant enzyme activities, and metabolic/transcriptomic profiles, with a focus on lipid metabolism. Verification of carbonate system parameters confirmed that the experimental conditions successfully established the intended alkalization gradients. Physiological resilience was observed in I. galbana during initial OAE treatment, whereas late-stage growth inhibition and partially increased enzyme activity reflected an adaptive stress response. However, substantial metabolic and transcriptional reprogramming was observed, particularly affecting metabolites associated with energy storage and lipid composition, and characterized by a resource allocation shift from growth/storage toward defense. These findings provide critical insights into the responses of marine primary producers to anthropogenic alkalization interventions and offer evidence to inform the ecological risk assessment of ocean negative emission technologies.