Differential responses of size-fractionated eukaryotic microalgae to ocean alkalinity enhancement in oligotrophic seawaters

Ocean alkalinity enhancement through olivine dissolution represents a critical CO 2 removal pathway. While laboratory experiments have provided insights into olivine dissolution impacts on eutrophic coastal seawater, ecological responses in offshore oligotrophic seawater remain poorly understood. This study investigated how olivine dissolution affects size-fractionated eukaryotic microalgae through shipboard incubations conducted in the South China Sea under near- in situ conditions. Our study demonstrated that olivine addition enhanced the richness and evenness of eukaryotic microalgae, promoted community network stability, and altered the direction of succession in the microbial community. Furthermore, olivine dissolution products modulated the activities of eukaryotic microalgae, which may further influence biological pump efficiency. Beyond the dissolution effects, undissolved olivine particles may also influence microbial community structure by providing attachment substrates and generating localized microenvironments at the particle–water interface. Additionally, olivine addition exerted significantly greater effects on community assembly processes in small-sized (0.2–20 μm) eukaryotic microalgae compared to larger size (>20 µm) fractions. IMPORTANCE Models and laboratory experiments suggest that adding the alkaline mineral olivine to seawater is an efficient method for enhancing the ocean’s uptake of atmospheric CO 2 . To further explore the ecological changes following olivine addition, we conducted a shipboard incubation experiment in the oligotrophic waters of the South China Sea. It was found that the dissolution of olivine in seawater affects the activity, community diversity, and stability of eukaryotic microalgae. Additionally, this study reveals that olivine dissolution has a greater impact on smaller-sized eukaryotic microalgae. These findings provide important understandings for the impact of the implementation of olivine-based ocean alkalinity enhancement on eukaryotic microalgae in the oligotrophic waters.