Influence of Inorganic Nutrients on a North Atlantic Community Response to Ocean Alkalinity Enhancement

Ocean Alkalinity Enhancement (OAE) is a promising carbon dioxide removal strategy, but its ecological impacts on marine microbial communities under varying nutrient condi-tions remain poorly understood. We conducted laboratory incubations using a natural North Atlantic microbial assemblage to investigate responses to OAE under both natural and nutrient-enriched regimes. We tracked phytoplankton and bacterioplankton dynam-ics, biomass, and leucine aminopeptidase (LAP) and alkaline phosphatase (ALP) activity as indicators of organic matter remineralization. OAE consistently reduced phytoplank-ton abundance in both nutrient regimes, likely due to CO₂ limitation, resulting in lower production of phytoplankton-derived organic matter. This reduction was reflected in de-creased LAP activity and shifts in the relative abundance of phytoplankton-associated bacterial taxa. These findings indicate that OAE can directly affect phytoplankton through carbonate chemistry alterations, with potential microbial responses largely mediated by changes in organic matter availability. While short-term microbial disruptions were modest, the ecological consequences of altered bloom dynamics should be carefully con-sidered in future OAE deployment strategies.