Site-specific seawater conditions modulate bacterial community responses to ocean alkalinity enhancement

Ocean alkalinity enhancement (OAE) based on alkaline mineral addition is a promising marine carbon dioxide removal (mCDR) strategy. However, knowledge remains limited on how OAE affects microbial communities across heterogeneous ocean environments, which play essential roles in marine biogeochemical cycles. In this study, we investigated the responses of the bacterial community and the carbonate system to OAE, achieved through olivine addition, in Qingdao (temperate) and Sanya (tropical), China. On day 9, relative to controls, olivine addition increased total alkalinity (TA) by 58 μmol/kg in Qingdao and 165 μmol/kg in Sanya, accompanied by concurrent increases in dissolved inorganic carbon (DIC) of 46 μmol/kg and 111 μmol/kg, pH of 0.012 and 0.047, and decreases in p CO 2 of 3 μatm and 35 μatm, respectively. The direction of microbial succession was consistent, though the rate was lower in the experimental groups than in the control groups. Olivine promoted bacterial network stability in Qingdao but not in Sanya. In Qingdao, olivine addition appeared to alleviate competition among microorganisms, resulting in significantly higher bacterial richness coupled with slightly lower deterministic processes. In contrast, in Sanya, olivine addition appeared to impose stronger selective pressure on microorganisms, as indicated by slightly lower richness and significantly higher deterministic processes. These results indicate that the ecological influences of olivine are governed by site-specific environmental factors, highlighting the necessity of assessing the in situ ecological impacts of OAE across diverse deployment locations. Synopsis The responses of microorganisms and the carbonate system to OAE vary across different zones, highlighting the critical influence of ocean heterogeneity in OAE deployment.