Here, we used outdoor microcosm exper- iments to investigate the effec t s of abrupt limestone-inspired and NaOH alkalinity additions of ∼750 μmol/kg, reflecting model-predicted ocean alkalinity enhancement (OAE) scenarios that produce severe localized impacts (e.g. large variations in pH and Omega). We assess the response of seasonal marine microbial communities (phytoplankton, bacteria) and viruses from the Santa Barbara Channel, analyzed by high-throughput amplicon sequencing and flow cytometry.This dataset is included in the OA-ICC data compilation maintained in the framework of the IAEA Ocean Acidification International Coordination Centre (see https://oa-icc.ipsl.fr). Original data were downloaded from Dryad Digital Repository (see Source) by the OA-ICC data curator. In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2024) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation by seacarb is 2026-07-03.