Focusing on Mg-ACC nanoparticles formed at a Mg ratio of 70:30, we observe a drastic slowing of reaction kinetics by four orders of magnitude with a slight acidification from pH 8.8 to 8.7. This change also results in a 100 % increase in the RSD (from 10 % RSD at pH 8.8 to 20 % RSD at pH 8.7). This suggests that even slight ocean acidification could potentially have a tremendous impact on biomineralization pathways that involve ACC nanoparticle-based attachment mechanisms.