As the atmospheric CO2 concentration rises, more CO2 will dissolve in the oceans. The consequences of the resulting drop in pH for marine biodiversity remain unclear. This especially applies to bacteria, as their pH tolerance is broad in relation to the predicted acidification range. At the community level however, even slight changes in pH might favour distinct bacterial groups, leading to a compositional shift. So far, studies looking at effects of ocean acidification on microbial communities mainly featured large-scale mesocosms. Here, complex factors come into play like induced phytoplankton blooms. To look only at the direct pH effects on microbial diversity, we conducted small-scale laboratory acidification experiments. We sampled water at Helgoland Roads long-term sampling station, North Sea, and accounted for seasonality by repeating the experiment four times (spring, summer, autumn, winter). Different dilution procedures selected for certain bacterial growth strategies. We studied alpha- and beta-diversity obtaining community patterns with the molecular fingerprinting method automated ribosomal intergenic spacer analysis (ARISA). Although variation between replicates was considerable, distinct patterns were observed for the pH treatments. Subsequent multivariate analyses revealed significant shifts for certain dilution procedure and season combinations. This suggests that already relatively small changes in pH affect the bacterial community.