Marine bacteria are the main consumers of the organic matter and largely control ₂ and CO₂ budgets in the ocean. The present increase in anthropogenic O₂ emissions is constantly decreasing seawater pH. This ocean acidification is expected to affect enzymatic hydrolysis of organic compounds with so-far unknown consequences for microbial physiology, organic matter cycling and marine biogeochemistry. We studied the effect of ocean acidification on a natural plankton community during a large-scale mesocosm study in the Raunefjord (Norway). We determined the concentration and composition of dissolved organic matter, in particular combined carbohydrates and amino acid, as well as marine gel particles, bacterial growth and enzymatic rates of organic matter hydrolysis. Dissolved organic carbon concentrations were increasing over time in all treatments while dissolved organic nitrogen concentrations remained stable. We observed higher protein hydrolysis rates, gel particle concentrations, and bacterial cell numbers in the low pH treatments. We conclude that ocean acidification may favor bacterial growth and degradation activities and therewith potentially change organic matter composition and cycling in the future ocean.