According to the internet "Aqueous carbon dioxide, CO2 (aq), reacts with water forming carbonic acid, H2CO3 (aq). Carbonic acid may loose protons to form bicarbonate, HCO3- , and carbonate, CO32-. In this case the proton is liberated to the water, decreasing pH. The complex chemical equilibria are described using two acid equilibrium equations."
If you've ever drunk flat carbonated water, you should know the acidic flavour that carbonation imparts to water. I assume it has some impact on the flavour of beer, though presumably since beer already has noticeable flavour this difference is less pronounced.