Originally Posted by salmo
When CO2 react with water it forms unstable carbonic acid H2CO3 making environment slightly acidic. The pH is slightly below 7 , which is a neutral point . There is an equilibrium between CO2, H2O and carbonic acid.
fwiw, the pH of pure water in equilibrium with atmospheric CO2 is 5.65 - quite acidic - yet poorly buffered. Seawater has circumneutral pH because of all the CaCO3 in it, from rock weathering delivered by rivers, which also helps somewhat to buffer the water against the acidifying influence of atmospheric CO2.
ocean acidification will be a subject of intense study over the next decade or so. The chemistry behind it is very rudimentary and well understood (simple carbonate chemistry). What's not as well understood is how biological systems will cope. For example, many planktonic algae are able to form calcium-carbonate skeletons are pH well below what they should theoretically be able to do, by creating local "microenvironments" conducive to carbonate precipitation. So although it seems that biology has some tricks for dealing with low pH, we don't know the true limits to those tricks as pH continues to drop.