What is the charge on the polyatomic ion hydrogen carbonate?

We got $H C {O}_{3}^{-}$, i.e. the conjugate base of carbonic acid, ${H}_{2} C {O}_{3}$...
And bicarbonate ion has a Lewis structure of...$\left(O =\right) C \left(O H\right) \left({O}^{-}\right)$
${H}_{2} C {O}_{3}$ might be better represented as $C {O}_{2} \cdot {H}_{2} O$, i.e. dissolved carbon dioxide...the presence of greater concentrations of carbonic acid in the world's oceans might mean a drop in mean ocean $p H$...tho I haven't seen good data to support this phenomenon.
What general effect do you think a drop in $p H$ would have on the ocean's coral reefs, the which are insoluble $C a C {O}_{3} \left(s\right)$ to a first approx....??