Denitrification riddle

Hi,

I understand (I believe) how DSB and plenums work. By creating an anoxic
zone where bacteria are forced to pull oxygen out of the nitrate molecule,
leaving inert nitrogen behind.

What baffles me is if the oxygen cant get the anoxic zone, how in blazes
does the water and nitrate get there in order to be denitrified? Or is it
that the anoxic zone incubates this sort of bacteria, which then finds its
way into the water column, but still goes for the nitrate rather than
oxygen as thats all it knows?

Inquiring minds want to know.

-Phil