Anywhere where dissolved oxygen is depleted (anaerobic) bacteria can use nitrates instead of oxygen to respire. However in order for the process to work you need some kind of electron donor or carbon source such as methanol or ethanol. Or something like that, basically if you rock houses these anaerobic zones then denitrification could occur but not to an effective level without constant addition of a carbon source. This is why some people does vodka (ethanol).
All this theory about denitrification in the aquarium with the use of live rock and deep sand beds IMO is over rated, and I don't think much of any denitrification processes are occurring in our aquariums, in the ocean yes, but it's just not feasible in the aquarium.
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