Its possible. There is anoxic zones in the media. Where there is deplete of oxygen, thus allowing anaerobic bacteria to grow. Aerobic bacteria ( not sure how to spell ) will be on the outer side of the pores, consuming oxygen. That is why deep inside the pores, there is lack of oxygen.
Yes, I was thinking the same thing; somehow, the media must create localized anaerobic spots for denitrification to occur.
However, what would be the purpose (if anaerobic bacteria truly are present)? Denitrification would take the nitrates that are produced by the aerobic bacteria, and convert back to nitrites...which are then converted back to nitrates by the aerobic bacteria.
On top of this, the aerobic bacteria would be much in excess compared to the anaerobic bacteria, meaning nitrification would be very