You're right, O2 is by-product, not, no2. My mistake.
One of many dsb's I've come in contact with :
2 1/2 years old, 3 - 5 inch dsb, mainly oolitic mixed with caribsea #0050
Below sand bed, filled with bubble pockets and algal growth (cyno & coraline). WAY more than @ set-up to 1 year. He's always maintained a temp of 80 to 82 (lights & highly insulated new house keep it @ that temp), never has detectable nitrites, nitrates, ammonia, but, since removal of refugium, phosphates have crept up. This dsb is working!!!
I set up a tank for a friend. He was very disturbed by bubbles in his dsb too. He had switched over from fresh water because of HUGE growths of cyno. Of course, he'd tried a plant tank, failled, switched over to ciclids, leaving substrate for plants still in place. The rotten roots and oxygen poor substrate led to diseaster. Needless to say, he was very concerned this was going to happen to his new reef tank. As the tank had only been up for a couple days, with zero life, I stirred the dsb for him, releasing all trapped air. As bio-load increased, feeding increased, his dsb matured, making more bubbles. Cyno was present on glass in dsb, again, both factors concerned him. He called me again. This time I did tons of dsb research on the web. The most info I could find was written by Shimek. All the info I needed was on his site. Since then, I've heard him speak at several differrent functions. This guy knows his sand. go to
http://rshimek.com/reef/sediment.htm
scroll down to to 1st set of pictures (showing aging dsb)
The two dsb stories are the same person, same tank.