[Subsea]

In the oceans, the only natural nutrient export is denitrification chemistry with faculative bacteria in a low oxygen enviroment. The oxygen in the NO3 (nitrate) molecule is consumed by faculative bacteria with water and free nitrogen gas to go back to the athmosphere.
Just as carbon dioxide exchanges between the ocean and the athmosphere so does nitrogen. Free nitrogen gas in the athmosphere and dissolved nitrogen gas in the water is a nutrient input through a process called nitrogen fixation, which is accomplished mainly by cynobacteria.
When I started in this hobby, DSB and live rock were the accepted methods to perform denitrification. I used the Jaubert Plenum method with a 6" DSB of crushed coral with a 2mm-5mm diameter. This large diameter substrate extended the faculative zone deeper into the substrate with a larger population of faculative bacteria. This method was differrent from the DSB methods proposed by Ron Schmeck, PHD marine biologist. While there was some denitrification chemistry going on, nutrient recycling was its main purpose. Micro fauna and fana absorbed nutrients and reproduced. This feeds the corals and other filter feeders.

PS. Since those early days, it has been shown that nitrification and denitrification can happen in close proximity of each other. While I have one DSB in operation for 20 years, I would not set up a tank with one again.