PNS Bacteria: Denitrification Powerhouses

Every aquarist possesses a baseline knowledge of nitrification. This is the oxidization of ammonia into nitrate by certain aerobic bacteria and archaea. Those metabolic processes are summarized here:

NH3 + 1.5 O2 NO2- + H+ + H20

NO2- + ½ O2 NO3

Elevated nitrate concentrations are the inevitable result of nitrification. Chronically-high nitrate levels can induce unpleasant algae blooms as well as stress fish and invertebrates that are adapted to oligotrophic (i.e. nutrient-poor) ecosystems such as shallow tropical coral reefs. Conventional methods for removing nitrate include water changes and chemical treatments, both of which are generally costly and laborious.

Natural methods of nitrate removal are comparatively inexpensive and simple. One particularly effective tool is denitrification. Denitrification is an anaerobic process which involves the reduction of nitrite, nitrate, nitric oxide and nitrous oxide into dinitrogen (N2) gas. This requires a series of interdependent reactions which are enzymatically specific to individual species/strains. The generalized equation for the denitrification of nitrate is:

2 NO3− + 10 e− + 12 H+ → N2 + 6 H2O

There is mounting evidence that denitrification was one of the earliest metabolic strategies that has emerged on this planet and that it has been the subject of numerous vertical and horizontal genetic adaptations/refinements throughout evolutionary history. As a result, an assortment of marine and freshwater microbes species conduct denitrification in order to derive energy in areas devoid of oxygen and in excess of nitrogenous compounds. Through this respiratory strategy, denitrifying species conquer and cleanse anoxic sediments and the inner crevices of rockwork. Through their metabolic activity they play a silent, but invaluable, role in driving the world’s carbon, nitrogen, sulfur and phosphorus cycles.

The enzymes behind these reactions have been classically generalized as “nitrite reductase,” “nitrate reductase” and “nitrous oxide reductase,” but in reality, there is a huge swath of enzymatic