Denitrifying Bacteria

Denitrifying bacteria form a necessary part of the process known as denitrification as part of the nitrogen cycle, their primary purpose being to metabolise nitrogenous compounds, with the assistance of the nitrate reductase enzyme, to turn oxides back to nitrogen gas or nitrous oxides for energy generation. This process takes place only in the absence of oxygen, as most denitrifying bacteria are facultative aerobes (prefers to use oxygen as their terminal electron acceptors), however, they could also utilize nitrate instead. Therefore, denitrification can only be performed under anaerobic conditions. This is the main reason why denitrification mostly occurs in the deep substrata of the soil, or in areas of stagnant water.

The process of denitrification lowers the fertility of the soil and thus is less common in areas where the land is rather well-cultivated. But this loss of nitrogen to the atmosphere can eventually be regained via introduced food and water, as part of the nitrogen cycle. Some nitrogen may also be fixated by species of nitrifying bacteria and the cyanobacteria.

Denitrification also becomes the key pathway for dissimilative nitrate reduction, which is the process in which nitrates are reduced from the soil, the former being highly toxic for living organisms. Denitrification tends to produce large amounts of by-products.

The most common denitrification process is basically outlined below, with the nitrogen oxides being converted back to gaseous nitrogen (as opposed to that of nitrifying bacteria):

2 NO₃- + 10 e- + 12 H+ → N₂ + 6 H₂O

As can be seen, the result is one molecule of nitrogen (consisting of two atoms) and six molecules of water.

Denitrifying bacteria themselves include several species of pseudomonas, alkaligenes and bacillus. By their activity the losses of nitrogen into the atmosphere is roughly balanced by that which is released into the soil by nitrifying bacteria, forming a relatively balanced cycle.