Denitrification

Denitrification refers to the conversion of the nitrate ( NO3- ) bound nitrogen to molecular nitrogen (N2 ) and nitrogen oxides by certain heterotrophic and autotrophic bacteria some of which are therefore referred to as denitrification. The process serves the bacteria to produce energy. Here, in the absence of molecular oxygen (O2 ) ( anoxic conditions ) different oxidizable substances ( electron donors ), such as organic matter, hydrogen sulfide (H2S) and molecular hydrogen ( H2), oxidized with nitrate as an oxidant (oxidizing agent ). So the process is a way of energy metabolism, namely an oxidative energy metabolism.

Reactions

The process is bound to the membranes of the bacteria, in its course, energy is conserved in the form of a proton - concentration difference between the separated through the membrane areas. This is therefore a form of anaerobic respiration, which is also referred to as nitrate respiration.

The individual steps of the multi -step reaction catalyzed by the metalloenzymes nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase:

( 1) nitrate:

(2) nitrite:

(3) nitric oxide reductase:

( 4) Nitrous oxide reductase:

Since the redox potentials of all individual steps of denitrification are positive, these bacteria reduce nitrate can be used as an electron acceptor ( oxidant) for their oxidative energy metabolism (oxidative phosphorylation ) when no or only limited molecular oxygen (O2) is available ( anoxic or hypoxic conditions ). In the oxidation of one mole of glucose with nitrate maximum of 2670 kJ of energy released ( ΔG0 '= -2670 kJ ) is.

The originating from the oxidation of organic or inorganic substances electrons (e - ) and the derived therefrom hydrogen ( H) is transferred by electron and hydrogen carriers, which differ according to the different enzymes and bacteria. In general, cytochromes and quinones serve as electron or hydrogen carrier. The electron transport leads by means of the chemiosmotic coupling for the synthesis of ATP and thus to energy conservation. In addition to N2, however, a smaller amount of the intermediate N2O is also always ( gaseous) released. This process takes place everywhere in nature, where nitrate and denitrifying oxidizable organic matter under anoxic or hypoxic conditions are available ( eg, marshes, soils, sediments in rivers and lakes). Certain bacteria, molecular hydrogen can ions ( Fe2 ) and methane with nitrate ( NO3- ) to form molecular nitrogen (N2 ) are oxidized (H2), hydrogen sulfide (H2S ), ammonium (NH4 ), iron ( II).

Denitrifying

Examples of denitrifying bacteria are:

• Paracoccus denitrificans ( autotrophic oxidation of H2 or thiosulfate ( S2O32 ) )
• Thiobacillus denitrificans ( autotrophic oxidation of sulfide ( S2 - ) or thiosulfate ( S2O32 ) )
• Pseudomonas stutzeri ( heterotrophic, oxidation of organic matter )
• Vogesella Indigofera

In general, the ability to de-nitrification in the prokaryote is widespread; Clusters there are in the alpha, beta and gamma - class of Proteobacteria.

Ecological and industrial importance

The bound in the nitrate nitrogen by denitrification to molecular nitrogen (N2 ) implemented, ie converted into a form that ( lat. sluggish, inactive ) is largely inert and by most living things not as a nutrient (nitrogen source) can be used. In water and soil it is therefore no longer available for the purposes of a fertilizer and no longer relevant to the environment. The resulting molecular nitrogen ( N2) escapes mostly in the atmosphere in which he is the main component anyway. Denitrification and discovered only recently anammox process are the only metabolic pathways, in which fixed nitrogen is excreted again in the molecular form, and are therefore an essential part of the nitrogen cycle.

Technically denitrification in wastewater treatment in sewage treatment plants is used for the elimination of nitrate. It can also be used for the removal of nitrate in the drinking water (see Water Treatment ). As a reductant ( electron donor ) while alcohol, rare molecular hydrogen is often used.