Nitrification

As nitrification is called the bacterial oxidation of ammonia ( NH3) to nitrate ( NO3- ). It consists of two connected sub-processes: in the first portion of ammonia is oxidized to nitrite, which is oxidized to nitrate in the second partial process. Both sub-processes provide for the participating organisms sufficient energy for growth and other life processes. In the nitrogen cycle of ecosystems, nitrification plays an important role as it transfers the released by decomposers of dead biomass ammonia into nitrate again. This produces nitrogenous mineral nutrients for plants. Sergei Winogradsky was the first to realize that the nitrification is a concerted process, participate in the different groups of bacteria. He described in his ( from today's perspective ) classical publications for nitrification not only the sub- processes, including nearly all known genera until today were first described. Winogradsky It used the prefix nitroso together as a unifying element in the generic name of the organisms involved in the first part of the process. The genera of the second thread given the prefix nitro in the genus name.

First part process

The first sub-process consists of the oxidation of ammonia with molecular oxygen to form nitrite (Equation 1). Under standard conditions (see energy metabolism ) are per mole of unreacted ammonia 235 kJ of energy released, according to the change of the free energy: DG ° '= -235 kJ / mol.

The bacteria responsible for the first sub-process are referred to as ammonia -oxidizing bacteria and nitrite bacteria. All representatives are aerobic and obligately chemolithoautotrophic. The following species, identifiable by the word part nitroso, including:

• Nitrosomonas
• Nitrosospira
• Nitrosovibrio
• Nitrosolobus
• Nitrosococcus

The oxidation of ammonia by means of molecular oxygen is carried out in two steps. In the first step (Equation 2), the ammonia to hydroxylamine by the enzyme Ammoniummonooxygenase (AMO) is oxidized. In this reaction, an oxygen atom of the oxygen molecule is incorporated into hydroxylamine, the other reduced to water. In the second step (eq. 3) is oxidized to hydroxylamine, nitrite, catalysed by the oxidoreductase hydroxylamine ( HAO ). 4 moles of the obtained from the oxidation of hydroxylamine electrons ( reaction 2), 2 mol of electrons for AMO reaction ( reaction 1) was used, about 1.7 moles of oxygen electrons are transferred by cytochrome C (Equation 4), the remaining 0.3 mol of the electron flow for the production of NAD (P) H in the decline in the electron transport.

Second part process

The second part of the process consists in the oxidation of nitrite to nitrate with molecular oxygen in the oxidized under standard conditions per mole of nitrite 76 kJ of energy is released ( the free energy change DG ° ' is -76 kJ / mol).

This reaction is catalyzed by the enzyme Nitritoxidase. The bacteria responsible for the second part of the process are referred to as nitrite -oxidizing bacteria and nitrate bacteria. All representatives are aerobic and up to Nitrobacter obligate chemolithoautotrophically. The following species, identifiable by the word part nitro, including:

• Nitrobacter
• Nitrospira
• Nitro Spina
• Nitrococcus

Overall implementation

Both sub-processes ( Equation 1 and Equation 5) sum to:

In this reaction under standard conditions per mole of ammonia oxidized 311 kJ of energy released ( the free energy change DG ° ' of -311 kJ / mol).

In nature, under normal conditions, representatives of both groups of bacteria, ammonia and nitrite oxidizer come together before and cooperate so that does not accumulate nitrite.

Physiology of nitrifying

The nitrifying bacteria ( nitrifying bacteria ) carry a chemolithoautotrophic metabolism: The inorganic nitrogen compounds serve as both electron donor ( Lithotrophie ) and together with oxygen O2 as an energy source ( Chemotrophie ). The heat released during the oxidation energy is required for the synthesis of ATP from ADP and phosphate. ATP is used mainly for the synthesis of biomass from carbon dioxide. The nitrifying bacteria able to satisfy their carbon demand of carbon dioxide alone. This means that they are autotrophic and operate so-called chemosynthesis. The carbon dioxide is assimilated through the Calvin cycle.

Ecological and industrial importance

Nitrifying bacteria are present in many aerobic ecosystems. The availability of the substrates required for their energy ammonia or ammonium or nitrite depends to a large degree by the ammonification. Many ammonium oxidizers can also use urea as the primary substrate. In natural systems, ammonium oxidation was also observed under conditions which were no longer tolerated by the investigated pure cultures. Thus, the ammonium oxidation in acid soils in cold ecosystems on the surface of acidic sandstones or in 50 - 60 ° C hot springs observed. In eutrophic systems, the nitrification lead to a significant consumption of oxygen, which denitrification can be promoted.

Nitrification is a production of acid (H - form, see equation 1). The pH value is reduced when the acid generated is not neutralized, for example by reaction with calcium carbonate ( CaCO3). The acid formed loaded, the buffer capacity of the water and can acidify the water or the ground. Since nitrifying bacteria metabolize only in the neutral to slightly alkaline range, the complete conversion of the fish toxic ammonium / ammonia can be prevented in sewage treatment plants ( autoinhibition ) by acidification. The nitric acid formed by nitrification can be destructive of mineral materials act (eg, building materials), especially on limestone by dissolving carbonates. Buildings are particularly affected in an environment that occurs in the ammonia and nitrification is possible, for example, waste water plants and animal stalls. Also on contact of carbonate building materials or sculptures with nitrogen oxides and water it often comes to the surface to destruction by nitrification: The nitrogen oxides with water form nitrous acid bacteria ( nitrite oxidizers ) is oxidized to nitric acid. Consequently, it results in the dissolution of carbonates and damage to the building or graphic work. In the sandstones of buildings, the outer regions with nitrifying bacteria are often interspersed. At the Cologne Cathedral ammonium oxidizers were (almost exclusively Nitrosovibrio ) detected in up to 15 cm depth in the sandstone. Along with the infestation of nitrifying bacteria was also the pH decreased in the stone, which by the time of the present in the sandstone dolomite ( CaMg ( CO3) 2 ) is dissolved.

In the aquarium based the main part of the filtering, the so-called biological filtration of the aquarium water to the nitrification.

Nitrification 4.33 g of oxygen (O2 ) per gram formed NO3 -N consumed ( oxygen serves as an electron acceptor ). Per gram by nitrifying bacteria formed NO3 -N Nitrifikatenbiomasse grows correspondingly 0.24 g chemical oxygen demand (COD) to ( cell yield, Eng. Yield ). 1.42 grams of COD correspond to a biotic gram dry weight.