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Biochemical mechanisms of nitrogen oxidizing bacteria
In nature it is possible to find various chemical elements in constant transformation due to the various processes that are carried out in it. This transformation can be observed in the cycles of nutrients, in which the relationship and interaction between the various living beings of the environment is reflected.
In this case, the nitrogen cycle is the set of changes that this has, being in the environment and through various organisms to be used. This cycle presents several processes that are carried by certain microorganisms, these are the assimilation, amonification, nitrification, denitrification, nitrogen fixation and anaerobic oxidation of the ammonia. In this case, all those who present nitrogen oxidation will be addressed to later explain the mechanisms that are involved in these.
To start talking about nitrogen oxidation it is necessary to start with the nitrification process. This consists of the aerobic oxidation of ammonia to nitrate in a series of 2 steps taken by a series of bacteria known as nitrifying bacteria. Because it is divided into 2 steps, 2 different types of bacteria, AOB and NOB, one for the oxidation of nitrite ammonia and another from nitrate nitrate, respectively, respectively are necessary. This process is presented both in nature and other processes. It is the one that occurs in the soils that is especially useful and relevant in the environment, this is because it is the one that provides the necessary nitrogen to the rest of organisms. There are many organisms that require a source of nitrogen for their growth, thus leaving the nitrification process as a regulator in obtaining or loss of the environment in general. We can also divide this process into nitritation (from ammonia to nitrite) and nitratation (from nitrate to nitrate), thus being for the independent use of these in reactors.
Nitritation occurs through the production of hydroxylamine under aerobic conditions through the following reactions: These reactions are catalyzed by 2 enzymes, these being monooxigenase ammonium and oxidorteductive hydroxylamine. The bacteria involved in these reactions are chimiolitoautotrophs, thus taking advantage of the oxidation of ammonia to obtain energy and have adequate growth. It is important to mention that there is a high oxygen consumption in these reactions, as well as the acidification of the environment due to the hydrogens generated. Likewise, it is possible to notice less growth of these bacteria compared to nitraters.
In this series of reactions, each of the enzymes performs a different job, being the first one responsible for the catalysis of ammonia A oxidation A for the production of hydroxylamine and water, thus leaving the second as in charge of oxidizing hydroxylamine to nitrite and release electrons in this process.
On the other hand, in nitratation energy is obtained by generating nitrate and using the oxidorted nitrite enzyme. The reaction in which it intervenes is as follows:
- AOB bacteria (ammonia-oxidizing bacteria/ammonia oxidizing bacteria). This kind of bacteria are gram-negative, located within the Nitrobacteriaceae family, being recognized for their ability to oxidize ammonia, some examples are: nitrosomonas, nitroseospira and nitrosococcus. It is in this kind of bacteria that we resume the first 2 reactions seen. The first of these is a reaction that absorbs or requires energy (endegonic). 2 electrons for ammonia oxidation are necessary, while the rest goes to the electron transport chain. It is the redox potential of NO2 /NH3 that allows the synthesis of a NADH through an inverted flow that uses ATP. In addition to this, an alternative to the first reaction of this process has been proposed, in which dioxide and nitrogen tetraxide are involved: this being a mechanism that does not include the presence of the enzyme ammonium monooxigenase. Obtaining organic carbon is obtained thanks to carbon dioxide fixation. The energy produced by these bacteria is hardly enough for its growth, this due to the amount of ATP that is necessary to generate the necessary reduction equivalents for this process. The monooxigenase ammonium enzyme has a certain specificity for the substrates you can use. On the other hand, these bacteria can oxidize and assimilate, but their growth can vary depending on the compounds that are in the environment, being unable to grow in methanol, propylene or benzene. Compounds such as acetate or pyravate can increase biomass, but they can also inhibit growth if their concentrations are very high.
- Nob bacteria (nitrite-oxidizing bacteria/bacteria oxidizing nitrites): This kind of bacteria is classified into 4: nitrobacter, nitrococcus, nitrospyra and nitrospine. It is worth mentioning that only Nitrobacter and Nitrospira have been found in soils. As in AOB bacteria, the reduction equivalents are generated by the inverted electron flow, consuming ATP in the process. A difference between these bacteria and AOB is that, although both use carbon dioxide fixing for growth, Nob bacteria are able to present myxotrophic and heterotrophic growths. Also, oxidorted nitrite enzyme provides bacteria for the ability to grow in anaerobic conditions. It is thanks to the metabolic versatility that these bacteria are more common than the AOB.
- Heterotrophic nitrification: This nitrification consists in the oxidation of reduced nitrogen organic and inorganic forms. One of the mechanisms used is similar to that registered in ammonia autotrophic oxidants, being in certain microorganisms, linked to aerobic denitrification.
- ANAMOX: This is the oxidation of ammonia in anxic conditions, which is carried out by strict anaerobic bacteria. The reaction that is carried out is as follows:
There are several genres that carry out this process, being some: Brocadia, Kounenia, Anammoxoglobus, Jettenia and Scalindua. This is done inside a compartment called Anamoxosoma, the point of this compartment is to protect the bacteria from any intermediary from the reactions that could be toxic to it. What happens in Anamox reactions is that it is reduced to nitric oxide thanks to the reductase nitrite to later react with ammonium and produce hydrazine with the help of the enzyme hydrazine dehydrolase. Finally the hydrazine is oxidized to nitrogen and releases electrons with the presence of dehydrogenase hydrazine.
Electrons released in this last reaction can be used in the electron transport chain of this compartment, thus generating ATP thanks to the atasas.
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