Microorganisms Involved in Denitrification Pathways

Heterotrophic denitrification

A large range of heterotrophic microorganisms can denitrify. Heterotrophs are organisms that require organic carbon supply for growth. Denitrifiers are very successful and are present in high numbers in both terrestrial and marine ecosystems in all climate zones of our globe. They include bacillus, paracoccus, pseudomonas, propionibacterium, and thiobacillus species. Most denitrifiers are facultative aerobes, which means that they gain energy using O2 as the electron acceptor (aerobic respiration), but are able to switch to anaerobic respiration and use NO3 as an electron acceptor when O2 becomes depleted. The enzymes required for deni-trification are activated by lowering the oxygen partial pressure. Denitrifiers are generally more competitive as aerobic heterotrophs in soil than they are as denitrifiers. However, the ability to respire using O2 or NO3 enables denitrifiers to survive and proliferate in a wide range of oxic, micro-oxic, and anoxic environments and adapt quickly in rapidly changing environments.

The final product of denitrification is not always N2, but could be N2O and sometimes even NO, depending on the organism involved and on the environmental conditions. Equally, NO3 does not need to be the terminal electron acceptor; instead, other intermediates may be used. For example, the bacteria Alcaligenes faecalis, Pseudomonas stutzeri, Paracoccus denitrificans, and Pseudomonas perfectomarinus were shown to grow in the laboratory using N2O as the electron acceptor.

Nitrifier denitrification

Nitrifier denitrification is a pathway carried out by the group of nitrifiers that perform the first step of nitrification, the ammonium oxidizers, which oxidize ammonium (NH|) to hydroxylamine (NH2OH) and then NO2-. Under O2 stress, these organisms then reduce NO- to gaseous forms of N (NO, N2O, N2):

NH2OH

The main nitrifying species are autotrophic bacteria; they use CO2 as a carbon source. Nitrifier denitrifiers can use O2 for two separate functions, as a substrate in the oxidation of NH^ and subsequently as a terminal electron acceptor in respiration. It has been suggested that this process can contribute significantly to the loss of NH| as NO and N2O from soil; however, it is difficult to really distinguish between autotrophic nitrifier denitrification and heterotrophic deni-trification. The nitrite reductase enzyme of nitrifers, and classic denitrifiers are biochemically and genetically similar. For example, similar spectroscopic characteristics, inhibition profiles, and reaction products were demonstrated for the copper-containing enzyme of Nitrosomonas europaea. Genetic similarities were observed between the nirKgene of a classic denitrifier and Nitrosomonas marina and other closely related ammonium oxidizers.

Chemodenitrification

Chemodenitrification is the chemical, nonenzymatic decomposition of intermediates from the oxidation

of ammonium to nitrite and nitrate (nitrification). This reaction usually occurs at low pH and may be facilitated by the presence of iron or amines and high levels of organic matter. The most important chemodenitrification reaction is the decomposition of HNO2 to form NO.

Soil pH is a controlling factor in chemodenitrification and above pH 5, this process is insignificant.

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