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Nitrobacter is a genus comprised of rod-shaped, gram-negative, and chemoautotrophic bacteria. The name Nitrobacter derives from the Latin neuter gendernoun nitrum, nitri, alkalis; the Ancient Greek noun βακτηρία,  βακτηρίᾱς, rod. They are non-motile and reproduce via budding or binary fission. Nitrobacter cells are obligate aerobes and have a doubling time of about 13 hours.

Nitrobacter play an important role in the nitrogen cycle by oxidizing nitrite to nitrate in soil and marine systems. Unlike plants, where electron transfer in photosynthesis provides the energy for carbon fixation, Nitrobacter uses energy from the oxidation of nitrite ions, NO2−, into nitrate ions, NO3−, to fulfill their energy needs. Nitrobacter fix carbon dioxide via the Calvin cycle for their carbon requirements. Nitrobacter belongs to the α-subclass of the Proteobacteria.

Morphology and Characteristics
Nitrobacter are gram-negative bacteria and are either rod-shaped, pear-shaped or pleomorphic. They are typically 0.5-0.9 x 1.0-2.0μm in size and have a intracytomembrane polar cap. Due to the presence of cytochromes c, they are often yellow in cell suspensions. The nitrate oxydizing system on membranes is cytoplasmic. Nitrobacter cells have been shown to recover following extreme CO2 exposure and are non-motile.

Phylogeny
16s rRNA sequence analysis phylogenetically places Nitrobacter within the class of Alphaproteobacteria. Pairwise evolutionary distance measurements within the genus are low compared to other genera, at less than 1%. Nitrobacter are also closely related to other species within the alpha subdivision, including the photosynthetic Rhodopseudomonas palustris, the root-nodulating Bradyrhizobium japonicum, Blastobacter denitrificans, and the human pathogens Afipia felis, and Afipia clevelandensis. Nitrobacter bacteria are presumed to evolve from photosynthetic ancestors, and there are evidences that the nitrification phenotype evolved separately from photosynthetic bacteria on multiple occasions for individual nitrifying genera and species.

All known nitrite-oxidizing prokaryotes are restricted to a handful of phylogenetic groups. Beyond that, nitrite oxidation is known to occur only in the genus Nitrospira of the phylum Nitrospirae, and the genus Nitrolancetus from the phylum Chloroflexi. Before 2004, nitrite oxidation was believed to only occur within Proteobacteria, it is likely that further scientific inquiry will expand the list of known nitrite oxidizing species even more. The low diversity of species performing nitrite oxidation notably contrasts with other processes associated with the nitrogen cycle in the ocean, such as denitrification and N-fixation, where a diverse range of taxa perform analogous functions. This might change as future research identifies new prokaryotic species.

Nitrification
Nitrification is a crucial component of the nitrogen cycle, especially in the oceans. The oxidation of nitrite (NO2-) into nitrate (NO3-) in nitrification is a crucial step, as photosynthetic organisms such as phytoplankton are only able to take up nitrogen in the form of nitrate. For this reason, nitrification is a source of nitrate for much of the planktonic primary production that occurs in the world's oceans. Nitrification is the source of half of the nitrate consumed by phytoplankton globally. Phytoplankton are major contributors to oceanic production, and are therefore important for the oceanic biological pump. The process of nitrification is crucial for separating recycled production from production leading to export. Biologically metabolized nitrogen returns to the inorganic dissolved nitrogen pool in the form of ammonia. Microbe-mediated nitrification converts that ammonia into nitrate, which can subsequently be taken up by phytoplankton and recycled.

The nitrite oxidation reaction performed by the Nitrobacter is as follows;

2NO2− + H2O → NO3− + 2H+ + 2e−

2H+ + 2e− + ½O2 → H2O

The Gibbs' Free Energy yield for nitrite oxidation is:

ΔGο = -74 kJ mol-1 NO2-

In the oceans, nitrite-oxidizing bacteria such as Nitrobacter are usually found in close proximity to ammonia-oxidizing bacteria. These two reactions together make up the process of nitrification. The nitrite-oxidation reaction generally proceeds more quickly in ocean waters, and it is therefore not a rate-limiting step in nitrification. For this reason, it is rare for nitrite to accumulate in ocean waters.

The two-step conversion of ammonia to nitrate observed in bacteria species such as Nitrobacter is puzzling to researchers. Complete nitrification, the conversion of ammonia to nitrate in a single step, has an energy yield (∆G°′) of −349 kJ mol−1 NH3, while the energy yields for the ammonia-oxidation and nitrite-oxidation steps of the observed two-step reaction are −275 kJ mol−1 NH3, and −74 kJ mol−1 NO2−, respectively. These values indicate that it would be energetically favourable for an organism to carry out complete nitrification from ammonia to nitrate, rather than conduct only one of the two steps. The evolutionary motivation for a decoupled, two-step nitrification reaction is an area of ongoing research. In 2015, it was discovered that the genus Nitrospira possesses all the enzymes required for carrying out complete nitrification in one step, suggesting that this reaction does in fact occur. This discovery raises questions about evolutionary capability of Nitrobacter to conduct only nitrite-oxidation.

Metabolism and Growth
Nitrobacter oxidize nitrite as a source of energy and reductant, and use CO2 as a carbon source. Nitrite is not a particularly favourable substrate from which to gain energy. Thermodynamically, the nitrite oxidation reaction gives a yield (∆G°′) of only -74  kJ mol−1 NO2−. As a result, Nitrobacter has developed a highly specialized metabolism to derive energy from the oxidation of nitrite.

Nitrobacter may reproduce by budding or binary fission. Carboxysomes, which aid carbon fixation, are found in lithoautotrophically and mixotrophically grown cells. Additional energy conserving inclusions are PHB granules and polyphosphates. When both nitrite and organic substances are present, cells can exhibit biphasic growth, first the nitrite is used and after a lag phase, organic matter is oxidized. Chemoorganotroph growth is slow and unbalanced, thus more poly-β-hydroxybutyrate granules are seen that distort the shape and size of the cells.

The enzyme responsible for the oxidation of nitrite to nitrate in Nitrobacter is Nitrite oxidoreductase (NXR), which is encoded by the gene nxrA. NXR is composed of two subunits, and likely forms an αβ-heterodimer. The enzyme exists within the cell on specialized membranes in the cytoplasm which can be folded into vesicles or tubes. The α-subunit is thought to be the location of nitrite oxidation, and the β-subunit is an electron channel from the membrane. The direction of the reaction catalyzed by NXR can be reversed depending on oxygen concentrations. The region of the nxrA gene which encodes for the β-subunit of the NXR enzyme is similar in sequence to the iron-sulfur centers of bacterial ferredoxins, and to the β-subunit of the enzyme nitrate reductase, found in Escherichia coli.

Ecology and Distribution
Nitrobacter are widely distributed in both aquatic and land environments. Nitrifying bacteria have an optimum growth between 25-30°C, and cannot survive past the upper limit of 49°C or the lower limit of 0°C, limiting their distribution even though they encompass a variety of habitats. Nitrobacter have an optimum pH between 7.3 and 7.5, and will die in temperatures exceeding 120 °F (49 °C) or below 32 °F (0 °C). According to Grundmann, Nitrobacter seem to grow optimally at 38 °C and at a pH of 7.9, but Holt states that Nitrobacter grow optimally at 28 °C and grows within a pH range of 5.8 -8.5 and has a pH optima between 7.6 and 7.8.

Nitrobacter 's primary ecological role is to oxidize nitrite to nitrate which is readily absorbed by plants. This role is also essential in aquaponics. Since all Nitrobacter are obligate aerobes, oxygen along with phosphorous tend to be the limiting factors of their capability to perform nitrogen fixation. The greater impact of Nitrosomonas and Nitrobacter in both oceanic and terrestrial ecosystems are due to their effect on the process of eutrophication. The distribution and differences in nitrification rate across the Nitrobacter genus may be attributed to differences in the plasmids amongst the species, as data presented in Schutt (1990) imply, habitat specific plasmid DNA was induced by adaptation for some of the lakes that were investigated. A follow up study performed by Navarro et al. (1995) showing a 60MDa plasmid and a 37 MDa plasmid in freshwater and sediment Nitrobacter populations. In conjunction with Schutts’ (1990) study, Navarro et al. (1995) illustrated genomic features that may play crucial roles in determining the distribution and ecological impact of Nitrobacter. Nitrifying bacteria in general tend to be less abundant than their heterotrophic counterparts, as the oxidizing reactions they perform have a low energy yield and most of their energy production goes toward carbon-fixation rather than growth and reproduction.

History
In 1890, Ukrainian-Russian microbiologist Sergei Winogradsky successfully isolated the first pure cultures of nitrifying bacteria which are capable of growth in the absence of organic matter and sunlight. The exclusion of organic material by Winogradsky in the preparation of his cultures is recognized as a contributing factor to his success in isolating the microbes. In 1891, English chemist Robert Warington proposed a two-stage mechanism for nitrification, mediated by two distinct genera of bacteria. The first stage is proposed as the conversion of ammonia to nitrite and the second the oxidation of nitrite to nitrate. Winogradsky named the bacteria responsible for the oxidation of nitrite to nitrate Nitrobacter in his subsequent study on microbial nitrification in 1892. Winslow et al. proposed the type species Nitrobacter winogradsky in 1917. The species was officially recognized in 1980.

Main Species

 * Nitrobacter winogradskyi
 * Nitrobacter hamburgensis
 * Nitrobacter vulgaris
 * Nitrobacter alkalicus