Nitrobacter

Nitrobacter is a genus comprising rod-shaped, gram-negative, and chemoautotrophic bacteria. The name Nitrobacter derives from the Latin neuter gender noun 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 into 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. 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 an intracytomembrane polar cap. Due to the presence of cytochromes c, they are often yellow in cell suspensions. The nitrate oxidizing system on membranes is cytoplasmic. Nitrobacter cells have been shown to recover following extreme CO2 exposure and are non-motile. 16s rRNA sequence analysis phylogenetically places Nitrobacter within the class of Alphaproteobacteria. Pairwise evolutionary distance measurements within the genus are low compared to those found in other genera, and are 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 and Blastobacter denitrificans, and the human pathogens Afipia felis and Afipia clevelandensis. Bacteria within the genus Nitrobacter are presumed to have arose on multiple occasions from a photosynthetic ancestor, and for individual nitrifying genera and species there is evidence that the nitrification phenotype evolved separately from that found in photosynthetic bacteria. All known nitrite-oxidizing prokaryotes are restricted to a handful of phylogenetic groups. This includes the genus Nitrospira within 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. The low diversity of species oxidizing nitrite oxidation 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. Nitrification is a crucial component of the nitrogen cycle, especially in the oceans. The production of nitrate (NO3−) by oxidation of nitrite (NO2−) by nitrification the process that produces the inorganic nitrogen that supplies much of the demand by marine oxygenic, photosynthetic organisms such as phytoplankton, particularly in areas of upwelling. For this reason, nitrification supplies much of the nitrogen that fuels planktonic primary production in the world's oceans. Nitrification is estimated to be the source of half of the nitrate consumed by phytoplankton globally. Phytoplankton are major contributors to oceanic production, and are therefore important for the biological pump which exports carbon and other particulate organic matter from the surface waters of the world's oceans. 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; NO2− + H2O → NO3− + 2H+ + 2e−