Originally all nitrifying bacteria were classified under the family Nitrobacteraceae. It wasn't until the ability to sequence the 16s rRNA gene that this family was broken up as the species were very diverse. N. mobilis is the only isolated nitrifier in the gammaproteobacteria class. (7)
The family Ectothiorhodospiraceae is composed mainly of halophilic and haloalkaliphilic bacteria. This family was originally created for purple sulfur bacteria with other bacteria being added later as the 16s rRNA genes were analyzed. Besides the anaerobic purple sulfur bacteria there are also aerobic chemolithotrophs such as Arhodomonas and Alkalispirillum. (10)
Description and Significance
Nitrococcus mobilis is found only in marine environments and is an obligate halophile. It is coccus in shape and has a flagellum to aid in movement. This bacterium is a nitrifier meaning it oxidizes nitrite to nitrate (HNO2 + H2O to HNO3). This is important in nitrogen cycling in marine environments. N. mobilis has been found at lower concentrations than other nitrifying bacteria such as Nitrosomonas species. Since plate count methods were used for determining the concentrations, there is criticism that these numbers are not exact. (1) N. mobilis also plays a small role in the marine carbon cycle as it fixes inorganic carbon which can then be utilized by herterotrophs.
N. mobilis has been cultured in a laboratory which provided a greater insight into its growth requirements. The cultured sample was found in the Pacific Ocean off of the Galapagos Islands. In culture N. mobilis requires media compromised of at least 70% sea water to grow. Just adding salts does not produce a media capable of culturing. The optimal temperature for growth was found experimentally to be 25° to 30°C and a pH of 7.8-8.0.
Methods for synthesizing polymers using enzymes found in microbes are being developed. Polymers can be synthesized using a thioester reaction with acetyl Co-A attached to different monomers. Therefore microorganisms with metabolic enzymes that use acetyl-co A as a substrate are being considered as possible mechanisms. N. mobilis was found to efficiently catalyze this reaction and is being considered along with various other bacteria.(9)
Fertilizer can also be synthesized in a more environmentally friendly manor by using ground fish, seaweed, and sea water innoculated by different microorganisms including N. mobilis, that participate in the nitrogen cycle. These organisms incorporate atmospheric nitrogen into a form that can be used by crops. (11)
N. mobilis contains a single circular chromosome that is approximately 25.4 kilobase pairs large. The 16s rRNA gene, used in classification, is 1483 base pairs long. The guanine/cytosine concentration is 60%. (5)
N. mobilis has been found to have participated in horizontal gene transfer of a carboxysomal operon with Paulinella chromatophora which is an amoeba. The amoeba's DNA has evidence of several horizontal gene transfers. The DNA of Paulinella chromatophora shows evidence that it may have been one of the first eukaryotes as it is compromised of several bacterial operons. (2)
Cell Structure, Metabolism and Life Cycle
Interesting features of cell structure; how it gains energy; what important molecules it produces.
Nitrococcus mobilis is an obligate chemoautotroph. The energy required for metabolism comes from chemical reactions of reducing nitrogen from nitrite to nitrate. They are aerobic as oxygen is required in the oxidation reaction. Inorganic carbon in the form of carbon dioxide is fixed through the Calvin Cycle (8). The energy from reducing nitrogen is very small, therefore 100 moles of nitrite must be oxidized to fix one molecule of carbon dioxide. The majority of the energy produced by the cell (80%) is used for carbon fixation. This inhibits fast growth of the cells. (8)
The reduction of nitrite occurs in a single reaction with the transfer of two electrons to water. This yields nitrate and two protons. The reaction is catalyzed by nitrite oxidoreductase. Cytochrome oxidase then catalyzes a reaction between the protons released from the oxidation reaction and oxygen to produce water. Therefore, for two molecules of nitrite oxidized, one molecule of oxygen is required. (8)
When free floating in the ocean, Nitrococcus mobilis exists as a single or diploid cell. If the cells can form a biofilm then several hundred will form aggregates on a slime layer.
The cell is a cocci with a diameter of 1.5 to 1.7 micrometers after division (it elongates to 3.5 micrometers just before division (6). Nitrococcus mobilis is the only nitrifier that is coccus. A 3-4 micrometer long flagella aids in mobility when the cell is free floating. If part of a biofilm, the flagella is lost and the only mode of movement is by rolling, which often lacks a direction. Nitrococcus mobilis is gram negative as it has an outer cell membrane surrounding a multilayered cell wall. In the cytosol of Nitrococcus mobilis is a unique cytomembrane system. This tubular system is continuous with the plasma membrane. The plasma membrane will invagenate, at what appears to be randomly distributed, to form the tubes. The tubes themselves are 6-8 nm in diameter and are formed by a double tracked cytomembrane. Electron microscopy has shown that the tubes do have contents in them. Other distinct structures in the cytosol are inclusion bodies. These are aggregates of insoluble proteins. It is unclear if the insolubility is due to misfolding or if the proteins are in different states. The particles are surrounded by a single layered membrane. The most prominent inclusion bodies are hexagonal in shape and have a diameter of approximately 100 nm. A single cell can contain as many as 200 of these particles. The cells also have glycogen storage areas and poly-beta-hydroxybutyrate granules.
Nitrification is completed with the use of nitrite oxidoreductase. This enzyme is located on the inner cytoplasmic and intracytoplasmic membranes (1).
The fatty acid composition of the membranes is unique from other nitrifying bacteria. The majority of the membrane is composed of vaccenic acid (18:1 cis11) (42-52%). There is also a higher concentration of hexadcanoic acid (16:0) and cis-9-hexadecenoic acid (16:1 cis9) then other nitrifiers at 27% and 20% respectively. The fatty acid profiles can change depending on the exact environmental conditions the bacterium is living in. (3)
The reactions carried out by Nitrococcus mobilis have been found to be similar to other nitrifying bacteria by comparing nitrite oxidoreducatase used in the different species (1).
Ecology and Pathogenesis
Nitrococcus mobilis has no known pathogenicity. There are also no known symbiotic relationships.
N. mobilis plays a role in the marine nitrogen cycle as it converts nitrite to nitrate. It, Nitrospina gracilis, and Nitrospira marina are the only halophiles capable of using nitrite as a substrate for energy. (7) Nitrite rarely collects in the oceans due to these nitrifiers who rapidly oxidize it further. Nitrite comes from the oxidation of ammonium by marine organisms such as .....(8).
As multiple nitrifiers are found in the ocean, N. mobilis must use a resource more efficiently then the others. One way is to use light wavelengths not optimized by other bacteria. N. mobilis does use a broad range of wavelengths but its optimal productivity occurs at 325, 375, and 475 nm. Nitrococcus mobilis has been found to be more sensitive to light than Nitrobacter sp. in laboratory settings which may explain why Nitrobacter sp. have been found at higher concentrations in marine ecosystems (3). This sensitivity also may determine where in the water column N. mobilis resides. If it is too close to the surface than it can experience UV rays. The bacterium also must take into account the oxygen profile which decreases with depth. Therefore, the location of N. mobilis is optimized between light intensity, oxygen concentration, and other nutrient profiles.
N. mobilis also plays a small role in the marine carbon cycle as it fixes carbon dioxide released by heterotrophic organisms. This carbon then once again becomes available to the heterotrophs when they consume the bacterium. Once the bacterium dies, it can sink out of the photic zone of the ocean and provide a carbon source to species at those depths. The role of N. mobilis in the carbon cycle is much smaller than in the nitrogen cycle due to the amount of nitrogen that must be reduced to provide enough energy to fix carbon. (8)
Habitat; symbiosis; biogeochemical significance; contributions to environment.
If relevant, how does this organism cause disease? Human, animal, plant hosts? Virulence factors, as well as patient symptoms.
[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.
(1) Bartosch, Sabine, Wolgast, Iris, Spieck, Eva, and Bock Eberhard. "Identification of nitrite-oxidizing bacteria with monoclonal antibodies recognizing the nitrite oxidoreductase". Applied and Environmental Microbiology. 1999. Volume 65. Number 9. p. 4126-4133.
(2) Marin, Birger, Nowack, Eva, Glockner, Gernot, and Melkonian, Michael. "The ancestor of the Paulinella chromatophore obtained a carboxysomal operon by horizontal gene transfer from a Nitrococcus-like gamma-proteobacterium." BMC Evolutionary Biology. 2007. Volume 7. Number 85.
(3) Guerrero, Maria and Jones, Ronald. "Photoinhibition of marine nitrifying bacteria. 11. Dark recovery after monochromatic or polychromatic irradiation" Marine Ecology Progress Series. 1996. Volume 141. p. 193-198.
(6) Watson, Stanley W. and Mandel, Manley. "Comparison of the Morphology and Deoxyribonucleic Acid Composition of 27 Strains of Nitrifying Bacteria". Journal of Bacteriology. 1971. Volume 107. No. 2. p. 563-569.
(10) Touroval, Tatjana, Spiridonova, Elizaveta, Berg, Ivan, Slobodova, Natalia, Boulvgina, Eugenia, and Sorokin, Dimitry. "Phylogeny and evolution of the family Ectothiorhodospiraceae based on comparison of 16S rRNA, cbbL and nifH gene sequences". International Journal of Systematic and Evolutionary Microbiology. 2007. Volume 57. p. 2387-2398.
Page authored by Hanna Miller and Toni Miskovich, students of Prof. Jay Lennon at Michigan State University.