Nitrococcus mobilis
Classification
Domain: Bacteria
Phylum: Proteobacteria
Class: Gammaproteobacteria
Order: Chromatiales
Family: Ectothiorhodospiraceae
Genus: Nitrococcus
Species
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NCBI: Taxonomy |
Nitrococcus mobilis
Description and Significance
Describe the appearance, habitat, etc. of the organism, and why you think it is important.
Nitrococcus mobilis is found only in marine environments. The only laboratory cultured sample was found in the Pacific Ocean off of the Galapagos Islands. When cultured in the laboratory, it was found N. mobilis requires media compromising of at least 70% sea water to grow. Just adding salts does not give 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.
This bacteria is a nitrifier meaning it reduces nitrite to nitrate (HNO2 + H2O to HNO3). This is important in the nitrogen cycling in marine environments. Nitrospira gracilis is the only other nitrifier that is an obligate halophile. Nitrococcus mobilis has been found at smaller concentrations then other nitrifying bacteria such as Nitrosomonas species but since plate count methods have been used for determining, there is criticism that these numbers are not exact (1). The reactions carried out by Nitrococcus mobilis has been found to be similar to other nitrifying bacteria by comparing nitrite oxidoreducatase used in the different species (1).
Genome Structure
Describe the size and content of the genome. How many chromosomes? Circular or linear? Other interesting features? What is known about its sequence?
16S rRNA 1483 base pairs
Entire genome is 25.4 kbp.
The GC content of the genome is 60%. (5)
Nitrococcus has been found to have participated in horizontal gene transfer of a carboxysomal operon with Paulinella chromatophora which is an amoeba(2). 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.
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 the final electron acceptor. Inorganic carbon is its carbon source, making it an autotroph. The energy from reducing nitrogen is very small, therefore 100 mol 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.
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 devision 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 apart 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 tubuler 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)
Ecology and Pathogenesis
Nitrococcus mobilis has no known pathogenicity. There are also no known symbiotic relationships.
This bacteria has been found to use a broad range of wavelengths but its optimal productivity occurs at 325, 375, and 475 nm. Nitrococcus mobilis is more sensitive to light than Nitrobacter sp. which may explain why Nitrobacter sp. have been found at higher concentrations in marine ecosystems. Therefore, Nitrococcus mobilis must be able to more productively use a different resource for it to coexist with other nitrifying bacteria (3).
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.
References
Author
Page authored by Hanna Miller and Toni Miskovich, students of Prof. Jay Lennon at Michigan State University.
