Magnetospirillum magnetotacticum

Classification

Bacteria (Kingdom); Proteobacteria (Phylum); Alphaproteobacteria (Class); Rhodospirillales (Order); Rhodospirillaceae (Class); Magnetospirillum (Family); Magnetospirillum (Genus) [7]

Species

Magnetospirillum magnetotacticum (Synonym: Aquaspirillum magnetotacticum)

Description and Significance

Magnetospirillum magnetotacticum is a helical, magnetotactic, microaerophilic spirillum. It is typically isolated from the oxic-anoxic transition zone at the sediment-water interface of freshwater environments. [1], [6]. First identified by Robert Blakemore in the 1980’s, this microbe was initially called Aquaspirillum magnetotacticum. Later phylogenetic studies of the 16sRNA sequence resulted in the creation of a new and distinct genus, Magnetospirillum, under which A. magnetotacticum was renamed. [2]. It is a member of the magnetotactic bacteria (MTB) class, whose mobility is dependent upon the magnetic characteristics of its environment.[6] M. magnetotacticum remains one of the most important species of Magnetospirillum as it was the first to be discovered and is one of the few capable of laboratory cultivation. This is a biologically and commercially relevant microbe that produces high quality single magnetite crystals far superior to those produced industrially. Some of its many potential uses including: a contrast enhancer in MRI’s; geobiological tracers; models for biomineralization; a waste water treatment agent; cell separation imaging; the manufacturing of printing inks and magnetic tapes; and the magnetic targeting of pharmaceuticals [4], [3].

Genome Structure

Magnetospirillum Magnetotacticum has a circular genome, consisting of a total of 9,211,536 base pairs, with a G-C content of 66.43% . Protein coding genes comprise 99.08% of the genome, the rest coding for rRNA genes. There are no pseudogenes present in the genome. Though no extrachromosomal structures are found when analyzing undigested DNA, one study found possible evidence of circular plasmid of 40,000 base pairs that contains 16s rRNA, bra and por genes, made linear and thus detectable by enzyme digestion. An interesting feature of the chromosome is the clustering of genes that contribute this microbes magnetotaxicity. The genes bfr (coding for the iron the iron storage protien bacterioferritin) and magA (coding for the magnetosomal mambrane that surrounds magnetite crystals), are located maximally in the same 17% of the genome, though further studies expect to find that they are indeed much closer together. Also interesting is the overlapping of two bfr genes. Very few magnetotactic bacteria code for two bfr genes, and the few that do, have no overlap. The reason for the overlap is possibly to keep the two genes in proximity, or could play a role in the regulation of the amounts of bacterioferritin protein produced. Magnetospirillum Magnetotacticum was the first magnetotactic bacteria to be phylogenetically analyzed using 16s rRNA genes. This is because it was one of the first magnetotactic bacteria to be isolated in pure culture. Origional analysis placed the bacteria in the Aquaspirillum genus, however further studies have placed it in a new genus of Magnetospirillum. This study resulted in the following evolutionary distance tree.

Cell Structure, Metabolism and Life Cycle

M. magnetotacticum is a gram negative, helical (clockwise), motile, spirilla that possesses two bipolar flagella. The organism ranges from 0.2-0.4 by 4.0-6.0µm and exhibits a wavelength of 1-2µm. M. magnetotacticum is a freshwater heterotroph capable of utilizing a variety of compounds as carbon and energy sources; among these, intermediates of the tricarboxylic acid cycle and other carbohydrates. [1]. M. magnetotacticum’s vast biosynthetic capabilities allow it to reduce nitrate to nitrous oxide and convert ammonium ions to ammonia. Nitrate and ammonium ions act as nitrogen sources for the organism. Nitrate is the terminal electron acceptor during M. magnetotacticum's metabolism. M. magnetotacticum is also able to reduce iron [8] during the formation of the magnetite (Fe₃O₄) crystals, which are attributed to its unusual magnetotactic behavior. The magnetite synthesized by these microbes is packaged into magnetosomes that range from 40 to 50nm and arrange in chains that establish a magnetic dipole in the organism. [1], [4]. The magnetosomes align at the cell's long axis [3] and are visible as dark chains within the organism when observed under various microscopic conditions. The proper alignment of the magnetosomes maximizes the magnetic dipole in the organism allowing it to efficiently swim along the Earths magnetic field lines. [5] As a member of the bacterial kingdom, M. magnetotacticum divides via binary fission. These bacteria generally form long chains of helices. In older cultures, coccoid bodies are abundant. [1]

Ecology and Pathogenesis

Magnetospirillum Magnetotacticum was first found by R.P. Blakemore in 1975, isolated from the microarobic zone of a pond. These bacteria, as well as many other magnetotactic bacteria, live in the sediments of fresh water or wet soils. They predominate at the water-sediment interface, an oxygen concentration transition zone, which is consistent with there microaerophillic (sometimes anaerobic) lifestyle. Optimal oxygen conditions for Magnetospirillum magnetotacticum is 3-5% saturation. The environment also has an abundance of dissolved organic acids, utilized by the organism for energy. The main contribution of M. magnetotacticum to the environment is its production of magnetosomes, and the magnetofossils that are preserved after the death of the organism. These particles of magnetite significantly contribute to the magnetization of soils and sediments. Magnetospirillum magnetotacticum also fixes nitrogen, contributing to the availability of nitrogen in wet soils and sediments. As far as current research goes, Magnetospirillum magnetotacticum has no important symbiotic relationships. This organism is of biogeochemical significance because of its role in iron cycling. It is especially of interest because, understanding the process by which magnetite is formed, along with the overall metabolism can illustrate how the biogeochemical cycles of nitrogen, carbon and iron can be influenced by one organism. Magnetospirillum magnetotacticum is of geological significance because it leaves a detectable fossil remain, and can be used as a geobiological tracer.

References

1. Maratea, D., and Blakemore, R.P., 1981. Aquaspirillum magnetotacticum sp. nov., a Magnetic Spirillum. International Journal of Systematic Bacteriology. Vol 31.4:452-455.
2. Brenner,D. J., Bergey, D. H., Garrity, G. M., Krieg, N. R., Bergey's manual of systematic bacteriology Ed.2.Springer.2005. p:30-31.
3. "Magnetospirillum magnetotacticum" http://genome.jpi-psf.org/draft-microbes/magma/magma.home.html
4. Schülwer, D., 1999. Formation of Magnetosomes in Magnetotactic Bacteria. J. Molec. Microbioloby. Biotechnol. 1(1):79-86
5. Blakemore, R.P., and Frankel. 1981. Magnetite and magnetotaxis in microorganisms. Bioelectromagnetics. Vol 10.3:223-237
6. "Magnetotactic bacteria" http://en.wikipedia.org/wiki/Magnetotactic_bacteria
7. "Magnetosprillum magnetotacticum MS-1": http://www.uniprot.org/taxonomy/188
8. "Magnetosprillum magnetotacticum MS-1": http://genamics.com/cgi-bin/genamics/genomes/genomesearch.cgi?field=ID&query=955

Author

Page authored by Susan Jarosz and Megan Hull, students of Prof.Jay Lennon at Michigan State University.