Magnetospirillum magneticum

A Microbial Biorealm page on the genus Magnetospirillum magneticum

Classification

Higher order taxa

Cellular organisms; Bacteria; Proteobacteria; Alphaproteobacteria; Rhodospirillales; Rhodospirillaceae; Magnetospirillum; Magnetospirillum Magneticum

Species

NCBI: Taxonomy

"Magnetospirillum magneticum"

Description and significance

Magnetotactic bacteria was first described in 1963 by Salvatore Bellini of the University of Pavia. Upon observations of bog sediments, he identified a bacteria that was uniquely oriented in the aquatic environment due to magnetic field lines. Richard Blakemore observed a similar phenomenon in 1975 at the Woods Hole Oceanographic Institution and subsequently wrote the first peer-reviewed report of magnetotactic bactera in Science. The observations of these two eary developers later became the hallmarks of magnetotactic bacteria species such as Magnetospirillum magneticum (MM). MM is an aquatic, motile, gram negative bacteria that generally inhabits chemically stratified water columns or sediments. It contains magnetite filled magnetosomes, uses flagella for motility, and grows most efficiently in the transition zone beween aerobic upper waters and anaerobic lower regions of most bodies of water. In the water, MM migrates along the geomagnetic field lines, swimming parallel or antiparallel to them in search of its optimum living environment. Although it was once considered a mere curiosity, MM has relevance in a number of fields including nanobiotechnology, biomineralization, biogeochemistry, and geomicrobiology. It also contributes to the global iron cycle by actively converting iron to magnetite or gregite within magnetosomes. MM has even been used to claim the existence of life on Mars due to the similarities between magnetite particle shapes found in MM and on the Martian meteorite ALH84001. While this issue has been hotly contested, it illustrates the breadth of application of Magnetospirilum magneticum to modern life.

Genome structure

Magnetospirillum magneticum contains a wild type genome of approximately 4,967,148 bp with a single circular chromosome and a recently discovered cryptic plasmid pMGT. The plasmid was discovered in the MGT-1 strain and had the purpose of developing an improved expression system in the AMB-1 strain(see current research). Protein coding sequences comprise approximately 88% of the genome with a GC content of 65% and an average ORF of 961 bp. It also contains 33 insertion sequences, of interest because they contain lower than average GC content, suggesting horizontal gene transfer from other bacteria or phage. Nine of these regions are known to contain phage related proteins, while the others are still being studied. . Although the picture above appears to show a long chain of magnetosomes floating in the cytoplasm, this is not the case. The MamAB operon encodes MamK, an active protein that is the most likely candidate for the actin-like filamentous cytoskeleton that anchors magnetosomes to the cell membrane. As these magnetosomes contain magnetite which is derived from iron, there must be a way for the cell to take in iron. The magA gene accounts for this behavior, encoding a protein that appears to construct a H/Fe(II) antiporter on the cytoplasmic membrane. As an important side note, cells with the magA gene only take up iron when ATP is supplied, so it appears to be an energy dependent mechanism.

Cell structure and metabolism

Magnetospirillum magneticum is a gram negative, spiral shaped, aquatic, mesophillic bacterium. It prefers temperatures between 25-40 C and is microaerophillic, requiring a lower level of oxygen than is available at normal sea level on earth. The main structures of interest in MM are the magnetosomes, magnetite containing vesicle invaginations of the inner cell membrane stabilized by cytoskeletal filaments. Magnetosomes are surrounded by a lipid bilayer and usually contain 15-20 crystals of magnetite, each about 50 nm in length, but these figures can vary with environmental conditions. As mentioned in the genome section, the magnetosomes allign themselves linearly and are anchored to the membrane by MamK. When working together, the combination of each tiny magnetic dipole enables the magnetosome chain to act as a compass to orient the bacteria in the geomagnetic field. Physically, the magnetic field imparts a torque on the stationary magnetosome chain, allowing it to pivot and allign the cell with magnetic field lines. The magnetosome points the bacteria in the direction of optimal growth envronment, but flagella are necessary for the motility to travel there. MM primarily utilizes iron reduction to derive energy, but it can also undergo some respiratory nitrate reduction. MM is a facultative anaerobic bacterium, which means it has metabolic pathways for both aerobic and anaerobic processes, however, its growth tends to be inhibited at higher oxygen concentrations. More specifically, when the bacterium encounters highly insoluble Fe(III) molecules in the aqueous environment, it releases high affinity ligands called siderophores to deliver the complexed iron to receptor proteins on the outer cell membrane. From there, a sequence of energy dependent carrier proteins bring the iron into the cell. High volumes of iron must be taken in by the bacterium in order to create the necessary proton motive force across the membrane. This allows utilization of the proton gradient for energy production. In this way, formation of magnetosomes drive metabolism because magnetosomes allow movement into better growth environments with more iron to reduce.

Ecology

The ecology of Magnetospirillum magneticum has been largely unstudied and it is unknown specifically how long they have been on the earth. MM are known to contribute to the global iron cycle by taking in iron and converting it to magnetite(Fe3O4) or greigite (Fe3S4) within magnetosomes. Also, MM found in marine environments have a high concentration of sulfur in their sediments, suggesting that they thrive in low oxygen environments. While these facts constrain the habitat of MM to poorly oxygenated water with abundant iron reserves, the extent of interaction it has with other organisms remains unknown.

Pathology

Magnetospirillum magneticum has no known pathogenic activity and is well regarded by many scientists because it is novel and safe to work with.

Application to Biotechnology

Does this organism produce any useful compounds or enzymes? What are they and how are they used?

Application to Biotechnology

Magnetospirillum magneticum has a number of applications to biotechnology, including its use in DNA/RNA isolation/purification procedures and its possible use as a alternative power source. Particles from the MM have been used for immunoassay experiments with endocrine disruptors, mRNA, and DNA strands. Theree are two basic modes of magnetic affinity: the direct method and the indirect method. The direct method involves a high affinity ligand directly coupled to the magnetic particle with an affinity for the target The indirect method is slightly more complicated as it utilizes a free affinity ligand which interacts with the target in suspension before the magnetic particle is able to recognize and bind to it. Most notably in a physiologic sense, these methods have been developed to isolate hemoglobin from the blood. This procedure relies on the fact that hemoglobin has four heme groups, each with an Fe(II) molecule within the porphyrin ring. The main drawback of these approaches is the cost, as many of the magnetic carriers and adsorbents deveolped are still rather expensive. While the use of MM to generate power may seem far fetched, Kartik Madiraju, a 16-year old high school student from Montreal was able to generate about half the voltage of an AA battery with 1/5 ounce of magnetotactic bacteria. This process utilized Faraday’s law of electromagetic induction, essentially spinning the cells with two metal electrode strips above and below in order to induce a magnetic field and electric current. The electrical parameters generated by the model were not near functional cacpacity for real world applications, however, future testing may improve the electrical output.

Current Research

Iron Uptake Systems The iron uptake systems of Magnetospirillum magneticum have been studied in detail using standard DNA microarray apparatus. These studies yielded five main types of expression for the genes feo, tpd, and ftr which all encode ferrous iron transporters. Each type was either upregulated or downregulated based on the iron concentration in the immediate environment except for type E, which were not responsive to the level of iron at all. It was shown that magnetosome constructing cells upregulated ferrous iron transport and downregulated ferric iron transport in iron rich conditions. Low oxygen conditions were also shown to facilitate iron transfer. The simultaneous uptake of iron and formation of magnetite suggested that these uptake systems may act as iron supply lines, enabling the synthesis of magnetosomes.

Siderophores in Magnetospirillum Magnetium Siderophores are low molecular weight transport molecules that target specific ligands with high afinity and bring them back to a cell. The first siderophore activity detected in magnetotactic bacteria was identified in Magnetospirillum magnetium by analysis of a late exponential phase free culture supernatant. This particular siderophore appeared to be targeting the highly insoluble Fe(III) to return it to the bacterium. It was shown to relieve growth inhibition of MM, as it increased the cellular assimilation of iron. The structure of the iron binding siderophore was determined by NMR spectral analysis to be 3,4 dihydroxybenzoic acid.

pMGT Plasmid The first Magnetospirillum magneticum small cryptic plasmid, pMGT, was identified in the MGT-1 strain. It is the first plasmid of its kind to surface in magnetotactic bacteria and is very stable under a range of physical conditions. pMGT has a nucleotide sequence of 3,741 bp with two potential open reading frames and a GC content of 62-65%. The GC content is of note because it is slightly lower than that found in the main MM genome. The purpose of the plasmid is to improve gene expression in the MM AMB-1 strain. Due to the fact that pMGT exists in MM MGT-1 strains, it must utilize a shuttle vector, pUMG, to accomplish its purpose of improving the MM AMB-1 strain expression.

References

1.Matsunaga, Okamura, Fukuda, Wahyudi, Murase, and Takeyama “Complete Genome Sequence of the Facultative Anaerobic Magnetotactic Bacterium Magnetospirillum sp. strain AMB-1,” DNA Research, 12, 157–166 (2005).

2.Frankel and Bazylinski, “Magnetosome Mysteries-Despite Reasonable Progress Elucidating Magnetotactic Microorganisms, Many Questions Remain,” ASM News Volume 70, Number 4, 176-183, (2004).

3.Safarik and Safarikova, “Magnetic techniques for the isolation and purification of proteins and peptides,” BioMagnetic Research and Technology, 2:7, November 2004.

4.Davila, Winklhofer, McKay, “Multicellular Magnetotactic Prokaryote As a Target For Life Search On Mars,” Lunar and Planetary Science XXXVIII, (2007).

5.Suzuki, Okamura, Calugay, Takeyama, and Matsunaga, “Global Gene Expression Analysis of Iron-Inducible Genes in Magnetospirillum magneticum AMB-1,” Journal of Bacteriology, 188, 6., 2275–2279, March 2006.

6.Okamura, Takeyama, Sekine, Sakaguchi, Wahyudi, Sato, Kamiya, and Matsunaga, “Design and Application of a New Cryptic-Plasmid-Based Shuttle Vector for Magnetospirillum magneticum,” Applied and Environmental Microbiology, 69, 7, 4274-4277, July 2003.

7.Calugay, Takeyama, Mukoyama, Fukuda, Suzuki, Kanoh, and Matsunaga, “Catechol Siderophore Excretion by Magnetotactic Bacterium Magnetospirillum magneticum AMB-1,” Journal of Bioscience and Bioengineering, Vol. 101, No. 5, 445-447, 2006.

8.Komeili, Li, Newman, and Jensen, “Magnetosomes Are Cell Membrane Invaginations Organized by the Actin-Like Protein MamK,” Science, Vol. 311, 242-244, January 2006.

9.NCBI Genome Project: Magnetospirillum magneticum: http://www.nci.nlm.nih.gov/entrez

10.NCBI Taxonomy Browser: Magnetospirillum magneticum: http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/

11.Leahy, Stephen, “Power Up With Magnetic Bacteria,” Wired Beta, May, 2006.

12.Wikipedia: http://en.wikipedia.org/wiki/Magnetotactic_bacteria

Edited by Adam Wheeler; student of Rachel Larsen and Kit Pogliano