Magnetotactic bacteria (or MTB) are a polyphyletic group of bacteria that orient themselves along the magnetic field lines of Earth's magnetic field. Discovered in 1963 by Salvatore Bellini and rediscovered in 1975 by Richard Blakemore, this alignment is believed to aid these organisms in reaching regions of optimal oxygen concentration. To perform this task, these bacteria have organelles called magnetosomes that contain magnetic crystals. The biological phenomenon of microorganisms tending to move in response to the environment's magnetic characteristics is known as magnetotaxis. However, this term is misleading in that every other application of the term taxis involves a stimulus-response mechanism. In contrast to the magnetoreception of animals, the bacteria contain fixed magnets that force the bacteria into alignment—even dead cells are dragged into alignment, just like a compass needle.Dusenbery, David B. (1996). "Life at Small Scale", pp. 100-101. Scientific American Library, New York. .
Introduction
The first description of magnetotactic bacteria was in 1963 by Salvatore Bellini of the University of Pavia. While observing bog sediments under his microscope, Bellini noticed a group of bacteria that evidently oriented themselves in a unique direction. He realized these microorganisms moved according to the direction of the North Pole, and hence called them "magnetosensitive bacteria". The publications were academic (peer-reviewed by the Istituto di Microbiologias editorial committee under responsibility of the Institute's Director Prof. L. Bianchi, as usual in European universities at the time) and communicated in Italian with English, French and German short summaries in the official journal of a well-known institution, yet unexplainedly seem to have attracted little attention until they were brought to the attention of Richard Frankel in 2007. Frankel translated them into English and the translations were published in the Chinese Journal of Oceanography and Limnology.
Richard Blakemore, then a microbiology graduate student at the University of Massachusetts at Amherst, working in the Woods Hole Oceanographic Institution in whose collections the pertinent publications of the Institute of Microbiology of the University of Pavia were extant, observed microorganisms following the direction of Earth's magnetic field. Blakemore did not mention Bellini's research in his own report, which he published in Science, but was able to observe magnetosome chains using an electron microscope. Bellini's terms for this behavior, namely , or , and (Bellini's first publication, last page), went forgotten, and Blakemore's "magnetotaxis" was adopted by the scientific community.
These bacteria have been the subject of many experiments. They have even been aboard the Space Shuttle to examine their magnetotactic properties in the absence of gravity, but a definitive conclusion was not reached.
The sensitivity of magnetotactic bacteria to the Earth's magnetic field arises from the fact these bacteria precipitate chains of crystals of magnetic minerals within their cells. , all magnetotactic bacteria are reported to precipitate either magnetite or greigite. These crystals, and sometimes the chains of crystals, can be preserved in the geological record as magnetofossils. The oldest unambiguous magnetofossils come from the Cretaceous chalk beds of southern England, though less certain reports of magnetofossils extend to 1.9 billion years old Gunflint chert. There have also been claims of their existence on Mars based on the shape of magnetite particles within the Martian meteorite ALH84001, but these claims are highly contested.Cat Faber, Living Lodestones: Magnetotactic bacteria , Strange Horizons, 2001
Biology Several different morphologies (shapes) of MTB exist, differing in number, layout and pattern of the bacterial magnetic particles (BMPs) they contain. The MTBs can be subdivided into two categories, according to whether they produce particles of magnetite () or of greigite (), although some species are capable of producing both. Magnetite possesses a magnetic moment with three times the magnitude of greigite.
Magnetite-producing magnetotactic bacteria are usually found in an oxic-anoxic transition zone (OATZ), the transition zone between oxygen-rich and oxygen-starved water or sediment. Many MTB are able to survive only in environments with very limited oxygen, and some can exist only in completely anaerobic environments. It has been postulated that the evolutionary advantage of possessing a system of magnetosomes is linked to the ability to efficiently navigate within this zone of sharp chemical gradients by simplifying a potential three-dimensional search for more favorable conditions to a single dimension. (See for a description of this mechanism.) Some types of magnetotactic bacteria can produce magnetite even in anaerobic conditions, using nitric oxide, nitrate, or sulfate as a final acceptor for electrons. The greigite mineralizing MTBs are usually strictly anaerobic.
It has been suggested MTB evolved in the early Archean Eon, as the increase in atmospheric oxygen meant that there was an evolutionary advantage for organisms to have magnetic navigation. Magnetosomes first evolved as a defense mechanism in response to the increase of reactive oxygen species (ROS) that resulted from the Great Oxygenation Event. Organisms began to store iron in some form, and this intracellular iron was later adapted to form magnetosomes for magnetotaxis. These early MTB may have participated in the formation of the first eukaryotic cells. Biogenic magnetite similar to that found in magnetotactic bacteria has been also found in higher organisms, from euglenoid algae to trout. Reports in humans and pigeons are far less advanced.
Magnetotactic bacteria organize their magnetosomes in linear chains. The magnetic dipole moment of the cell is therefore the sum of the dipole moment of each BMP, which is then sufficient to passively orient the cell and overcome the casual thermal forces found in a water environment. In the presence of more than one chain, the inter-chain repulsive forces will push these structures to the edge of the cell, inducing turgor.
Nearly all of the genes relevant to magnetotaxis in MTB are located in an approximately 80 kilobase region in the genome called the magnetosome island. There are three main operons in the magnetosome island: the mamAB operon, the mamGFDC operon, and the mms6 operon. There are 9 genes that are essential for the formation and function of modern magnetosomes: mamA, mamB, mamE, mamI, mamK, mamM, mamO, mamP, and mamQ. In addition to these 9 genes that are well conserved across all MTB, there are more than 30 total genes that contribute to magnetotaxis in MTB. These non-essential genes account for the variation in magnetite/greigite crystal size and shape, as well as the specific alignment of magnetosomes in the cell.
The diversity of MTB is reflected by the high number of different morphotypes found in environmental samples of water or sediment. Commonly observed morphotypes include spherical or ovoid cells (cocci), rod-shaped (bacilli), and spiral bacteria of various dimensions. One of the more distinctive morphotypes is an apparently multicellular bacterium referred to as multicellular magnetotactic bacteria (MMB).
Regardless of their morphology, all MTB studied so far are motile by means of flagella and are gram-negative bacteria of various phyla. Despite the majority of known species being Pseudomonadota, e.g. Magnetospirillum magneticum, an alphaproteobacterium, members of various phyla possess the magnetosome gene cluster, such as Candidatus Magnetobacterium bavaricum, a Nitrospira. The arrangement of flagella differs and can be polar, bipolar, or in tufts. The first phylogenetic analysis on magnetotactic bacteria using 16S rRNA gene sequence comparisons was performed by P. Eden et al. in 1991.
Another trait that shows considerable diversity is the arrangement of magnetosomes inside the bacterial cell. In the majority of MTB, the magnetosomes are aligned in chains of various lengths and numbers along the cell's long axis, which is magnetically the most efficient orientation. However, dispersed aggregates or clusters of magnetosomes occur in some MTB, usually at one side of the cell, which often corresponds to the site of flagellar insertion. Besides magnetosomes, large inclusion bodies containing elemental sulfur, polyphosphate, or poly-β-hydroxybutyrate are common in MTB.
The most abundant type of MTB occurring in environmental samples, especially sediments, are coccoid cells possessing two flagellar bundles on a somewhat flattened side. This "bilophotrichous" type of flagellation gave rise to the tentative genus "Bilophococcus" for these bacteria. In contrast, two of the morphologically more conspicuous MTB, regularly observed in natural samples, but never isolated in pure culture, are the MMB and a large rod containing copious amounts of hook-shaped magnetosomes (Magnetobacterium bavaricum).
Magnetism The physical development of a magnetic crystal is governed by two factors: one is moving to align the magnetic force of the molecules in conjunction with the developing crystal, while the other reduces the magnetic force of the crystal, allowing an attachment of the molecule while experiencing an opposite magnetic force. In nature, this causes the existence of a magnetic domain, surrounding the perimeter of the domain, with a thickness of approximately 150nm of magnetite, within which the molecules gradually change orientation. For this reason, the iron is not magnetic in the absence of an applied field. Likewise, extremely small magnetic particles do not exhibit signs of magnetisation at room temperature; their magnetic force is continuously altered by the thermal motions inherent in their composition. Instead, individual magnetite crystals in MTB are of a size between 35 and 120nm, that is; large enough to have a magnetic field and at the same time small enough to remain a single magnetic domain.
The inclination of the Earth's magnetic field in the two respective hemispheres selects one of the two possible polarities of the magnetotactic cells (with respect to the flagellated pole of the cell), orienting the biomineralisation of the magnetosomes.
Aerotaxis is the response by which bacteria migrate to an optimal oxygen concentration in an oxygen gradient. Various experiments have clearly shown that magnetotaxis and aerotaxis work in conjunction in magnetotactic bacteria. It has been shown that, in water droplets, one-way swimming magnetotactic bacteria can reverse their swimming direction and swim backwards under reducing conditions (less than optimal oxygen concentration), as opposed to oxic conditions (greater than optimal oxygen concentration). The behaviour that has been observed in these bacterial strains has been referred to as magneto-aerotaxis.