Magnetotactic bacteria

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 Blakmore, 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. 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 Blakmore, 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. 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 Microbiologia's 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 Italian: batteri magnetosensibili, French: bactéries magnétosensibles or bactéries aimantées, German: magnetischen empfindlichen Bakterien and English: magnetosensitive bacteria (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. To date, 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. 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 (Fe3O4) or of greigite (Fe3S4), 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 § Magnetism 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 Proterozoic Era, as the increase in atmospheric oxygen reduced the quantity of dissolved iron in the oceans. 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 produce their magnetic particles in 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.