Magnetotactic bacteria
The organisms in this group each act like a living compass, aligning their bodies to the earths magnetic field using a chain of 10-20 organelles called magnetosomes. These magnetosomes are made of the iron rich minerals magnetite and/or greigite. There have also been observations of a combination of greigite and pyrite. The bacteria are grouped depending on the composition of the magnetosomes; all magnetite (Fe3O4), all greigite (Fe3S4)/greigite and pyrite (FeS2), or a mixture of these two groups. These bacteria are found in water and sediment where there is a transition from an oxic to an anoxic environment. The magnetosomes aid in navigating back to the transition zone if the bacteria is outside of these optimal conditions. The magnetic minerals that comprise magnetosomes align the bacteria to geomagnetic field, where the bacteria will then move up or down along this field to reach the transition zone in the sediment/water column. If in the northern hemisphere, it would swim forward in a downward direction if situated in an oxic environment, and backwards in an upward direction if it was in an anoxic environment. In the southern hemisphere the opposite occurs due to the reversed inclination of the magnetic field. To move backwards or forwards they have flagella, where turning it clockwise will move the cell in one direction and anti-clockwise will move it in the other direction.
The magnetosome production process is still not fully understood and appears to vary from species to species. The basic understanding is that these bacteria will take the more abundant Fe3+ ions in their environment into their cells. Proteins will then convert it to Fe2+ to be used for the magnetosomes. This biotic process produces crystals that are regular in size and shape, and are chemically pure. However the size and shape is different across species and is determined by genetics. The image sets show two species and their different magnetosome shapes. Each image set has an example morphological model of the magnetosome's shape. These models have been produced from a series of tilted TEM images and their electron diffraction pattern. Single examples from these series are images C) in both image sets and along with their associated diffraction pattern (the dotted pattern).
-JC
Reference: https://go.nature.com/2B7bZrd https://bit.ly/2vQHTDF Image credit: https://bit.ly/2OxZ2Zp https://bit.ly/2MKtUFC
