Magnetotactic bacteria (MTB) produce organelles known as magnetosomes which are magnetic magnetite or greigite nanocrystals encased in a membrane. These organelles allow them to align along magnetic field lines, a behaviour known as magnetotaxis. This trait is thought to aid the bacteria in locating and maintaining an optimal position for growth, at an oxic-anoxic interface. MTB have a wide distribution, occurring in most habitats including extreme environments, such as, hot springs and hypersaline waters. These bacteria are also very important in the cycling of key elements including sulphur, nitrogen, carbon and especially iron.
MTB are a very diverse group of Gram-negative bacteria but only occur in minor populations. Currently all known MTB are found within the domain Bacteria, connected with three phyla. Alphaproteobacteria class represent the most dominant proportion of uncultured MTB, which have incorporated sulphur globules into their cells. Therefore, using reduced sulphur as an electron source is thought to be a common feature. Also the ability of nitrogen fixation was found in all MTB tested although one strain was found not to encode the gene.
In the majority of MTB, magnetosomes are aligned in varying lengths of chains, the most efficient orientation in terms of magnetic potential. This structure is thought to be under specific genetic control. Genes for bio-mineralization of crystals are organised in clusters within the genomes of MTB and are common in all, some keep them as a genomic island. However, it is unclear how environmental factors affect the production of these minerals. The presence and location of the genes within MTB genomes can contribute to the understanding of the origin and evolution of magnetosomes.
The origin of this organelle seems to be monophyletic, vertical gene transfer seems to mainly responsible for the distribution of this trait whereas distribution among closely related groups is attributed to horizontal gene transfer. If this was true and the common ancestor to all MTB then this suggests that the common ancestor of all Proteobacteria was magnetotatic or carried similar genes for this trait. The evidence shown suggests that when Proteobacteria originated approximately 2.5-3 billion years ago, there was low levels of oxygen. Therefore magnetotaxis would have been important in locating reactive oxygen species but when oxygen levels increased, this behaviour was no longer needed and the magnetosome genes were lost in some bacteria.
They are still many questions unanswered: This trait is also found in the domain eukarya, used for orientation, navigation and location of other organisms but how did they develop this ability? Is this trait found in the domain Archaea? Why do some bacteria still have this trait if they use sulphur compounds instead of reactive oxygen species? Do these organelles have another function and if not, why waste energy on its production?
Lefevre C.T. and Wu L. (2013) Evolution of the bacterial organelle responsible for magnetotaxis. Trends in Microbiology 21: 534-543