Prokaryotes that exhibit magnetotaxis, collectively known as the magnetotactic bacteria, are those whose direction of motility is influenced by the Earth’s geomagnetic and externally applied magnetic fields. These ubiquitous, aquatic microorganisms represent a morphologically, phylogenetically, and physiologically diverse group that biomineralize unique organelles called magnetosomes that are responsible for the cells’ magnetotactic behavior. Magnetosomes consist of magnetic mineral crystals, either magnetite (Fe3O4) or greigite (Fe3S4), each enveloped by a phospholipid bilayer membrane that contains proteins not present in other membranes. While there are several different magnetite and greigite crystal morphologies, mature crystals of both minerals are always in the single magnetic domain size range, about 35–120 nm, thus having the highest possible magnetic moment per unit volume. In most magnetotactic bacteria, magnetosomes are arranged as a chain within the cell thereby maximizing the magnetic dipole moment of the cell causing the cell to passively align along magnetic field lines as it swims. Magnetotaxis is thought to function in conjunction with chemotaxis in aiding magnetotactic bacteria in locating and maintaining an optimal position in vertical chemical concentration gradients common in stationary aquatic habitats, by reducing a three-dimensional search problem to one of a single dimension.
Although the detection of magnetotactic bacteria in samples collected from natural environments is relatively easy, the magnetotactic bacteria are a fastidious group of prokaryotes and special culture conditions are necessary for their isolation and cultivation. Phylogenetically, most known cultured and uncultured magnetotactic bacteria are associated with the Alpha-, Gamma-, and Deltaproteobacteria classes of the Proteobacteria phylum and the Nitrospirae phylum. All cultured species are either microaerophiles or anaerobes or both. Most cultured species of the Alpha- and Gammaproteobacteria classes are microaerophiles that grow chemolithoautotrophically using reduced sulfur compounds as electron sources and the Calvin-Benson-Bassham cycle or the reverse tricarboxylic acid cycle for autotrophy and chemoorganoheterotrophically using organic acids as electron and carbon sources. Those in the Deltaproteobacteria are sulfate-reducing anaerobes that only grow chemoorganoheterotrophically. Almost all cultured species exhibit nitrogenase activity and thus fix atmospheric nitrogen and many denitrify. Magnetotactic bacteria thus show a great potential for iron, nitrogen, sulfur, and carbon cycling in natural environments.
Genetic determinants for magnetosome synthesis, the mam and mms genes, are organized as clusters in the genomes of all magnetotactic bacteria examined. These clusters are in close proximity to each other within the genomes and are surrounded by genomic features that suggest that magnetosome genes are organized as a magnetosome gene island that might be transmitted to many different bacteria through horizontal gene transfer. Through the development of genetic systems in some magnetotactic bacteria, the functions of several magnetosome membrane proteins in the biomineralization of the magnetite magnetosome chain have been demonstrated although the roles of most remain unknown.
Bacterial magnetosomes have novel physical and magnetic properties and also geological significance and have been used in a large number of commercial and medical applications.