Badwater Basin in Death Valley sits 282 feet below sea level and is known for its extensive salt flats and brackish water. The harsh environment is forbidding to all but the specially adapted and salt-tolerant. And recently added to its collection of atypical life-forms is a group of greigite-producing magnetic bacteria, which were isolated from a sample of brackish spring water and described in a report published in Science in late December.
Magnetic bacteria orient and navigate along magnetic fields and are guided in their movements by intracellular organelles known as magnetosomes, of which there are two types—those that contain nanocrystals of the iron-oxide mineral magnetite (Fe3O4), and those that contain crystals of the iron-sulfide mineral greigite (Fe3S4). The magnetosomes form chains along the cell's plasma membrane and are fixed into a permanent magnetic dipole. The strength of the magnetic dipole (the magnetic dipole moment) is such that the entire organism is always oriented along the geomagnetic field, switching its orientation only when a stronger field is applied. Magnetic bacteria have one of two polarities, North or South, which is dictated by the orientation of the magnetic dipole.
The Badlands microorganisms are unique from most previously known magnetic bacteria in that they contain both magnetite and greigite magnetosomes, as opposed to just one or the other. The proportion of the two magnetosomes varies with the chemical features of the environment. For example, when hydrogen sulfide is allowed to accumulate in the bacterial growth medium, the predominant magnetosomes are greigite-producing. When hydrogen sulfide concentrations decrease, the cells contain primarily magnetite-producing magnetosomes. The ability to switch between the two forms of biomineralization may be the result of two different magnetosome gene clusters—one for magnetite and one for greigite—that occur in the organism's genome.
The Science study demonstrated that the greigite-producing bacteria can thrive in an anaerobic environment with a liquid medium for sulfate-reducing bacteria. Hence, sulfate appears to be an environmental factor that determines whether the newly discovered bacteria produce greigite. This observation indicates that the new organisms constitute a group of sulfate-reducing bacteria, which phylogenetic analyses placed within the class Deltaproteobacteria.
Deltaproteobacteria also contains a group of greigite-producing multicellular prokaryotes known as many-celled magnetotactic prokaryotes, or MMPs. MMPs are obligately multicellular and consist of 10 to 60 genetically identical cells, shaped into a hollow ball. The cells reproduce together, with all the individual cells dividing at the same time; when a cell is removed from the MMP, it dies. Despite their differences in cellular behavior, MMPs and the newly discovered Badlands microogranisms both exhibit motility in response to magnetic fields, under the direction of greigite magnetosomes.
The discovery of the new greigite-producing bacteria could have impacts in nanotechnology and biotechnology. Magnetite, for example, is being explored for various applications, such as the development of novel drug delivery systems. Similar investigations for greigite had been delayed in part by the lack of a biological source that could be grown in culture.
Kara Rogers is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is a member of the National Association of Science Writers and author of Science Up Front on the Britannica Blog. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.