I've mentioned magnetic bacteria a couple of times now, so I got quite excited when Lucas Brouwers alerted me to a recent paper in Science (ref below) that explored a whole new group of magnetic bacteria. As I've covered before, these magnetotactic bacteria contain small nanoparticles of magnetic material which allow them to swim along magnetic field lines.
It isn't just one clear species of bacteria that has magnetotactic ability, rather there are several different groups of bacteria of different shapes and sizes. Some of these are large multicellular bacterial groups, while others are single-celled large and rod-shaped. It is these large rod-shaped bacteria that the paper has been exploring, putting together a comprehensive description of them as a group.
Samples of magnetotactic bacteria were collected from brackish water in the wonderfully named Death Valley Park in California. While many different types of magnetic bacteria were found, the rod-shaped ones were by far the dominant group. Having isolated a sample of exclusively rod-shaped bacteria, researchers could then examine the physical properties. All of the bacteria had a flagellum, a single little 'tail' used for propulsion. They contained different forms of the internal magnetic nanoparticles as well; some of the nanoparticles were made up of magnetite (Fe3O4 - iron and oxygen complex) and some made up of greigite (Fe3S4 - iron and sulfur complex). The greigite nanoparticles were a range of different shapes while the magnetite crystals formed bullet shaped particles.
The type of nanoparticle that formed was found to be strongly related to the outside surroundings. When the bacteria were put into high sulphur growth medium, more of the greigite nanoparticles were formed. Conversely, when the hydrogen sulfide concentration was artificially reduced, more magnetite crystals formed.
These samples were collected from brackish water rather than the marine environment that most magnetotactic bacteria are found in, which means that magnetic bacteria have diversified into a greater range of habitats than previously thought. Although some of the bacteria isolated contained exclusively magnetite or greigite nanoparticles, it seems a fairly safe bet that under different environmental circumstances they would be able to change which nanoparticle was made, to reflect the available elements. In both cases, an iron compound is required, but there seems to be no appreciable fitness difference between bacteria with the iron-sulfur compounds, or the iron-oxygen ones.
Genetic analysis showed that the genes for producing magnetite and greigite appear to be in two separate groups or clusters on the genome. Comparing these clusters to other bacteria that produce magnetite or greigite nanoparticles supports the idea that one contains genes that code for proteins required for manufacturing the bullet-shaped magnetite nanoparticles, while the other contains genes related to greigite production. Keeping the two different types on two seperate gene clusters allows them to be regulated differently, and respond individually to different environmental stimulus to ensure that whatever the surrounding environment, the bacteria will always be able to swim along geomagnetic field lines.
Credit link for fig. 1
Ref: Lefevre, C., Menguy, N., Abreu, F., Lins, U., Posfai, M., Prozorov, T., Pignol, D., Frankel, R., & Bazylinski, D. (2011). A Cultured Greigite-Producing Magnetotactic Bacterium in a Novel Group of Sulfate-Reducing Bacteria Science, 334 (6063), 1720-1723 DOI: 10.1126/science.1212596