The evolutionary advantage that magnetic bacteria enjoy has always been something of a conundrum, but now, scientists at the Naval Research Laboratory (NRL) and Purdue University have shed at least some light on the mystery. Their description of how being magnetic assists the bacteria to navigate appears in the Biophysical Journal. Found in a variety of aquatic environments, magnetic bacteria grow strings of microscopic magnetic particles called magnetosomes. When placed in a magnetic field, these make the bacteria align like tiny compass needles, a phenomenon call magnetotaxis. The strain of bacterium the NRL team studied, Magnetospirillum magneticum, was originally found in a pond in Tokyo.
The researchers didn't originally set out to uncover the bacterium's evolutionary provenance. Rather, they were using genetic engineering to create a strain of the bacteria that become magnetic only when exposed to specific toxic chemicals, with the goal of using them as living chemical sensors that could be easily integrated with other technologies. As a first step, they created a strain that couldn't make magnetosomes, and was therefore not magnetic.
Lloyd Whitman from NRL, who led the research team, explained that "during the course of our research, we realized that nobody had ever really demonstrated that being magnetic actually helps the bacteria." "Genetic modification allowed us to directly observe differences in behavior between magnetic and non-magnetic versions of the same bacterium," added Purdue's Bruce Applegate.
Magnetic bacteria prefer to live far below the water's surface, where oxygen is scarce. In the past, scientists had suspected that being magnetic helps a bacterium find the oxygen concentrations it prefers more quickly by swimming only up and down relative to the Earth's magnetic field, rather than randomly in all directions.
Now, with both magnetic and non-magnetic versions of the same bacteria, the researchers were able to test this hypothesis. And it seems that being magnetic does indeed help the bacteria move to preferred low-oxygen environments, but how magnetism actually does this is still not clear, appearing to be more complex than simply swimming up and down. "We determined that being magnetic actually makes the bacteria much more sensitive to oxygen when in a magnetic field, so that they swim away from oxygen at much lower concentrations," said NRL researcher Paul Sheehan. And it seems the stronger the magnetic field, the bigger the effect.
The scientists are baffled as to how the magnetic field has this effect on the bacteria, and are currently conducting additional experiments to help answer that question. Interestingly, the effect was too small for the researchers to measure in the Earth's natural (but weak) magnetic field. "Therefore," mused NRL's Whitman, "the advantage to these bacteria in nature must be very small. But over millions of years, this very subtle advantage has somehow produced bacterial magnetism."
Source: Naval Research Laboratory
