Magnetic bacteria and their unique superpower attract researchers

As a graduate student in the 1970s, microbiologist Richard Blakemore probably wasn’t expecting to discover a new bacterial species with a never-before-seen ability. While studying bacteria that live in muddy swamps, he observed that some tended to swim reliably toward the same geographical direction. Even when he rotated the microscope, they persisted in wiggling toward one direction. After confirming that their swimming behaviors were unaffected by light, Blakemore suspected they might be responding to the weak magnetic fields naturally present on Earth.

After further tests and observations, Blakemore confirmed the bacteria were reacting to magnetism. He published a landmark paper in the journal Science in 1975 introducing magnetotactic bacteria to the wider world. Later, researchers realized that another scientist, Salvatore Bellini, had previously discovered magnetotactic bacteria, but his work received scant attention because it hadn’t been archived.

In the decades since, scientists have continued to study how these tiny magnetic creatures behave. Of course it’s just cool to learn more about these unique single-celled organisms. But researchers like me are also figuring out ways to harness their magnetic properties in medical and other engineering applications.

[embedded content] Watch magnetotactic bacteria dance as the magnetic field around them changes direction.

What makes them living magnets?

You’ve probably stuck a magnet to the metal door of a refrigerator before. This unique group of prokaryotes basically contain super tiny versions of those fridge magnets. They pack either iron-oxide or iron-sulfide molecules into highly dense structures known as magnetic nanoparticles.

Each nanoparticle is about 100,000 times smaller than a grain of rice. Magnetotactic bacteria produce them in different shapes: bullet, rectangular and spherical. Researchers aren’t sure of a reason for this variation, but a possible explanation is that differently shaped particles can interact differently with magnetic fields.

By clustering and aligning in chains, these magnetic nanoparticles enable magnetotactic bacteria to respond even to the weak magnetic fields of the Earth – a strength of about 0.5 Gauss, as opposed to the 100 Gauss of a refrigerator magnet.

Where did magnetotactic bacteria come from?

There are two main proposals for how magnetotactic bacteria emerged on Earth.

The first hypothesis suggests that this group of bacteria evolved a couple billion years ago, in a time of increasingly abundant oxygen. As the oxygen reacted with iron, the amount of iron dissolved in the oceans decreased.

A lipid membrane (looks like a translucent cloud in this image) wraps around magnetic nanoparticles to form a magnetosome in a magnetotactic bacterium. Tay et al., Advanced Functional Materials, 2017, CC BY-ND
Living things need iron for metabolic activities such as respiration, so bacteria started storing it to prevent coming up short in times of scarcity. But high concentrations of freely diffusing iron are toxic for cells.

The idea is that evolution favored bacteria that wound up crystallizing iron into nanoparticles and wrapped a lipid membrane around them to form magnetosomes.

An alternative explanation is more widely accepted by biologists. It’s based on the observation that magnetotactic bacteria grow best in environments like the swamps where they were first discovered – places with very limited oxygen, at concentrations as low as 1 to 2 percent.

As a magnetotactic bacterium moves through a swampy bog, it’s likely to encounter sand or soil particles that could obstruct its path. A bacterium can actively use its flagellum – a whip-like appendage that propels it while swimming – to move past these sediments to reach its preferred growth environment.

But in some cases, the flagellum might not be powerful enough. Magnetic particles can provide some additional force for these bacteria, allowing them to make use of Earth’s magnetic field for navigation and a little extra thrust forward. Magnetosomes allow for more effective navigation.

Magnetotactic bacteria use Earth’s magnetic field to locate an environment where they can flourish. Nature Education, CC BY-NC-ND

Isolating and using magnetic genes in the lab

For many years, scientists have been trying to determine whether animals including bees, sea turtles, sharks and pigeons are magneto-sensitive. Could this possible sense – called magnetoreception – help them with amazing feats of navigation? So far studies have been mostly inconclusive.

Studying simpler organisms like the magnetotactic bacteria might be one way to better understand how genes regulate biomagnetism.