Bacteria containing chains of magnetic crystals (magnetosomes) that align with Earth's magnetic field, making them swim toward optimal oxygen levels. They're living compasses that biomineralize perfect magnetic nanoparticles.
Frank Mickoleit, CC BY-SA 3.0, via Wikimedia Commons
Living magnets of Rio
In the marine sediments near Rio de Janeiro, a unique form of life navigates using an internal compass. These are magnetotactic bacteria (MTB), a diverse group of microorganisms that biomineralize magnetic nanoparticles inside their cells. This ability was first detailed in 1975 by Richard P. Blakemore, who observed bacteria in sediment actively moving in response to the planet's magnetic field. The bacteria inhabit a very specific layer in the water column or sediment known as the oxic-anoxic transition zone (OATZ). This is a narrow band where oxygenated water meets oxygen-free water, and the bacteria use their magnetic sense to stay precisely within this zone, which is the perfect environment for their survival.
The source of this ability is a series of organelles called magnetosomes. Each magnetosome is a membrane-enclosed, perfectly formed crystal of a magnetic iron mineral, usually magnetite (Fe₃O₄) or greigite (Fe₃S₄). Inside the bacterium, 15 to 20 of these magnetosomes are arranged in a chain, much like a string of pearls. This chain acts as a single, rigid magnetic dipole—effectively a compass needle which passively aligns the entire bacterium with Earth's magnetic field lines. The magnetic force is strong enough that even dead bacteria will align with the field.
Navigating by inclination
The bacteria's movement is a combination of this passive magnetic alignment and active swimming with their flagella, a behavior termed magneto-aerotaxis. The Earth's magnetic field is not perfectly horizontal; it inclines downwards in the Northern Hemisphere and upwards in the Southern Hemisphere. In Rio de Janeiro, the inclination is about 25 to 30 degrees south. The local MTB are "south-seeking," meaning their internal compass orients them to swim against the magnetic field lines. This orientation guides them downward, away from the oxygen-rich surface waters and toward the microaerophilic conditions of the OATZ. This reduces their search for optimal oxygen levels from a three-dimensional problem to a simple one-dimensional swim up or down the magnetic field lines.
The biomineralization process is under precise genetic control, resulting in magnetic crystals of a uniform size and shape, typically between 35 and 120 nanometers. This uniformity has attracted interest for applications in nanotechnology and medicine. Researchers are exploring the use of isolated magnetosomes for targeted drug delivery, as contrast agents in magnetic resonance imaging (MRI), and in cancer therapy through a technique called magnetic hyperthermia.
💡Fun Facts
Fossilized magnetosomes, known as magnetofossils, have been found in the geologic record, with the oldest robust examples dating back to the Cretaceous period.
Salvatore Bellini at the University of Pavia in Italy first observed and published on these "magnetosensitive bacteria" in 1963, but his work was not widely recognized at the time.
Researchers studying sediments near the geomagnetic equator in Brazil found roughly equal populations of north-seeking and south-seeking bacteria in the same samples.
The genetically controlled process of biomineralization produces magnetite crystals of a higher quality and uniformity than can be created synthetically.
Not publicly accessible. The bacteria inhabit offshore marine sediments.
Admission
Free
Accessibility
The habitat of these bacteria is not accessible to the public. Research on local populations is conducted by institutions like the Federal University of Rio de Janeiro.
