What Is Magnetospirillum and Why Is It Magnetic?

The genus Magnetospirillum includes a group of bacteria with the ability to navigate using the Earth’s magnetic field. First described in the 1970s, these microorganisms captured scientific interest due to their internal magnetic particles. This trait allows them to orient themselves and move along magnetic field lines, helping them find ideal living conditions in their aquatic homes. Their discovery opened a new window into how life can interact with planetary magnetic forces.

Defining Characteristics of Magnetospirillum

Magnetospirillum species are prokaryotic, meaning their cells lack a true nucleus and other membrane-bound organelles. These bacteria have a spiral, or helical, shape and are propelled by flagella located at both ends of the cell. Their cell wall structure is Gram-negative, a classification with implications for their biology and interaction with the environment.

These organisms are classified within the Alphaproteobacteria. Metabolically, they are microaerophilic, thriving in environments with very low oxygen concentrations, though some are facultative anaerobes capable of surviving without oxygen. Their metabolism is primarily respiratory, and they use organic acids as their source of energy and carbon.

The Magnetosome: A Bacterial Compass

The magnetic ability of Magnetospirillum is due to specialized structures called magnetosomes. These are organelles, each consisting of a magnetic crystal enclosed within a lipid bilayer membrane. This membrane is formed by the cell, demonstrating biological control over the formation of these internal structures. The magnetosomes are arranged in a precise chain within the bacterium’s cytoplasm.

This chain of magnetosomes acts like the needle of a compass. The aligned crystals create a magnetic dipole for the bacterium, strong enough to orient the cell with the Earth’s geomagnetic field. This alignment allows the bacterium to use its flagella to swim in a directed manner along these field lines, a behavior known as magnetotaxis. The crystals are most commonly made of magnetite (Fe₃O₄), the most magnetic naturally-occurring mineral on Earth.

This navigation system provides a significant advantage. In the stratified layers of water where these bacteria live, the Earth’s magnetic field lines are inclined, pointing downwards in the Northern Hemisphere and upwards in the Southern Hemisphere. By following these lines, the bacteria can efficiently move vertically to locate and maintain their position within the narrow zone that has the optimal low-oxygen concentration they require.

Natural Habitats and Ecological Significance

Magnetospirillum are found in aquatic environments, including the shallow waters and sediments of ponds, lakes, and some marine settings. They are most abundant at the oxic-anoxic transition zone (OATZ), a narrow interface where oxygen-rich water meets oxygen-depleted water or sediment.

In these environments, they are active participants in biogeochemical cycles. By taking up iron from their surroundings to build their magnetosomes, they participate in the iron cycle. Some species are also capable of using reduced sulfur compounds, contributing to the sulfur cycle. As part of the diverse microbial community at the OATZ, their presence can indicate the specific chemical conditions present in an aquatic system.

Scientific Interest and Biotechnological Prospects

Magnetospirillum is a model organism for scientific study. Researchers are interested in the process of biomineralization—how the bacterium forms uniform magnetite crystals within its magnetosomes. Understanding the genetic and biochemical pathways involved could provide insights into creating novel nanomaterials. The mechanism of magnetotaxis is also studied to learn how biological systems can sense and respond to magnetic fields.

The uniform magnetic nanoparticles they produce have potential in biotechnology. Scientists are exploring their use in targeted drug delivery, where drugs attached to magnetosomes can be guided to a specific location in the body using external magnets. Another medical application is in magnetic hyperthermia, where the particles could be used to generate heat to destroy cancer cells.

Magnetosomes are also being investigated as contrast agents to improve the resolution of magnetic resonance imaging (MRI). Other proposed applications include their use in cell separation techniques and in the bioremediation of environments contaminated with heavy metals. While many of these applications are still in development, they highlight the bacterium’s broad potential.