Biologically produced crystals help some bacteria detect Earth's magnetic field to simplify navigation.
Introduction
At winter’s end, somewhere in the Arctic, you look around to see the night sky dance with green bands of light that reflect on the snow below, painting the white landscape a deep jade.
The aurora, or northern or southern lights, occur when high-energy particles from solar flares smash into gas molecules in our atmosphere. And the light these collisions produce travels along invisible magnetic field lines that wrap Earth in a spaghetti network called the magnetosphere.
When high energy particles collide with molecules in our atmosphere, the northern and southern lights ignite the sky along Earth’s fluctuating magnetic field lines.
In this computer model of Earth (oriented so its axis is vertical), blue magnetic field lines point inward and orange point outward. The dense tangle of lines exist within Earth’s core.
The magnetosphere exists because our planet is a giant magnet. Scientists think electric currents generated by molten metal lava in the earth’s outer crust creates the field. We’re lucky it exists because it blocks out high levels of radiation that would otherwise penetrate our atmosphere and wreak havoc on all life. It also enables us to orient ourselves on the landscape using compasses.
Some of the first forms of life on Earth also made use of this phenomenon and evolved to have tiny, nanoscale compass needles embedded inside them. Some of these magnetotactic bacteria still follow Earth’s invisible magnetic field lines to this day.
The Strategy
The magnet-directed movement of these aquatic bacteria is called “magnetotaxis.” It doesn’t pull bacteria along geomagnetic field lines like a magnet attracts metal, but only aligns their single-celled bodies with those lines. The bacteria must still wiggle their flagella to move forward or backward.
Magnetotactic Bacteria Under a Microscope
In this video, a researcher uses a hand magnet to induce sudden changes in direction of swimming by magnetotactic bacteria.
The bacteria swim through saturated soils and mud at the bottom of bodies of freshwater to find areas which contain less dissolved oxygen, in which they thrive. Scientists think that their magnet-directed mode of movement simplifies scouting for these regions because it reduces the number dimensions in which the bacteria can search.
Magnetotactic bacteria have magnetosomes, which are organelles with fatty membranes surrounding magnetic, nanosized crystals. The crystals vary in length from 35 to 120 nanometers, and each type of bacteria usually makes either magnetite (Fe3O4) or greigite (Fe3S4). Scientists have discovered only one type of bacteria that makes both crystal types. The shapes of the crystals vary across organisms but are usually cubes, rectangular prisms, or pointed forms.
A chain of twelve magnetite (Fe3O4) nanoparticles acts as a tiny compass needle. (© 2010 Nature Education All rights reserved)
Most magnetotaxic bacteria have 10 to 20 magnetosomes that connect into chains. The total magnetism of the chains is the sum of what individual crystals contribute. The additive effect forms a type of compass needle with greater sensitivity to geomagnetic field lines. Single crystals might not have enough magnetic strength to work. Without enough magnetic force to overcome thermal and other mechanical forces in the water, bacteria would simply align themselves in their surroundings like most other bacteria—randomly.
The Potential
Magnetite nanoparticles, like those found in magnetotactic bacteria, could become critical components of many nanotechnological innovations. Nanopumps could deliver drugs to specific cells in the body to improve their healing powers. Nanomachines might seek out and destroy individual tumor cells to treat people with cancer. Nanogenerators could create electricity from kinetic energy extracted from water molecules in ocean waves or air molecules in gusting wind.
