Magnetotactic bacteria get their name from ‘magneto taxis’ or the ability to align and coordinate along the earth’s magnetic field lines. They build nano-sized magnetic crystals called magnetosomes in their cells which act like tiny magnetic compasses. They use the resulting magnetic moment generated from these magnets to steer them toward favourable living conditions in their habitats.
Scientists are still deciphering the cellular mechanisms of how these bacteria synthesize these nanocrystals. So let us delve into the world of these fascinating magnet manufacturers and get acquainted with them.
Magnetotactic bacteria are prokaryotes, single-celled microorganisms without a distinct nucleus or well-defined organelles in their cells. Instead, their genetic material floats about the cytoplasm (cell fluid). These bacteria live in freshwater and marine habitats in oxic-anoxic regions, that is, where there is an oxygen gradient in the water. As anaerobes, they do not require oxygen for cell functions and thrive in oxygen-depleted zones in the water sediments. The bacterial cell tapers into a tail-like extension called a flagellum, which helps the organism swim around in its habitat. Magnetotactic bacteria are expert swimmers, clocking double the speeds of E coli bacteria.
Blakemore found that each bacterium contains several nano-sized magnetic crystalline structures called magnetosomes. The magnetosomes are aligned in a chainlike manner along the central axis of the cell. When strung together, the chain behaves like a magnet with poles. This living compass orients along the earth’s magnetic field lines. Research has shown that due to the magnetic moment, a magnetic torque develops in the cell, which gives a ‘push’ to the flagellum to rotate it clockwise or anticlockwise. This rotation propels the cell’s movement. Magnetotactic bacteria are often found swimming from high to low oxygen gradients in their environments, northwards in the Northern hemisphere, southwards in the southern parts, and in both directions in the equatorial regions.
The cutting-edge technology
These bacteria are skilled engineers employing complex molecular gymnastics to synthesize magnetic nanocrystals. Iron is the chief element for making them.
Iron plays a vital role in several metabolic pathways in organisms and magnetotactic bacteria, too, require this essential nutrient to grow and proliferate. However, iron is not available in a stable state in their living environments due to a lack of oxygen. (In aerobic or oxygen-rich environments, iron is present in the ferrous state (Fe2+), which quickly gets oxidized to the ferric (Fe3+) state). Therefore, these bacteria have devised an alternative iron-absorbing technique.
Research has shown that these bacteria release particular iron-binding molecules called siderophores into the vicinity. The siderophores have a high affinity for Fe3+ ions and latch to them, after which they are absorbed into the bacterial cell. Once inside, the ferric ions are cleaved from these molecules. Studies indicate that synthesizing magnetosomes (from the absorbed iron) involves several steps, some of which remain unclear. However, broadly, the bacteria,
form a bio-vesicle in the cytoplasm
transport iron into these vesicles, and
biomineralize the iron into magnetic nanocrystals and then into magnetosomes.
Over the decades, scientists studied these bacteria in greater detail to understand how these living magnets function. They found that the bacteria use the magnetosome chain to decipher the orientation of the earth’s magnetic field.
The chain generates a magnetic field of about 0.5 Gauss, creating a magnetic moment (energy) in the organism. As the magnetosome chain is stacked along the central axis of the cell, a torque develops, which they use to ‘push’ the flagellum and rotate it clockwise or anticlockwise (depending on the orientation). Thus the flagellum is steered, which propels the organism to move.
Depending on which hemisphere the bacteria live in, they swim up or down in search of anoxic regions.
Research has found that the bacteria exhibit photokinesis or increased swimming speed in red light ranges and slowing down in green light conditions. However, they avoid intense UV light regions.
Researchers observed magnetosomes under an electron microscope and found them to have two-dimensional uniform crystalline structures. The Mam gene is responsible for forming the membrane proteins and cytoskeleton. The magnetosome’s membrane proteins determine the morphology of the crystals.
Four types of Mms proteins are involved in the crystallization process to form these nanostructures. Different species make different shaped crystals – rods, spheres, cubes, elongated octahedrons, cuboctahedra or prismatic.
Researchers have isolated the magnetosomes from the bacteria and found them to be highly crystalline and stable particles of 30-100 nanometres. Unfortunately, our most advanced technology cannot still reproduce these crystals’ structural finesse and stability. However, science has found that magnetosomes have potential in targeted therapies, especially for cancer. Hence, they are mass-producing magnetosomes growing bacteria cultures.
Key Players of Earth’s Iron Cycle
Magnetosomes are stable particles and retain their structure and composition even outside the bacterial cell.
Magnetotactic bacteria consist of up to 3% iron by dry weight (10-13 to 10-15 grams of iron per cell).
Studies have found that magnetotactic bacteria play a crucial role in iron recycling, replenishing up to 10% of the mineral in their environment.
When the bacteria die, magnetosomes deposit in the sediments as magneto-fossils or are ingested by other organisms completing the cycle.
Several experiments show magnetosomes have potential in new-age cancer therapies as external magnetic fields can manipulate magnetosomes. They are non-toxic to humans and do
not degrade in body fluids.
Researchers propose that magnetosomes can be injected into the body close to the tumour region, guided to the cancer site with external magnets and at the site, made to perform cancerinhibiting actions like,
Localized radiation (hyperthermia) – apply a safe level of alternating magnetic field the magnetosomes will rapidly align-realign to the field-- the nanoparticles expend energy in
this process -- dissipated heat kills cancer cells.
Localized chemotherapy – manipulate the outer layer of magnetosomes coat with drugs -- steer to cancer site -- unload drugs directly to cancer cells.