Chemotaxis is not the only mode of navigation available to your cell. For a clue to others, think about how we orient ourselves in the world. In addition to our nose sniffing out food, we also use our eyes to sense light. Similarly, some photosynthetic bacteria have evolved phototaxis (ordered movement in response to light).
We may also use an external tool to navigate unfamiliar surroundings: a compass. Believe it or not, some bacteria have one, too. Magnetotactic species like this Magnetospirillum magneticum have evolved specialized structures called magnetosomes. They are pockets of inner membrane filled with crystals of a magnetic iron mineral like magnetite. The cell organizes the magnetosomes into a line using filaments of a cytoskeletal protein called MamK (named for its association with magnetosome membranes) (⇩), which is related to eukaryotic actin. The linear chain of magnetosomes functions like the needle of a compass, aligning the cell in a magnetic field.
Magnetosomes first form with the creation of a pocket of inner membrane, as you can see in this cell. Multiple short chains of magnetosomes may form, which are then organized into a single chain by MamK filaments. As mineralization begins, the pockets enlarge to accommodate the growing crystals, ultimately reaching ~60 nm in diameter. Magnetosome chains are inherited by daughter cells through division. Segregation is relatively easy: the chain spans the division plane so that the inward-growing cell wall simply splits the chain between magnetosomes, delivering half the chain to each daughter.
Much remains mysterious about these structures. For instance, what forms the membrane pockets? An even bigger question is how cells use their compasses. One theory is that magnetosomes guide the aquatic bacteria vertically to the optimal height in the water column for e.g. a certain oxygen level. However, these species are also found at the magnetic equator, where the Earth’s field is horizontal. Another hypothesis is that magnetic orientation improves the efficiency of navigation by buffering the cell against Brownian motion that would knock it off course.
MamK, like its eukaryotic homologue, actin, polymerizes into double-stranded filaments like this one from Magnetospirillum magneticum [70]. Unlike eukaryotic actin, though, the two strands are aligned in register, rather than being staggered so that the bumps and grooves would pack more tightly.
