Multicellular magnetotactic bacteria (white dots) move through an artificial pore space (black/red grid). Image credit: Petroff et al. (CC BY 4.0)
Animals have developed a variety of strategies to detect direction, from specialised cells and organs to complex brain structures. This can be particularly difficult for microorganisms, such as bacteria, which often live in microenvironments where gravity cues are generally negligible.
For example, magnetotactic bacteria – bacteria that sense and orient along magnetic fields – live in waterlogged environments and instead use the geomagnetic field, the Earth’s magnetic field, to navigate. They contain magnetic crystals that act like a compass needle, aligning the cells with magnetic field lines. Because geomagnetic field lines are rarely parallel to the sediment surface, these bacteria move vertically through sediments. This motion allows them to position themselves at precise depths, follow changing gradients, and move between layers with distinct chemical conditions.
However, moving vertically through the sediment is challenging. First, a microbe must couple its motion to some external field with up-down asymmetry. Several options exist, including the gravitational field and gradients of chemicals or light. It must then balance motion along field lines with the need to move around obstacles.
Despite their similar behaviors, magnetotactic bacteria show a large diversity in their morphological and motility traits. Petroff et al. sought to understand how natural selection has shaped these traits to balance vertical motion with efficient obstacle avoidance.
Using a combination of theory and experiment, the researchers explored how swimming speed, magnetic moment and size of an organism affect navigation through porous environments. They measured the average speed of the magnetotactic bacterium Magnetoglobus through an artificial pore space in a controlled magnetic field.
The results showed that efficient navigation requires an optimal combination of swimming speed, magnetic moment and body size. Movement along magnetic lines was fastest when the distance a microbe swims before realigning with the magnetic field matches the pore size. Microbes that reorient too quickly get trapped; those that align too slowly wander. Additionally, because the efficiency of magnetotaxis only depends on the ratio of environmental factors and bacterial phenotype, there is no single characteristic that can be tuned to allow optimal navigation. Rather, optimality is achieved when different phenotypic characteristics balance one another. For example, small cells with weak magnetic moments perform as well as large cells with strong ones.
Magnetotactic bacteria have found a remarkable solution to navigating complex environments. Natural selection has shaped their phenotypes so that the environmental randomness is mirrored in their trajectories. Similar strategies may also apply to artificial navigation in complex settings, such as robots designed to move through rubble. To this end, it could be useful to design magnetotactic robots.
