Earth’s magnetic field extends more than 65,000 kilometers above the surface of our planet, reaching far beyond our atmosphere and streaming out into space. Meanwhile, down on the planet’s surface, in the narrow band of conditions between aquatic oxic and anoxic zones (characterized by the abundance and absence, respectively, of molecular oxygen) in sediments or the water column, microscopic bacteria follow Earth’s magnetic signals as they shuttle chemicals across the gap.
Li et al. investigate the chemistry and cellular structure of one kind of these magnetic field–following bacteria, called Candidatus Magnetobacterium casensis (Mcas), to illuminate the role of magnetic navigation, or magnetotaxis, in aquatic chemical cycling.
The researchers found that Mcas’s internal structure is closely linked to its shuttling function. The bacteria produce iron-containing magnetite crystals that act as compasses, guiding the bacterium’s navigation. Mcas cells also form sulfur-carrying globules and storage pods of nitrate called vacuoles, often at different times.
In the researchers’ model, Mcas cells carry substances through their environment like a person organizing a house. The oxic attic is full of oxygen and nitrate, whereas sulfur and iron are stored in the anoxic basement. Mcas bundles up sulfur in the basement and carries it to the attic, where the bacterium can combine sulfur and oxygen to make sulfite and sulfate, producing energy in the process. While it is upstairs, Mcas packs vacuoles with nitrate, then carries them down to the basement to aid in reactions that gather up sulfur and iron and perhaps to produce more energy through anoxic reactions.
Rather than moving freely in any direction, Mcas cells save energy by following only the oxygen gradient up and down magnetic field lines—the staircase between the basement and the attic—using their iron-containing compasses as a guide.
These tiny reorganizations happen in aquatic environments around the globe and ultimately contribute to global biogeochemical cycles, the authors note, with Mcas and similar magnetotactic bacteria efficiently shuttling sulfur, iron, carbon, nitrogen, and phosphorus—all key elements for life on Earth. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2020JG006012, 2020)
—Elizabeth Thompson, Science Writer