China News Service, Beijing, September 30 (Reporter Sun Zifa) The geomagnetic field is like a "coordinate network" covering the world, continuously providing directional information to organisms. As a "living compass", how do magnetotactic bacteria build magnetic bonds? It has always attracted academic attention.
Li Jinhua, a researcher from the Institute of Geology and Geophysics of the Chinese Academy of Sciences, led and collaborated with collaborators from Germany, Australia, the United Kingdom, France, Canada and other countries to construct a cross-scale research framework of "particle magnetization-magnetic linkage response-cell connection" using a single-cell strain of magnetotactic Vibrio (WYHV-1) discovered in Weiyang Lake, Xi'an.
Magnetosome structure, magnetism and magnetic field-dependent intercellular magnetic connections of Magnetotactic Vibrio WYHV-1. Photo courtesy of Institute of Geology and Geosciences, Chinese Academy of Sciences
This important research result spans the three scales of magnetic particles, magnetosome chains, and cells and expands the classic single magnetic domain model. The related paper was published online in the Proceedings of the National Academy of Sciences (PNAS) in the early morning of September 30, Beijing time.
Magnetosomes “do not take the usual path”
Li Jinhua said that magnetotactic bacteria synthesize nano-magnetite magnetosomes in their cells and arrange them into chains, oriented along the geomagnetic field, to efficiently find suitable ecological niches in the oxidation-anoxia transition zone. The classic model holds that magnetotactic bacteria rely on single-domain magnetosomes with regular morphology and uniform magnetization to obtain stable magnetic moments.
However, in nature, some magnetotactic bacteria can synthesize particles that are relatively large in size and unevenly magnetized. Can these "atypical" magnetosomes maintain effective magnetic functions? Can the magnetic effects of magnetosome chains cross cell boundaries? Direct evidence has long been lacking.
The special feature of WYHV-1 studied this time is that the magnetosomes it synthesized are "unconventional". These magnetosomes are elongated prismatic magnetites with an average length of about 105 nanometers and a width of about 81 nanometers. More than three-quarters of the particles are 90 to 150 nanometers long, which is significantly larger than most reported prismatic magnetosomes from magnetotactic bacteria. What's even more unique is that there are symmetrical pits at both ends of the crystal, shaped like a pair of "dimples" with an average depth of about 11 nanometers. It has been confirmed through multiple methods that the pits are an inherent feature of the crystal.
What are the magnetic consequences of large size and pits? Theoretically, isolated large-sized magnetite particles tend to form non-uniform magnetization instead of ideal single magnetic domains. This time, the research team directly "saw" the magnetic moment arrangement at the nanometer scale: at the chain end or local collapse, a single magnetosome is often in a single vortex state, and the magnetic moment curls around a vortex core about 15 nanometers wide, and is not all in the same direction.
However, when these particles are closely arranged into chains with a spacing of about 2 nanometers, the strong intra-chain magnetostatic coupling significantly suppresses the local inhomogeneity, making most of the particles in the chain tend to an "effective single magnetic domain-like" state, and the entire magnetic chain forms a stable magnetic moment along the chain axis. Micromagnetic simulations further confirmed that particles of about 40 nanometers to 85 nanometers are single magnetic domains, and large particles of 90 nanometers to 150 nanometers are mostly single vortices. After forming a chain, the chain level cooperatively "tames" the vortex, and the entire chain is the source of stable magnetic response.
In addition, magnetosome chains mediate intercellular connections. In the hanging drop experiment, when cells are locally enriched and an external magnetic field of approximately 0.1–1 millitela is applied (approximately 2–20 times the local geomagnetic field), WYHV-1 cells will connect end to end along the magnetic field to form chain-like aggregates of 20 microns–50 microns or even longer. The study explains this as magnetic field-dependent physical connections between otherwise independent cells.
Reasonably explain the hanging drop experiment
Li Jinhua pointed out that in order to test its magnetic basis, the research team established single-chain and head-to-tail double-chain models. Calculations show that the stray field at the chain end attenuates with distance, but is still higher than the local geomagnetic field strength (about 0.000045 Tesla) in the range of 100 nanometers to 1000 nanometers; the double-chain static magnetic energy gain exceeds the room temperature thermal disturbance kinetic energy.
The model suggests that when two living cells are about 2000 nanometers apart, nearly one cell length, the inter-chain magnetic interaction may still resist Brownian perturbations. This provides a reasonable explanation for the hanging drop experiment: the external field first roughly aligns the cells, and the stray field at the chain end then promotes end-to-end attraction, connection, and reconnection. At this time, the magnetosome chain is not only an intracellular "compass", but also a magnetic link that transmits magnetic force between cells.
The classical single magnetic domain model expanded in this study shows that magnetotactic bacteria do not need each magnetosome to be in an ideal uniform magnetization state. Large-sized single vortex particles can still obtain stable axial magnetic responses through chain-level cooperation. Therefore, the functional evaluation unit should be expanded from isolated particles to complete magnetic chains and even cell-cell interactions.
At the same time, WYHV-1 also provides a modern biological reference for the prismatic magnetite with pits at both ends in the geological record, which has implications for the identification of magnetosome fossils and the interpretation of paleoenvironments and paleomagnetic records.
Li Jinhua said that based on the results of this research, in the future, direct observation of living cells in a simulation system close to the geomagnetic field intensity, and through magnetism-electrochemistry-chemistry coupling experiments, will hopefully reveal whether this inter-cell magnetic connection has real ecological functions. (over)
