Magneto-aerotaxis in magnetotactic bacteria

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Magnetotactic bacteria orient in magnetic fields with the help of a dedicated organelle, the magnetosome. Magnetosomes are typically organized in a chain, which acts as an intracellular compass needle. In this way, their swimming, powered by their flagella, is guided by the magnetic field; the bacteria can be understood as self-propelled compass needles. Magnetotaxis is usually intertwined with the tactic response to oxygen gradients, as magnetotactic bacteria use the vertical component of the Earth magnetic field to navigate towards the oxic-anoxic transition zone near the bottom of layered aquatic environments. In this project, we use a combined theoretical and experimental approach for the quantitative characterization of magneto-aerotactic behaviors to understand the complex interplay of the responses to magnetic field and to oxygen gradients. In addition, we aim at understanding how this interplay depends on the architecture of the cells magnetic moment and motility apparatus. In the first funding period, we have discovered several new magneto-aerotactic behaviors and classified these behaviors with a theoretical model of directional swimming based on either the oxygen gradient or the magnetic field or both. Moreover, we have developed methods for the theoretical and experimental study of the motion of single bacteria in 3D. In the second funding period, we will combine these methods towards a quantitative understanding of magneto-aerotaxis and the underlying swimming properties. We will track individual bacteria in three dimensions and quantitatively characterize their motility (speed, rate and angle of directional changes, shape of the trajectories) under different conditions (different oxygen concentrations, homogeneous and gradient, different magnetic field strength and inclination relative to the oxygen gradient) and for different strains of magnetotactic bacteria. These results will be used to inform a model for magneto-aerotaxis based on active Brownian particles with multiple internal states. The model will be used to predict the spatial profiles of the bacterial density in oxygen gradients (aerotactic band) for different magnetic field strengths. These predictions will be tested against experimental density profiles. Moreover, we will use Stokesian dynamics simulations to simulate the swimming of magnetotactic bacteria for different cellular architectures to get insight into the coordination of flagella and the effect of the relative orientation of the magnetic moment and the flagellum or flagella. The trajectories from these simulations will be compared to the experimental ones from the 3D tracking. The combination of our experimental approaches and theoretical description on multiple levels will lead to a comprehensive quantitative picture of magneto-aerotactic motility and more generally shed light on the integration of different signals for directional motility of biological and synthetic microswimmers.