Environment-dependent swimming strategy of Magnetococcus marinus under   magnetic field
  Magnetotactic bacteria (MTB) are fascinating micro-organisms which possess embodied biomineralized nanomagnets providing them the ability to orient with the Earth's magnetic field. This property is presumably related to an evolutionary advantage in finding the oxic-anoxic interface along the up and down direction in aquatic environments. So far the magnetic field response by MTB, called magnetotaxis, has been well described by a paramagnetic model where bacteria orient passively along the field lines according to a purely physical mechanism where magnetic torque and orientational Brownian noise compete. Here we demonstrate using Magnetococcus marinus strain MC-1 as MTB model that magnetotaxis shows more complex behaviors, which are affected by environmental conditions of different types. Indeed while MC-1 swimmers are found to essentially obey the paramagnetic paradigm when swimming in their growth medium, they exhibit a run-and-tumble dynamics in a medium devoid of energy source. Tumbling events are found to provide isotropic reorientation capabilities causing the cells to escape from their prescribed field direction. This behavior has a major influence on the capabilities of the cells to explore their environment across field lines and represents an alternative search strategy to the back-and-forth motion along field-imposed tracks. Moreover, we show that aside chemical conditions, steric/geometrical constraints are also able to trigger tumbling events through obstacle encountering. Overall, physico-chemical environmental conditions appear to be important parameters involved in the swimming properties of MTB. Depending on environmental conditions, the run-and-tumble mobility may provide advantages in the search for nutrient or ecological niche, in complement to classical magnetotaxis. 
Nicolas Waisbord, Christopher Lef\`evre, Lyd\'eric Bocquet, Christophe   Ybert, C\'ecile Cottin-Bizonne
cond-mat.soft cond-mat.stat-mech physics.flu-dyn