Configuration of redox gradient determines magnetotactic polarity of the marine bacteria MO-1.

Magnetotactic bacteria are capable of aligning and swimming along the geomagnetic field lines; such a behaviour is called magnetotaxis. Previous studies reported that bacteria in the northern hemisphere migrate preferentially towards the North Pole of the Earth's magnetic field (north-seeking, NS), whereas those in the southern hemisphere swim towards the South Pole (south-seeking, SS). The orientated swimming is thought to guide bacteria migrating downward to the favourable microaerobic or anaerobic regions in stratified water column or sediments. Recent identification of SS populations in northern hemisphere challenged the model of the adaptive value of magnetotaxis. To seek explanation for the apparent discrepancy, we analysed magnetotaxis polarity of axenic cultures under simulated growth conditions in hypomagnetic, northern-hemisphere-like or southern-hemisphere-like magnetic fields. We found that NS and SS cells could obviously coexist in hypomagnetic field and even, when the oxidation-reduction gradient configuration is suitable, in the geomagnetic field. These results reveal the selectivity of the redox gradient configuration on magnetotactic polarity of the cells and reconcile the discrepancy of the early reports.