Magnetotactic bacteria (MTBs) synthesize magnets in their cytoplasm and, as a result, align with external magnetic fields. The strains of MTBs we are interested in also self-propel by rotating their back flagella, and as a result swim preferentially along magnetic lines. We investigate what happens when we place dense suspensions of such swimmers in confined geometries under an external field. In a rectangular channel, we observe experimentally the emergence of plumes of bacteria at the scale of the confinement. This new collective motion can be modelled by considering the two opposite forces generated by the propulsion of each bacterium. In the sphere, on the other hand, previous experiments showed the formation of a global vortex with a preferred direction orthogonal to the magnetic field. This is the result of a series of symmetry breakings that can only be accounted for by combining gravity and swimmer chirality, as well as the flow from the force-based propulsion.