Magnetotactic bacteria provide some excellent examples of convergent evolution.  In particular the ability to synthesize iron compounds has evolved at least twice, respectively employing iron oxide (magnetite) and iron sulphide.

The ability of some bacteria to secrete tiny crystals of iron compounds that they employ to navigate along the magnetic field lines of the Earth has been known for some time.  Evidently the magnetic polarity enable the bacteria to adjust their position in the sediment so they are in the appropriate redox environment. It is less appreciated, however, that the magnetotactic bacteria also provide some excellent examples of convergent evolution, in particular regarding iron compound synthesis and coloniality. 

Iron compounds and the magnetosome

The ability to synthesize these iron compounds has evolved at least twice in the magnetotactic bacteria, respectively employing iron oxide (magnetite) and iron sulphide in the α and δ sub-divisions respectively of the proteobacteria.  The tiny grains of mineral are housed in a so-called magnetosome, that involves proteins that otherwise are typical of the eukaryotic cytoskeleton, and recall the organelles of the latter group. The synthesised crystals of magnetite or other iron mineral are employed to orientate the bacterium in the ambient magnetic field and so allow it to find the optimum position in an area of variable redox (or other chemocline) boundaries.

Colonial magnetotactic bacteria

Magnetotactic bacteria have independently evolved a form of “multicellularity” or coloniality, forming balls of cells that are strikingly similar to colonial green algae such as Eudorina, and also propel themselves through the water by placing the bacterial flagellae on the outside.