Symbiosis and mutualism are drivers of life diversification and sources of biological innovation in the microbial world. Recently, magnetotactic assemblies composed of a flagellated protist and ectosymbiotic bacteria biomineralizing magnetic crystals have been discovered. Their mutualistic symbiosis relies on a collective magnetotaxis coupled to a hydrogen-based syntrophy. This new form of cooperation challenges our view of magnetic biomineralization in prokaryotes and magnetoreception in eukaryotes, but it sill remains poorly understood. This project is based on the assumption that these magnetotactic symbioses emerged several times in the course of microbial eukaryotes evolution as an adaptation to anoxic environments. Thus, we propose to test this hypothesis by (1) characterizing the diversity and ecology of the magnetotactic holobionts, (2) identifying the functional bases of the symbiosis and (3) deciphering the origin and evolutionary history of the magnetotactic symbiosis.