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Summary

Multicellular magnetotactic bacteria (MMB) are unique microorganisms typically comprised of 10–40 bacterial cells arranged around a central acellular compartment. Their life cycle has no known unicellular stage and division occurs by separation of a single MMB aggregate into two identical offspring. In this study, South-seeking multicellular magnetotactic bacteria (ssMMB) were enriched from a New England salt marsh. When exposed to light, ssMMB reversed their magnetotactic behaviour to become North-seeking. The exposure time needed to generate the reversal response varied with light wavelength and intensity. Extensive exposure to light appeared to be lethal. This is the first report of a Northern hemisphere MMB displaying South-seeking behaviour and the first time a MMB is found to exhibit photo-magnetotaxis. We suggest that this mechanism enables ssMMB to optimize their location with regard to chemical gradients and light intensities, and propose a model to explain the peculiar balance between photo- and magnetotaxis.