Magnetotactic bacteria (MTB) represent a large diversity of microorganisms found in several prokaryotic taxa. Geochemical data, as well as phylogenetic studies, suggest a very early emergence of these bacteria, around 3 Ga. MTB produce intracellular organelles, called magnetosomes, that contain nanocristals of magnetite (Fe3O4) or greigite (Fe3S4) allowing their orientation along the Earth’s magnetic field. They are ubiquitous in fresh and marine waters but thrive mainly at redox interfaces because of their anaerobic or oxygen tolerant metabolisms. Such environments correspond to water-sediments boundaries or chemoclines in water column of meromictic3 lakes. Lake Pavin is a meromictic lake in the French Massif Central containing a large amount of MTB of the Magnetococcaceae family at the redox transition between oxic and anoxic waters.
The study of MTB can help to better understand biomineralization mechanisms and to develop medical applications such as the use of nanocristals in cancer therapies. During this thesis, chemical and Fe isotope compositions of biosynthesized magnetite will be analyzed in order to develop new biological MTB tracers and better constrain biomagnetite formation pathways. Following the discovery of a unique mass independent fractionation (MIF) on 57Fe of the cuboctahedral magnetite in Magnetospirillum magneticum (Amor et al., Science 2016), I will use another MTB species, Magnetovibrio blakemorei that synthesizes elongated prismatic magnetite. In parallel, I will extend the analyzes to Lake Pavin, considered here as a model for natural environments. The methodology of prospection and development of a magneto-trap based on chemotactic and magnetotactic behaviors of MTB and cultures processes are presently developed in order to collect a sufficient amount of magnetosomes for isotopic analysis by MC-ICPMS and elemental composition analysis by ICP-MS.
3 Meromictic: characterizes a stratified lake which upper part, called mixolimnion, is regularly mixed and oxygenated while the lower part, called monimolimnion, remains isolated with the development of anoxic conditions