Magnetotactic bacteria accumulate a large pool of iron distinct from their magnetite crystals

Magnetotactic bacteria (MTB) are ubiquitous aquatic microorganisms that form intracellular nanoparticles of magnetite (Fe3O4) or greigite (Fe3S4) in a genetically controlled manner. Magnetite and greigite synthesis requires MTB to transport a large amount of iron from the environment which is subsequently concentrated in organelles called magnetosomes for crystal precipitation and maturation. X-ray absorption analysis of MTB suggests that the intracellular iron is mainly contained within the crystals, thus preventing potential toxic effects of free iron. In contrast, recent mass spectrometry studies suggest that MTB may contain a large amount of iron that is not precipitated in crystals. Here, we attempt to resolve these descrepancies by performing chemical and magnetic assays to quantify the different iron pools in the magnetite-forming strain Magnetospirillum magneticum AMB-1 cultivated at varying iron concentrations. AMB-1 mutants showing defects in crystal precipitation were also characterized following the same approach. All results show that magnetite represents at most 30 % of the total intracellular iron under our experimental conditions. We further examined the iron speciation and subcellular localization in AMB-1 using the fluorescent indicator FIP-1 that is designed for detection of labile Fe(II). Staining with this probe suggests that unmineralized reduced iron is found in the cytoplasm and associated with magnetosomes. Our results demonstrate that, under our experimental conditions, AMB-1 is able to accumulate a large pool of iron distinct from magnetite. Finally, we discuss the biochemical and geochemical implications of these results. ImportanceMagnetotactic bacteria (MTB) are a group of microorganisms producing iron-based intracellular magnetic crystals. They represent a model system for studying iron homeostasis and biomineralization in bacteria. MTB contain an important mass of iron, about 10 to 100 higher than other bacterial model such as Escherichia coli, suggesting efficient iron uptake and storage systems. Accordingly, MTB have been proposed to significantly impact the iron biogeochemical cycle in sequestering a large amount of soluble iron into crystals. Recently, several studies proposed that MTB could also accumulate iron in a reservoir distinct from their crystals. Here, we present a chemical and magnetic methodology for quantifying the fraction of the total cellular iron contained in the magnetic crystals of the magnetotactic strain Magnetospirillum magneticum AMB-1. Comparison of the mass of iron contained in the different cellular pools showed that most of the bacterial iron is not contained in AMB-1 crystals. We then adapted protocols for the fluorescent detection of Fe(II) in bacteria, and showed that iron could be detected outside of crystals using fluorescence assays. This work suggests a more complex picture for iron homeostasis in MTB than previously thought. Because iron speciation controls its solubility, our results also provide important insights into the geochemical impact of MTB. A large pool of unmineralized iron in MTB could be more easily released in the environment than magnetite, thus limiting iron sequestration into MTB crystals.