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 (Fe₃0₄) or greigite (Fe₃S₄) in a ge-netically controlled manner. Magnetite and greigite synthesis requires MTB to trans-port a large amount of iron from the environment. Most intracellular iron was pro-posed to be contained within the crystals. However, recent mass spectrometry studies suggest that MTB may contain a large amount of iron that is not precipi-tated in crystals. Here, we attempted to resolve these discrepancies by performing chemical and magnetic assays to quantify the different iron pools in the magnetite-forming strain Magnetospirillum magneticum AMB-1, as well as in mutant strains showing defects in crystal precipitation, cultivated at various iron concentrations. All results show that magnetite represents at most 30% of the total intracellular iron under our experimental conditions and even less in the mutant strains. We further examined the iron speciation and subcellular localization in AMB-1 using the fluores-cent indicator FIP-1, which was designed for the detection of labile Fe(ll). Staining with this probe suggests that unmineralized reduced iron is found in the cytoplasm and associated with magnetosomes. Our results demonstrate that, under our experi-mental 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. IMPORTANCE Magnetotactic bacteria (MTB) produce iron-based intracellular mag-netic crystals. They represent a model system for studying iron homeostasis and biomineralization in microorganisms. MTB sequester a large amount of iron in their crystals and have thus been proposed to significantly impact the iron biogeochemi-cal cycle. Several studies proposed that MTB could also accumulate iron in a reser-voir distinct from their crystals. Here, we present a chemical and magnetic method-ology for quantifying the iron pools in the magnetotactic strain AMB-1. Results showed that most iron is not contained in crystals. We then adapted protocols for the fluorescent Fe(ll) detection in bacteria and showed that iron could be detected outside crystals using fluorescence assays. This work suggests a more complex pic-ture for iron homeostasis in MTB than previously thought. Because iron speciation controls its fate in the environment, our results also provide important insights into the geochemical impact of MTB.