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. Most intracellular iron was proposed to be contained within the crystals. However, recent mass spectrometry studies suggest that MTB may contain a large amount of iron that is not precipitated 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 fluorescent indicator FIP-1, which was designed for the 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.

中文翻译：

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趋磁细菌积聚了大量不同于其磁铁矿晶体的铁。

趋磁细菌（MTB）是普遍存在的水生微生物，它们以遗传控制的方式形成磁铁矿（Fe ₃ O ₄）或钙铁矿（Fe ₃ S ₄）的细胞内纳米颗粒。磁铁矿和钙铁矿的合成需要MTB从环境中运输大量铁。建议将大多数胞内铁包含在晶体中。但是，最近的质谱研究表明，MTB可能含有大量的铁，这些铁没有沉淀在晶体中。在这里，我们试图解决由进行化学和磁检测这些差异来量化磁铁矿形成应变不同的铁池磁螺magneticum在各种铁浓度下培养的AMB-1以及在晶体沉淀中显示缺陷的突变菌株中。所有结果表明，在我们的实验条件下，磁铁矿最多占总细胞内铁的30％，而在突变菌株中则更少。我们使用荧光指示剂FIP-1进一步检查了AMB-1中的铁形态和亚细胞定位，该指示剂旨在检测不稳定的Fe（II）。用该探针染色表明未矿化的还原铁存在于细胞质中，并与磁小体有关。我们的结果表明，在我们的实验条件下，AMB-1能够积聚大量不同于磁铁矿的铁。最后，我们讨论了这些结果对生化和地球化学的影响。