Magnetotactic bacteria (MTB) produce intracellular magnetite (Fe3O4) nanoparticles in a genetically controlled manner. They may represent some of the oldest biomineralizing organisms available in the geological record, but identification of their fossils remains highly debated. While organic molecules are degraded during diagenesis and metamorphic processes, MTB magnetite nanocrystals can be efficiently preserved in the rock record and are referred to as magnetofossils. Experimental work on the freshwater bacterium Magnetospirillum magneticum strain AMB-1 has demonstrated specific minor and trace element patterns distinct from those of abiotic magnetite, and were proposed as a tool for magnetofossil identification. These promising geochemical signatures need to be validated in diverse MTB strains to be used for paleontological reconstruction. Here, we cultivated a marine MTB (Magnetovibrio blakemorei strain MV-1) under various chemical conditions to test possible generalization of this new proxy. MV-1 was grown under various Fe concentrations (50, 100 and 150 μM) and redox states using either Fe(II)-ascorbate or Fe(III)-citrate as Fe sources. The chemical compositions of the growth media and extracted magnetite crystals were determined by ICP-MS analyses to quantify the partitioning of trace and minor elements between magnetite and solution. Results show that partition coefficients do not depend at first order on the Fe concentration and redox state, a crucial conclusion for potential application to natural systems. A comparison of the two strains shows that MV-1 magnetite generally contains higher concentrations of impurities than AMB-1 magnetite. However, a number of elements possess similar partition coefficients and may represent useful chemical proxies for testing the biological origin of magnetite. These consistent elements can be separated into three groups. The first group is composed of elements (Co, Mn, Pb, Sr) highly depleted in MTB magnetite relative to abiotic magnetite. The second group contains elements with similar partitioning in MTB and abiotic magnetite, including Ca and Li. This group may serve as a reference for constraining a paleo-fluid composition. The last group contains elements (Mo, Sn, Se) enriched in MTB magnetite relative to abiotic magnetite. Such enrichments might be related to biological function of those elements. Chemical patterns determined from laboratory experiments therefore represent promising chemical proxies to identify MTB magnetite in the rock record but now need to be tested in modern natural environments, where MTB and surrounding solution can be jointly collected.

中文翻译：

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确定磁铁矿的痕量和微量元素中的含量作为趋磁细菌的生物特征


趋磁细菌 （MTB） 以遗传控制的方式产生细胞内磁铁矿 （Fe3O4） 纳米颗粒。它们可能代表了地质记录中一些最古老的生物矿化生物，但对其化石的鉴定仍然存在很大争议。虽然有机分子在成岩作用和变质过程中会降解，但 MTB 磁铁矿纳米晶体可以有效地保存在岩石记录中，被称为磁熔硅。对淡水细菌磁螺菌 AMB-1 菌株 AMB-1 的实验工作已经证明了不同于非生物磁铁矿的特异性微量元素模式，并被提议作为磁化石鉴定的工具。这些有前途的地球化学特征需要在不同的 MTB 菌株中得到验证，以用于古生物学重建。在这里，我们在各种化学条件下培养了一种海洋 MTB （Magnetovibrio blakemorei 菌株 MV-1），以测试这种新代理的可能泛化。MV-1 在各种 Fe 浓度（50、100 和 150 μM）和氧化还原状态下生长，使用 Fe（II）-抗坏血酸酯或 Fe（III）-柠檬酸盐作为 Fe 源。通过 ICP-MS 分析测定生长培养基和提取的磁铁矿晶体的化学成分，以量化磁铁矿和溶液之间痕量元素和微量元素的分配。结果表明，分配系数在一阶上不依赖于 Fe 浓度和氧化还原状态，这是可能应用于自然系统的关键结论。两种菌株的比较表明，MV-1 磁铁矿通常比 AMB-1 磁铁矿含有更高浓度的杂质。 然而，许多元素具有相似的分配系数，可能代表测试磁铁矿生物起源的有用化学代理。这些一致的元素可以分为三组。第一组由相对于非生物磁铁矿在 MTB 磁铁矿中高度贫乏的元素（Co、Mn、Pb、Sr）组成。第二组包含在 MTB 和非生物磁铁矿中具有相似分配的元素，包括 Ca 和 Li。该组可以作为限制古流体成分的参考。最后一组包含相对于非生物磁铁矿富含 MTB 磁铁矿的元素（Mo、Sn、Se）。这种富集可能与这些元素的生物学功能有关。因此，从实验室实验中确定的化学模式代表了在岩石记录中识别 MTB 磁铁矿的有前途的化学代理，但现在需要在现代自然环境中进行测试，在那里可以共同收集 MTB 和周围溶液。