Iron is a micronutrient critical to fundamental biological processes including respiration and photosynthesis, and it can therefore impact primary and heterotrophic productivity. Yet in oxic environments, iron is highly insoluble, rendering it, in principle, unavailable as a nutrient for biological growth. Life has “solved” this problem via the invention of iron chelates, known as siderophores, that keep iron available for microbial productivity. In this work, we examined the impact of siderophore synthesis on the speciation, mobility, and bioavailability of iron from rock-forming silicate minerals—shedding new light on the mechanisms by which microbes use mineral substrates to support primary productivity, as well as the consequent effects on silicate dissolution. Growth experiments were performed with Shewanella oneidensis MR-1 in an oxic, iron-depleted minimal medium, amended with olivine minerals as the sole source of iron. Experiments included the wild-type strain MR-1, and a siderophore synthesis gene deletion mutant strain (ΔMR-1). Relative to MR-1, ΔMR-1 exhibited a very pronounced growth penalty and an extended lag phase. However, substantial growth of ΔMR-1, comparable to MR-1 growth, was observed when the mutant strain was provided with siderophores in the form of either filtrate from a well-grown MR-1 culture, or commercially available deferoxamine. These observations suggest that siderophores are critical for S. oneidensis to acquire iron from olivine. Growth-limiting concentrations of deferoxamine amendments were observed to be ≤5–10 µM, concentrations significantly lower than previously recorded as necessary to impact mineral dissolution rates. X-ray photoelectric spectroscopy analyses of the incubated olivine surfaces suggest that siderophores deplete mineral surface layers of ferric iron. Combined, these results demonstrate that low micromolar concentrations of siderophores can effectively mobilize iron bound within silicate minerals, supporting very significant biological growth in limiting environments. The specific mechanism would involve siderophores removing a protective layer of nanometer-thick iron oxides, enhancing silicate dissolution and nutrient bioavailability.

中文翻译：

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硅酸盐矿物质作为限制性营养素的直接来源：铁载体的合成和摄取促进橄榄石和微生物生长中三价铁的生物利用度

铁是对包括呼吸和光合作用在内的基本生物过程至关重要的微量营养素，因此它可以影响初级和异养生产力。然而，在有氧环境中，铁是高度不溶的，因此原则上不能作为生物生长的营养物质。生命通过铁螯合物（称为铁载体）的发明“解决”了这个问题，铁螯合物使铁可用于微生物生产力。在这项工作中，我们研究了铁载体合成对造岩硅酸盐矿物中铁的形态、迁移率和生物利用度的影响——揭示了微生物利用矿物基质支持初级生产力的机制，以及随之而来的对硅酸盐溶解的影响。进行生长实验Shewanella oneidensis MR-1 在含氧、贫铁的基本培养基中，用橄榄石矿物质作为唯一铁源进行修正。实验包括野生型菌株 MR-1 和铁载体合成基因缺失突变菌株 (ΔMR-1)。相对于 MR-1，ΔMR-1 表现出非常明显的生长惩罚和延长的滞后期。然而，当突变菌株以来自生长良好的 MR-1 培养物的滤液或市售去铁胺的形式提供铁载体时，观察到 ΔMR-1 的大量生长，与 MR-1 的生长相当。这些观察结果表明铁载体对S. oneidensis至关重要从橄榄石中获取铁。观察到去铁胺修正剂的生长限制浓度≤5–10 µM，浓度显着低于先前记录的影响矿物溶解速率所需的浓度。对培养的橄榄石表面的 X 射线光电光谱分析表明，铁载体消耗了三价铁的矿物表面层。综上所述，这些结果表明，低微摩尔浓度的铁载体可以有效地动员硅酸盐矿物中结合的铁，支持在限制环境中非常显着的生物生长。具体机制将涉及铁载体去除纳米厚的氧化铁保护层，增强硅酸盐溶解和营养生物利用度。