Iron (Fe) is one of the most important micronutrients for most life forms on earth. While abundant in soil, Fe bioavailability in oxic soil is very low. Under environmental conditions, bacteria need to acquire sufficient Fe to sustain growth while limiting the energy cost of siderophore synthesis. Biofilm formation might mitigate this Fe stress, since it was shown to accumulate Fe in certain Gram-negative bacteria and that this Fe could be mobilized for uptake. However, it is still unclear if, and to what extent, the amount of Fe accumulated in the biofilm can sustain growth and if the mobilization of this local Fe pool is modulated by the availability of environmental Fe (i.e., Fe outside the biofilm matrix). Here, we use a nondomesticated strain of the ubiquitous biofilm-forming soil bacterium Bacillus subtilis and stable Fe isotopes to precisely evaluate the origin of Fe during growth in the presence of tannic acid and hydroxides, used as proxies for different environmental conditions. We report that this B. subtilis strain can accumulate a large quantity of Fe in the biofilm, largely exceeding Fe associated with cells. We also report that only a fraction of biofilm-bound Fe is available for uptake in the absence of other sources of Fe in the vicinity of the biofilm. We observed that the availability of environmental Fe modulates the usage of this pool of biofilm-bound Fe. Finally, our data suggest that consumption of biofilm-bound Fe relates to the efficacy of B. subtilis to transport Fe from the environment to the biofilm, possibly through siderophores.

中文翻译：

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枯草芽孢杆菌可调节其对生物膜结合铁的使用，以响应环境中铁的可用性。

铁（Fe）是地球上大多数生命形式中最重要的微量营养素之一。虽然土壤中铁丰富，但在有氧土壤中铁的生物利用度非常低。在环境条件下，细菌需要获取足够的铁来维持生长，同时限制铁载体合成的能源成本。生物膜的形成可能会减轻这种铁的胁迫，因为它被证明会在某些革兰氏阴性细菌中积累铁，并且可以动员这些铁吸收。然而，目前尚不清楚生物膜中累积的铁量是否能够维持到什么程度，以及在多大程度上可以维持生长，以及该局部铁库的动员是否受环境铁（例如，生物膜基质外的铁）的可用性调节。 。在这里，我们使用无处不在的形成生物膜的土壤细菌枯草芽孢杆菌菌株稳定的铁同位素，可以精确地评估在鞣酸和氢氧化物存在下生长过程中铁的来源，这些鞣剂可以用作不同环境条件的代理。我们报告此枯草芽孢杆菌菌株可以在生物膜中积累大量的铁，大大超过与细胞相关的铁。我们还报告说，在生物膜附近没有其他铁源的情况下，只有一部分生物膜结合的铁可供摄取。我们观察到环境铁的可用性调节了生物膜结合铁池的使用。最后，我们的数据表明，与生物膜结合的铁的消耗与枯草芽孢杆菌将铁从环境运输到生物膜（可能通过铁载体）的功效有关。