Microorganisms produce soluble low-molecular-weight (LMW, <1 kDa) siderophores, one of the strongest Fe-binding agents, to respond to the scarcity of Fe in the ocean. The presence of siderophores in marine particles/colloids is mostly not considered. Here, experimental evidence is provided to suggest the possibility of siderophore incorporation into marine particles. An incubation experiment with a 59Fe-complexed desferrioxamine (DFO, siderophore-model compound) was conducted using natural seawater (<3 μm) at dark condition to examine the size re-distribution of DFO and its associated Fe during microbial growth. 59Fe and DFO in suspended particles/aggregates, colloids and dissolved phase were quantified after the incubation. Our results showed that ∼55% of the 59Fe, originally in the form of LMW DFO-Fe, was incorporated in the suspended particles/aggregates. Noticeably, a minor amount (0.395 ± 0.020%) of the DFO was incorporated into the particulate phase. This finding is novel in that while the DFO facilitating Fe incorporation into microbial biomass was released back into the dissolved phase, still a minor fraction of the siderophores could be “retained” in particles. This could have become cumulatively more important in more complex natural systems that involves the interplay between minerals, bacteria, phytoplankton and zooplankton. Furthermore, our results indirectly suggest a balance of two different mechanisms during the Fe-siderophore transport. Our results are in favor of the processes occurring outside of the cells (Fe dissociation from the Fe-siderophore complex followed by the microbial Fe uptake) but the second mechanism can also exist (uptake of intact Fe-siderophore complex into the microbial intracellular fractions), albeit to a lesser extent.

中文翻译：

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天然海水中异羟肟酸铁载体和相关Fe掺入海洋颗粒中

微生物产生可溶性低分子量（LMW，<1 kDa）铁载体，这是最强的铁结合剂之一，以应对海洋中铁的稀缺性。大多数情况下不考虑海洋颗粒/胶体中铁载体的存在。在这里，提供了实验证据来表明铁载体并入海洋颗粒的可能性。在黑暗条件下使用天然海水 (<3 μm) 进行了 59Fe 复合去铁胺（DFO，铁载体模型化合物）的孵育实验，以检查 DFO 及其相关 Fe 在微生物生长过程中的尺寸重新分布。培养后对悬浮颗粒/聚集体、胶体和溶解相中的 59Fe 和 DFO 进行定量。我们的结果表明，约 55% 的 59Fe，最初以 LMW DFO-Fe 的形式存在，被掺入悬浮颗粒/聚集体中。值得注意的是，少量 (0.395 ± 0.020%) 的 DFO 被掺入到颗粒相中。这一发现的新颖之处在于，虽然促进 Fe 掺入微生物生物质的 DFO 被释放回溶解相，但仍有一小部分铁载体可以“保留”在颗粒中。在涉及矿物质、细菌、浮游植物和浮游动物之间相互作用的更复杂的自然系统中，这可能变得越来越重要。此外，我们的结果间接表明在 Fe 铁载体运输过程中两种不同机制的平衡。