Iron plays a pivotal role in regulating ocean primary productivity. Iron is supplied from diverse sources such as the atmosphere and the geosphere, and hence iron biogeochemical research has focused on identifying and quantifying such sources of “new” iron. However, the recycling of this new iron fuels up to 90% of the productivity in vast oceanic regions. Evidence points to the key role of microbes in mediating this recycling, referred to as the “ferrous wheel”, that remobilises iron initially supplied to ocean biota. In the iron-limited subantarctic waters of the Southern Ocean, iron uptake is dominated by microbes smaller than 2 μm and exhibits seasonal and depth-related variations. The microbial community within the <2 μm size fraction comprises heterotrophic bacteria and picophytoplankton, both competing for iron. Here, we dissect the demand component of the ferrous wheel by separately assessing iron uptake by heterotrophic bacteria and photoautotrophic picophytoplankton. To explore the seasonal and depth-related variability in iron uptake, the influence of light on iron uptake in both bacterial and phytoplankton communities was examined. We observed that picoeukaryote phytoplankton demonstrated iron uptake rates 10 times greater than those observed in bacteria when normalized to biomass. Light was shown to stimulate iron uptake by 8- to 16-fold in phytoplankton and by 4- to 8-fold in heterotrophic bacteria. These results highlight the unexpectedly significant role of picoeukaryotic phytoplankton in driving the speed of the ferrous wheel, with implications for iron recycling across diurnal cycles, different oceanic depths, and seasonally.

中文翻译：

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微生物对铁的竞争决定了铁在铁轮中的可利用性


铁在调节海洋初级生产力方面起着关键作用。铁的来源多种多样，例如大气和地圈，因此铁的生物地球化学研究侧重于识别和量化“新”铁的此类来源。然而，这种新铁的回收为广大海洋地区高达 90% 的生产力提供了燃料。有证据表明，微生物在介导这种循环中起着关键作用，这种循环被称为“铁轮”，它重新动员了最初供应给海洋生物群的铁。在南大洋铁含量受限的亚南极水域，铁的吸收以小于 2 μm 的微生物为主，并表现出季节性和与深度相关的变化。<2 μm 大小分数内的微生物群落包括异养细菌和微浮游植物，两者都竞争铁。在这里，我们通过分别评估异养细菌和光合自养瘦浮游植物对铁的吸收来剖析铁轮的需求成分。为了探索铁摄取的季节性和深度相关变化，研究了光对细菌和浮游植物群落铁摄取的影响。我们观察到，当标准化为生物量时，picoeukaryote 浮游植物的铁吸收率比在细菌中观察到的铁吸收率高 10 倍。光在浮游植物中刺激铁的吸收是 8 到 16 倍，在异养细菌中刺激铁的吸收是 4 到 8 倍。这些结果突出了 picoeukaryotic 浮游植物在驱动铁轮速度方面出乎意料的重要作用，对昼夜周期、不同海洋深度和季节性的铁循环产生影响。