Measurements of organic complexation of iron during the CARUSO-EISENEX experiment

The speciation of strongly chelated iron during the 22-day course of an iron enrichment experiment in the Atlantic sector of the Southern Ocean deviates strongly from ambient natural waters. Three iron additions (ferrous sulfate solution) were conducted, resulting in elevated dissolved iron concentrations (Nishioka, J., Takeda, S., de Baar, H.J.W., Croot, P.L., Boye, M., Laan, P., Timmermans, K.R., 2005, Changes in the concentration of iron in different size fractions during an iron enrichment experiment in the open Southern Ocean. Marine Chemistry, doi:10.1016/j.marchem.2004.06.040) and significant Fe(II) levels (Croot, P.L., Laan, P., Nishioka, J., Strass, V., Cisewski, B., Boye, M., Timmermans, K.R., Bellerby, R.G., Goldson, L., Nightingale, P., de Baar, H.J.W., 2005, Spatial and Temporal distribution of Fe(II) and H2O2 during EisenEx, an open ocean mescoscale iron enrichment. Marine Chemistry, doi:10.1016/j.marchem.2004.06.041). Repeated vertical profiles for dissolved (filtrate < 0.2 µm) Fe(III)-binding ligands indicated a production of chelators in the upper water column induced by iron fertilizations. Abiotic processes (chemical reactions) and an inductive biologically mediated mechanism were the likely sources of the dissolved ligands which existed either as inorganic amorphous phases and/or as strong organic chelators. Discrete analysis on ultra-filtered samples (< 200 kDa) suggested that the produced ligands would be principally colloidal in size (> 200 kDa-< 0.2 µm), as opposed to the soluble fraction (< 200 kDa) which dominated prior to the iron infusions. Yet these colloidal ligands would exist in a more transient nature than soluble ligands which may have a longer residence time. The production of dissolved Fe-chelators was generally smaller than the overall increase in dissolved iron in the surface infused mixed layer, leaving a fraction (about 13-40%) of dissolved Fe not bound by these dissolved Fe-chelators. It is suggested that this fraction would be inorganic colloids. The unexpected persistence of such high inorganic colloids concentrations above inorganic Fe-solubility limits illustrates the peculiar features of the chemical iron cycling in these waters. Obviously, the artificial about hundred-fold increase of overall Fe levels by addition of dissolved inorganic Fe(II) ions yields a major disruption of the natural physical-chemical abundances and reactivity of Fe in seawater. Hence the ensuing responses of the plankton ecosystem, while in itself significant, are not necessarily representative for a natural enrichment, for example by dry or wet deposition of aeolian dust.
Ultimately, the temporal changes of the Fe(III)-binding ligand and iron concentrations were dominated by the mixing events that occurred during EISENEX, with storms leading to more than an order of magnitude dilution of the dissolved ligands and iron concentrations. This had strongest impact on the colloidal size class (> 200 kDa-< 0.2 µm) where a dramatic decrease of both the colloidal ligand and the colloidal iron levels (Nishioka, J., Takeda, S., de Baar, H.J.W., Croot, P.L., Boye, M., Laan, P., Timmermans, K.R., 2005, Changes in the concentration of iron in different size fractions during an iron enrichment experiment in the open Southern Ocean. Marine Chemistry, doi:10.1016/j.marchem.2004.06.040) was observed.

南大洋大西洋海域为期22天的富铁实验过程中，强螯合铁的形态分布与周围天然水体存在显著差异。实验共开展三次硫酸亚铁溶液投加，提升了水体溶解态铁（dissolved iron）浓度（参考文献：Nishioka, J., Takeda, S., de Baar, H.J.W., Croot, P.L., Boye, M., Laan, P., Timmermans, K.R., 2005, 南大洋开放海域富铁实验中不同粒径组分的铁浓度变化，《海洋化学》，doi:10.1016/j.marchem.2004.06.040），同时检测到显著的亚铁离子（Fe(II)）水平（参考文献：Croot, P.L., Laan, P., Nishioka, J., Strass, V., Cisewski, B., Boye, M., Timmermans, K.R., Bellerby, R.G., Goldson, L., Nightingale, P., de Baar, H.J.W., 2005, EisenEx中Fe(II)与过氧化氢（H₂O₂）的时空分布——一项开放海域中尺度富铁实验，《海洋化学》，doi:10.1016/j.marchem.2004.06.041）。针对经0.2 μm滤膜过滤的溶解态三价铁结合配体（Fe(III)-binding ligands）的重复垂直剖面观测显示，铁富集投加诱导上层水体产生了螯合剂。溶解态配体的潜在来源包括非生物过程（化学反应）与诱导性生物介导机制，它们以无机非晶相或强有机螯合剂的形式存在。对超滤（ultra-filtered）样品（<200千道尔顿（kDa））的离散采样分析表明，所产生的配体主要为胶体粒径范围（>200 kDa且<0.2 μm）的组分，而铁投加前占主导的是可溶性组分（<200 kDa）。不过相较于停留时间更长的可溶性配体，这类胶体配体具有更强的暂态特性。表层投加铁的混合层中，溶解态铁螯合剂的产生量总体低于溶解态铁的总增量，导致约13%~40%的溶解态铁未被这些螯合剂结合，该组分被认为是无机胶体。此类无机胶体浓度意外地持续高于无机铁溶解度阈值，体现了该海域铁化学循环的独特特征。显然，通过投加溶解态无机亚铁离子使总铁水平提升近百倍，会严重扰乱海水中铁的天然物理化学丰度与反应活性。因此，后续浮游生物生态系统（plankton ecosystem）的响应虽本身意义重大，但未必能代表自然状态下的铁富集过程，例如通过风成粉尘（aeolian dust）的干、湿沉降所引发的富集。

最终，三价铁结合配体与铁浓度的时间变化主要受EISENEX实验期间的混合事件调控，风暴导致溶解态配体与铁浓度被稀释一个数量级以上。这种影响对胶体粒径组分（>200 kDa且<0.2 μm）最为显著，观测到胶体配体与胶体铁浓度均出现大幅下降（参考文献：Nishioka, J., Takeda, S., de Baar, H.J.W., Croot, P.L., Boye, M., Laan, P., Timmermans, K.R., 2005, 南大洋开放海域富铁实验中不同粒径组分的铁浓度变化，《海洋化学》，doi:10.1016/j.marchem.2004.06.040）。