Determination of natural Fe organic complexes in the surface waters of the Amundsen Sea

The distribution and biological availability of Fe is strongly controlled by its physical-chemical speciation within seawater, where colloids and Fe-organic complexes are dominant factors. In order to study the distribution and the biological availability of Fe the natural Fe organic complexes were determined in the surface waters of the Amundsen Sea (300 m). Methods Samples were collected using Go Flo bottles and filtered under ultra-clean conditions in flow benches (class 0). The concentration of iron binding ligands (organic compounds which strongly bind Fe) and their binding strength (conditional stability constant) were studied in 5 size classes here: unfiltered water, 0.2 μm filtered water, &lt; 1000 kDa (Stereapore, Mitsubishi-rayon Co. Ltd, Nishioka and al., 2000, 2005), &lt; 100 kDa and &lt; 10 kDa ultra-filtrated water (Sartorius, Vivaflow 50, Schlosser and Croot, 2008). The left-over fraction from the ultra filtrations (retentates) were also analyzed for dFe and ligand characteristics to ensure a mass balance calculation and validate the ultrafiltration method. The dissolved iron concentrations in all the size fractions (and retentates) were measured (see dFe measurement section) using a chemo luminescence method (FIA) with acidified samples (pH 1.8). Total iron concentrations will be measured 6-12 months after the acidification of the unfiltered sample. Ligand characteristics were determined by using a complexing ligand titration with addition of iron (between 0 and 10 nM of Fe added) in buffered seawater (mixed NH3/NH4OH borate buffer, 5 mM). The competing ligand 'TAC' (2-(2-Thiazolylazo)-p-cresol) with a final concentration of 10 μM was used and the complex (TAC)2-Fe was measured after equilibration (&gt; 15 h) by cathodic stripping voltammetry (CSV) (Croot and Johansson, 2000). The electrical signal recorded with this method (nA) was converted to a concentration of (TAC)2-Fe (nM). Subsequently, the ligand concentration and the binding strength were estimated using the non-linear regression of the Langmuir isotherm (Gerringa and al., 1995) and a newer \"Leo\" model currently built up (Gerringa et al, in prep). The voltammetric equipment consisted of a μAutolab potentiostat (Type II and III, Ecochemie, The Netherlands), a mercury drop electrode (model VA 663 from Metrohm). All equipment was protected against electrical noise by a current filter (Fortress 750, Best Power). Sampling statistics 26 stations were sampled on this cruise. These included 14 profiles and 5 stations where different size fractions were analyzed after 4 filtrations with different filter sizes (0.2 μm cut-off, 1000 kDa, 100 kDa and 10 kDa). In addition, 11 Fe/Ligand experiments were analyzed. Special attention was given to determine the iron binding ligands before and after incubation with and without artificial ligands in these experiments, to look at the response of algae and the change of the ligand characteristics during the incubations. Preliminary results An average ligand concentration of 0.789 nEq was found on the NBP0901 cruise, varying from 0.2 and 1.6 nEq of Fe. Highest ligand concentrations were found at 10 m depth followed by a minimum at 25 m. Concentrations increased with depth to become rather constant at 200 and 300 m. Low Fe binding strength of the ligands was found at 10 and 25 m in the polynya suggesting freshly produced ligands by organisms (phytoplankton or bacteria) or by a change in the ligand content and characteristics due to the biologic activity (the pool of strong binding ligands may be removed or used). The ratio ligand/dissolved iron (Fig. 38) clearly shows differences between the surface water and deeper samples. Very high ratios (10) were found in the surface waters of the Pine Island polynya due to the low dissolved iron concentration and high ligand concentrations. In the deep water (200 and 300 m) of the polynya and the circumpolar deep water upwelling in front of the PIG the ligand/dissolved iron ratio was close to 1 indicating a saturation of the ligands by iron and the possibility for iron to be removed from the water column by precipitation.海水中铁（Fe）的分布与生物可利用性，主要受其物理-化学形态调控，其中胶体与铁-有机络合物为关键控制因素。为探究铁的分布特征及生物可利用性，本研究针对阿蒙森海（Amundsen Sea）表层至300米水深的海水，开展了天然铁有机络合物的测定工作。 方法 样品采用Go Flo采水器（Go Flo bottles）采集，并于0级超净工作台（flow benches class 0）的超洁净环境中完成过滤。本次研究针对5个粒径/截留分子量分级样品，测定了铁结合配体（即强结合铁的有机化合物）的浓度及其结合强度（条件稳定常数，conditional stability constant），分级设置如下：未过滤海水、0.2 μm过滤海水、<1000 kDa级过滤海水（采用Stereapore膜，三菱丽阳有限公司（Mitsubishi-rayon Co. Ltd），参考文献Nishioka等，2000、2005）、<100 kDa级超滤海水及<10 kDa级超滤海水（采用赛多利斯Sartorius Vivaflow 50超滤装置，参考文献Schlosser与Croot，2008）。为验证超滤方法并完成质量平衡计算，同时对超滤过程产生的截留液（retentates）中的溶解态铁（dissolved iron, dFe）及配体特征进行了分析。所有分级样品（含截留液）的溶解态铁浓度均采用化学发光-流动注射分析法（FIA）测定，样品预先酸化至pH 1.8。未过滤海水样品酸化后6至12个月，将测定总铁浓度。 配体特征通过配体滴定法测定：向添加了0~10 nM铁的、采用5 mM氨-氢氧化铵-硼酸混合缓冲液配制的缓冲海水中，加入终浓度为10 μM的竞争配体TAC（2-(2-噻唑偶氮)-对甲酚，2-(2-Thiazolylazo)-p-cresol），平衡15小时以上后，采用阴极溶出伏安法（cathodic stripping voltammetry, CSV）测定(TAC)₂-Fe络合物浓度（Croot与Johansson，2000）。将该方法记录的电信号（纳安，nA）转换为(TAC)₂-Fe络合物的浓度（纳摩尔，nM），随后通过朗缪尔等温线（Langmuir isotherm）非线性回归（Gerringa等，1995）及当前正在构建的新型“Leo”模型（Gerringa等，待发表）估算配体浓度与结合强度。 伏安分析设备包括μAutolab型恒电位仪（II型与III型，荷兰Ecochemie公司）及Metrohm VA 663型滴汞电极，所有设备均通过Fortress 750型电流滤波器（Best Power公司）屏蔽电气噪声。 采样统计 本次航次共布设26个采样站位，包含14条断面剖面，其中5个站位分别采用4种不同过滤装置（0.2 μm截止孔径、1000 kDa、100 kDa及10 kDa）进行分级过滤后开展分析。此外，共完成11组铁/配体实验分析。本实验重点测定了添加与不添加人工配体的培养实验前后的铁结合配体含量，以探究藻类响应及培养过程中配体特征的变化。 初步结果 在NBP0901航次中，配体平均浓度为0.789纳当量（nEq），变化范围为0.2~1.6纳当量（以铁计）。配体浓度在10米水深处达到峰值，25米水深处降至最低；随水深增加，配体浓度逐渐升高，在200米与300米水深趋于稳定。在冰间湖（polynya）10米与25米水深处，配体的铁结合强度较低，这表明配体可能由浮游植物或细菌等生物新生成，或是因生物活动导致配体库与特征发生改变——强结合配体库可能被消耗或移除。 配体/溶解态铁比值（图38）清晰体现了表层海水与深层海水样品间的差异：派恩岛冰间湖（Pine Island polynya）表层海水的该比值高达10，这源于其较低的溶解态铁浓度与较高的配体浓度；而在冰间湖深层海水（200米与300米水深）以及派恩岛冰川（Pine Island Glacier, PIG）前缘上升的环极深层水中，配体/溶解态铁比值接近1，表明配体已被铁饱和，且铁有可能通过沉淀作用从水体中移除。