Organic Fe Speciation in the Southern Ocean

Objectives

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 over the whole water depth were determined in three different size fractions. Special attention was given in that distinct water masses present were sampled as well.

Samples were collected by an ultra-clean sampling system using 24 Go Flo bottles fixed on an all-titanium frame and with a Kevlar cable. The concentration of iron binding ligands (organic compounds which strongly bind Fe) and their binding strength (conditional stability constant) are studied in 3 size classes here: unfiltered water, 0.2 µm filtered water and smaller than 1000 KDa ultra-filtrated water.

Methods

General

Under ultra clean conditions the 0.2 µm filtered seawater was ultra-filtrated using polyethylene hollow-fiber filters as to make an operational defined distinction between large colloidal and small colloidal Fe including the “truly dissolved” Fe (1000 KDa nominal weight, Stereapore, Mitsubishi-rayon Co. Ltd, Nishioka and al., 2000, 2005). The dissolved organic iron (0.2 µm filtered) as well as the truly dissolved iron (< 1000 KDa) were analysed by Maarten Klunder and Patrick Laan using a chemo luminescence method (FIA) with acidified samples (pH 1.8). Total iron will be measured 6-12 months after the acidification of the unfiltered sample. The natural ligand characteristics were determined by doing a complexing ligand titration with addition of iron (between 0 and 8 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 (> 15 h) by cathodic stripping voltammetry (CSV) (Croot and Johansson, 2000). The electrical signal recorded with this method (nA) was converted as a concentration (nM), then the ligand concentration and the binding strength were estimated using the non-linear regression of the Langmuir isotherm (Gerringa and al., 1995).

The voltammetric equipment consisted of a µAutolab potentiostat (Type I, 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

Seven stations were sampled on the zero meridian transect with a maximal depth of 4500 m. A total of 140 samples on 56 depths were sampled (28 of unfiltered, 56 of 0.2 µm filtered and 56 of 1000 KDa ultra-filtered). Among them, 11 depths characterizing the most important water-masses were sampled twice and kept frozen for later analyses while back at NIOZ (for the study of kinetic exchange between the different forms of iron).

Two profiles were sampled in the Weddell Sea for a total of 46 samples (8 of unfiltered, 19 of 0.2 µm filtered and 19 of 1000 KDa ultra-filtered). 8 depths were also sampled twice to characterize important water-masses.

研究目的

海水中铁（Iron）的物理化学形态对其分布及生物可利用性具有极强的调控作用，其中胶体（colloids）与铁-有机络合物（Fe-organic complexes）是关键控制因素。为探究铁的分布与生物可利用性，本研究针对全水深范围内的天然铁有机络合物，设置3个不同粒径分级进行测定，并特别对采样覆盖的不同水团进行了针对性取样。

样品采集采用超洁净采水系统，搭载固定于全钛框架（all-titanium frame）的24个Go-Flo采水器（Go Flo bottles），并使用凯夫拉缆绳（Kevlar cable）进行部署。本研究针对3个粒径等级的样品开展分析：未过滤海水、0.2 µm过滤海水以及孔径小于1000 kDa的超滤液海水，重点测定其中的铁结合配体（iron binding ligands，即对铁具有强络合作用的有机化合物）浓度及其络合强度（conditional stability constant）。

研究方法

通用流程
在超洁净环境下，采用聚乙烯中空纤维过滤器（polyethylene hollow-fiber filters）对0.2 µm过滤后的海水进行超滤，以实现大胶体、小胶体与"truly dissolved"铁（"truly dissolved" Fe，标称分子量1000 kDa，Stereapore，三菱 rayon有限公司（Mitsubishi-rayon Co. Ltd），Nishioka等，2000、2005）之间的操作定义区分。Maarten Klunder与Patrick Laan采用酸化样品（pH 1.8）的化学发光分析法（chemo luminescence method，流动注射分析（Flow Injection Analysis, FIA）），分别对溶解态有机铁（0.2 µm过滤级）与真溶解铁（<1000 kDa级）进行检测。未过滤样品将在酸化后6~12个月内完成总铁含量的测定。

天然配体的特征通过络合配体滴定法（complexing ligand titration）确定：在以NH3/NH4OH硼酸盐缓冲液（NH3/NH4OH borate buffer，5 mM）缓冲的海水中，添加梯度浓度的铁（0~8 nM）进行滴定；使用终浓度为10 µM的竞争配体TAC（2-(2-噻唑偶氮)-对甲酚，2-(2-Thiazolylazo)-p-cresol, TAC），待体系平衡（>15 h）后，通过阴极溶出伏安法（cathodic stripping voltammetry, CSV）测定(TAC)₂-Fe络合物的信号（Croot与Johansson，2000）。将该方法记录的电流信号（纳安级，nA）转换为浓度（纳摩尔级，nM）后，通过Langmuir等温线（Langmuir isotherm）的非线性回归拟合，估算配体浓度与络合强度（Gerringa等，1995）。

伏安检测设备包括µAutolab恒电位仪（µAutolab potentiostat，I、II、III型，Ecochemie，荷兰）以及Metrohm公司VA 663型号滴汞电极（mercury drop electrode）。所有设备均通过电流滤波器（current filter，Fortress 750，Best Power）屏蔽电磁干扰。

采样统计
本研究在零子午面断面（zero meridian transect）上设置7个采样站位，最大采样水深达4500 m，共在56个深度层采集140个样品：未过滤样品28个、0.2 µm过滤样品56个、1000 kDa超滤样品56个。其中，11个表征主要水团的深度层进行了重复采样，样品冷冻保存，待返回荷兰皇家海洋研究所（NIOZ）后开展不同形态铁间动力学交换过程的后续分析。

另在威德尔海（Weddell Sea）采集2条剖面，共计46个样品：未过滤样品8个、0.2 µm过滤样品19个、1000 kDa超滤样品19个；其中8个深度层同样进行了重复采样，以表征关键水团特征。