The Role of Iron-binding Ligands in the Release of Iron from Dust, Minerals and Marine Particles in Seawater

Iron (Fe) is an essential trace nutrient for most known organisms due to its important role in many enzymatic processes. It is now well established that in many regions of the world’s oceans, the low Fe availability controls primary production, community structure and ecosystem functioning (Gledhill & Buck, 2012). Dissolved Fe(III) levels in the oceanic surface waters are extremely low (0.02 to 1 nM) (Vraspir & Butler, 2009) , with typically more than 99 % of dissolved Fe(III) bound to organic ligands (Gledhill & van den Berg, 1994; Rue & Bruland, 1995). In marine environments, under low Fe aerobic conditions bacteria often secrete siderophores to solubilise and sequester Fe(III) as one of several possible strategies to facilitate Fe uptake (Vraspir & Butler, 2009). Aeolian Fe deposition is a significant source of Fe to the remote regions of the ocean and is considered to have played a critical role in controlling the uptake of atmospheric CO2 through oceanic biological production during geological times (Islas et al., 2010). The fate of Fe during atmospheric transport and after deposition at the sea surface strongly depends on the mineralogy of aerosol particles (Kraemer, 2005). Several pathways such as thermal dissolution, mobilisation by organic ligands and photoreduction may be involved in the acquisition of Fe by marine organisms, often directly from particulate sources such as mineral dust (Kraemer, 2005) Organic ligands such as siderophores can have a strong influence on the solubility of iron oxides due to their high affinity for Fe. In remote oceanic regions siderophore-promoted dissolution of colloidal Fe is a critical process, as in those waters the input of new Fe is strongly determined by atmospheric deposition of Fe-bearing aerosols (e.g. dust) ( Borer, 2008). It has been found that siderophores can influence iron oxide dissolution by two mechanisms, a) ligand-controlled and b) light induced dissolution mechanisms (Kraemer et al., 1999 ;Borer et al., 2005). It has been observed that under atmospheric conditions, the presence of significant concentrations of dicarboxylic acids such as oxalic acid in marine aerosols may promote ligand-controlled and photoreductive dissolution mechanisms (Sempéré & Kawamura, 2003). The present study investigated the role of siderophores (DFB), oxalate and light in the dissolution of Fe from natural dust (Australian origin) and Fe-bearing minerals (goethite and lepidocrocite). The on-board experiments discussed in this thesis were conducted during the New Zealand leg of GEOTRACES GP 13 southwest Pacific Ocean cruise (2011) and the Fe Cycle III voyage (2012) in the southwest Pacific East of New Zealand. The results of our experiments indicated that the siderophore DFB promote Fe oxide dissolution from minerals as well as natural dust even at ambient pH under natural seawater conditions. The mineral dissolution experiments revealed that even at ambient seawater pH (pH ≈ 8) Fe(III)(hydr)oxides can undergo photoreductive dissolution in the presence of siderophores like DFB and organic species like oxalate. While comparing the minerals goethite and lepidocrocite, the strongest effect was observed for lepidocrocite, an intermediate phase in terms of thermodynamic stability. It was evident that the Fe dissolution is dependent on the type of mineral associated with the dissolution. Additionally, the present study investigated the release of DFe and the production of potentially both strong L1 and weaker L2-type Fe binding ligands during the particle remineralisation process using competitive ligand exchange- adsorptive cathodic stripping voltammetry (CLE-AdCSV) method and detected the possible siderophore-type compounds produced, using high performance liquid chromatography (HPLC) tandem muticollector inductively coupled plasma mass spectrometry(ICP-MS) method. Despite of the fact that previous bioremineralisation experiments have observed the release of DFe and the production of L2 throughout the experiment, we measured here, for the first time, the scavenging of DFe and consumption of ligand during the first three days of the Fe Cycle III bioremineralisation experiment. Although evidence had suggested that siderophores (potential L1-type ligands) are produced during bioremineralisation, in this study we were able to measure L1-type ligands electrochemically for the first time. During this study, it was observed that the siderophore-type ligand production is influenced not only by Fe limitation but also by other environmental factors such as nutrients, bacterial abundance and community structure