Nanoscale lignin particles as iron chelators in the ocean

Iron is essential for the growth and metabolism of all marine organisms. Yet at the high dissolved oxygen concentration found in the open ocean, iron exists primarily as Fe(III) which is highly reactive with respect to hydrolysis and adsorption. Due to the low solubility of iron(III)hydroxide combined with sedimentation of iron-rich particles, iron concentrations in the open ocean are extremely low and could even reach a point too low to sustain life. Fortunately, iron solubility is enhanced by organic ligands which keep the iron in solution and prevent precipitation and sedimentation of iron oxide minerals. The low concentrations of these ligands make them difficult to detect, and only a few marine Fe ligands have been structurally characterized so far. It has been shown that the activities of plankton have a profound influence through the release of different classes of important chelating ligands like siderophores, porphyrins, or exopolymeric substances. The present project focuses on humic nanoparticles, a yet unidentified class of iron chelators in the ocean. These substances are released by wetland ecosystems and transported into the sea by rivers. The main goal of the proposed project is to prove that a fraction of the dissolved Fe found in the surface ocean waters is associated with the dissolved lignins found in these ocean waters (at the typical oceanic iron and lignin concentrations). The corroboration of this hypothesis affords highly sensitive and selective methods for accurate quantification of iron and iron bearing compounds in seawater at naturally occurring concentration levels. Aiming at the discovery and investigation of low to ultralow concentrations of organic iron-complexes, we decided to combine elemental and molecular mass spectrometry orthogonally. We will apply the concept of general unknown screening via pre-separation followed by on-line enrichment and accurate mass spectrometry (LC-TOF-MS). Concomitantly, X-ray absorption fine structure (EXAFS) spectroscopy will be used to determine the binding mode and oxidation state of iron in the iron-binding lignin fractions.

Dissolved organic matter (DOM) is ubiquitous in aquatic environments and influences many biological processes. DOM is known to range over a wide distribution of size and polarity of its molecules. Major building blocks include organic acids, aromatic and aliphatic moieties as well as protein-derived and carbohydrate structures. The aim of this project was to contribute to a deeper understanding of those DOM constituents which make iron soluble and bioavailable in seawater. This question is of special significance, because the solubility of inorganic iron species in seawater is very low and hence the micronutrient iron represents a growth limiting factor in the worlds ocean, while on the other hand oceanic biomass production by phytoplankton has a key impact on global climate. It has been shown recently that peat-bog influenced waters contain relatively high concentrations of organic molecules which form strong complexes with iron and make it soluble in fresh water and seawater. These molecules are transported by rivers from land to the sea where they have a profound effect on the supply of iron to marine plankton organisms. At present, little is known about these DOM constituents. However, it is crucial to understand how the iron is bound and released from these complexes, which functional groups are involved and how the uptake of iron by plankton algae works. Model compounds enable a glimpse inside the chemistry of these natural chelators. We synthesized iron complexes with different coordination motifs and ligand scaffolds and investigated them for their suitability as model compounds. The results led to the conclusion that catechol-based ligand systems should be excellent scaffolds for modeling ironbinding DOM. Concomitantly, we investigated natural DOM via time-of-flight mass spectrometry. What we found is that the fraction of riverine iron which is able to withstand flocculation in the river water/seawater mixing zone is mainly bound to low molecular weight DOM of the lignin type.Further, a combined X-ray absorption spectroscopy and valence-to-core X-ray emission spectroscopy study at the Fe K-edge revealed that iron binding in DOM is only dependent on oxygen containing functional groups, such as carboxyl and phenol. Thus we chose guaiacylglycerol-?-guaiacyl ether as a model ligand and decided to modify the ?-O-4 backbone of guaiacylglycerol-?-guaiacyl ether by introduction of a free catecholic moiety. In a next step, the ability of our new model substance to supply marine algae with iron was investigated in algal batch cultures. Guaiacylglycerol-?-guaiacyl ether ligands bearing catecholic moiety showed very good impact on marine plankton algae comparable with natural iron-binding DOM. However, guaiacylglycerol-?-guaiacyl ether (non-bearing catecholic moiety) did not enhance the growth of the algae. These results suggest that catecholic moieties may play an important role in natural iron-binding DOM. Further, we found that Fe-centers in seawater- soluble Fe-DOM were mononuclear. This suggests that only mononuclear iron-DOM complexes can be transported across the estuarine mixing zone whereas polynuclear iron- DOM complexes (which are also present and abundant in river water) precipitate and tend toward sedimentation under the influence of increasing salinity. Further, we investigated the photoreduction mechanism of natural Fe(III)-DOM in oceanic conditions into bioavailable aquatic Fe(II) forms. Our results show that this mechanism may be a vital component of the upper-ocean iron cycle.