Bioinorganic nanoparticles as transport vehicles for iron

The increasing amount of CO 2 in the atmosphere from combustion of fossil fuels has become a serious environmental concern. Whether or not a detrimental CO 2 increase will occur in future decades depends on the natural carbon cycle which is highly influenced by processes that control the global iron budget in seawater. Several iron enrichment experiments have demonstrated iron`s role in controlling carbon uptake and hence regulating atmospheric partial pressure of carbon dioxide via the flux of carbon from the atmosphere to the deep sea. Iron presents a paradox. It is the fourth most abundant element in the Earth`s crust, and an essential element for all known living organisms, yet phytoplankton production in major ocean regions is restricted by extremely low iron availability. In many river waters, ferrous and ferric ions are present as suspended inorganic and bioinorganic colloidal particles. These particles are coagulated and settled as their freshwater carrier is mixed with seawater at the continental boundary. The global river input of bioavailable iron to the open ocean has therefore been estimated to be only of marginal importance. However, the iron concentration and speciation in a river is related to the types of soils in its watershed. Iron is essentially insoluble in soils, and there are only a few particular situations in soil systems through which it can be solubilized. In the soil type classified as Histosols, colloidal humic and fulvic acids are generated through incomplete decomposition of debris from sphagnum peat moss and other plants under waterlogged conditions. These naturally occurring metal chelating agents increase the tendency of iron dissolution in silicate weathering processes (leaching of Fe) and are responsible for the transport of iron in river waters. In preliminary experiments, we could show that they serve as transport vehicles for iron in coastal waters as well. In the present project, the global river input of bioavailable iron to the open ocean will be re-evaluated. By use of a 59Fe tracer method, the partitioning of the iron load from the suspended and dissolved mobile fraction to storage in the sediments will be measured with high accuracy as a function of salinity in mixtures of peat-bog influenced river waters with natural sea water. Second research need lies in the characterization of the unique organic nanoparticles which are leaching from these soils and serving as iron-carriers in river waters and coastal waters. The expected results will help to understand the role of the terrestrial ecosystem as a supplier of iron which is an essential nutrient for oceanic biota, and will also be important for estimating the anthropogenic impact on this iron source. Wetland ecosystems are prone to anthropogenic factors. By drainage, vast areas of wetlands have been and will be reclaimed for agricultural purposes. Even in protected areas, drought-induced acidification may lead to lower concentrations of organic colloids and iron in stream waters due to drainage measures and climate change.

Iron is an essential trace metal for nearly all living organisms yet phytoplankton production in major ocean regions is restricted by low iron availability due to the extremely low solubility of iron(III)hydroxide in seawater. Ultratrace amounts of various organic chelating ligands present in seawater complex more than 99% of the dissolved iron thereby preventing hydrolysis and sedimentation of the iron. These marine iron chelators increase the concentration and hence bioavailability of iron in the surface ocean by a factor of ~ 100 which is of vital importance for marine life and has also implications for the Earths climate system via the drawdown of CO2 from the atmosphere by phytoplankton. The extremely low concentrations of the marine chelators, however, make their direct detection a formidable task. Knowledge of the speciation of iron in seawater remains therefore an essential but elusive key component in our understanding of the biogeochemistry of iron in the ocean. The present project was focused on Fe-binding chelators in peat-bog influenced river waters and their possible significance as iron carriers from land to the sea. In this project, the use of a novel technique (field flow fractionation) to derive the data allowed us to improve our understanding of iron in aquatic systems. The project results showed clearly that iron can be carried from land to the sea by nano-particulate oligomeric lignin which is leaching out from acidic peat soils. A fraction of lignin and associated iron is not removed from solution when the ionic strength is increased beyond the level at which all other types of iron colloids are removed by coagulation during estuarine mixing with sea water. Our results suggest that gymnosperm as well as angiosperm tissues are contributors to the seawater-resistant iron- bearing lignin nanoparticles. Interestingly, the types of lignin components observed after CuO degradation of these nano-particulate phases correspond with the lignin degradation fractions found in the ocean. Lignins, which have no autochthonous source in the ocean, have been nevertheless found in low concentrations throughout the entire Arctic, Atlantic, and Pacific oceans. It is therefore tempting to speculate that peatland-derived iron-bearing lignin particles may have a sufficiently long half-life in ocean waters to sustain iron concentration in extended regions of the ocean. That means the marine lignins could be an important iron chelating ligand class in addition to the autochthonous marine chelating ligands. This project has contributed to deeper understanding of peatland ecosystems and their life-sustaining ecosystem services, providing additional scientific basis for natural infrastructure programms and climate change mitigation strategies as proposed by UNEP.