Can acidic polysaccharides exuded by diatoms function as iron ligands, increasing the iron availability for diatoms?
Final Report Abstract
This project combined field work and experiments to investigate the impact of polysaccharides released by diatoms on iron speciation. It was hypothesised that the input of iron into an iron limited system might promote the release of polysaccharides into the water and that these substances would act as iron ligands keeping iron in the surface water and bioavailable for phytoplankton over a longer time span. The marine iron cycle is very complex, and not well understood, but it is known that organic Fe-ligands play an essential role in keeping Fe bioavailable in the water. The hypothesis that diatoms release transparent exopolymer particles after an input of iron in iron limited systems could not be substantiated in the ship board experiments, although field data from EIFEX appeared to indicate such a reaction. Released polysaccharides of diatoms do, however, impact the iron cycling significantly and diatom exudates seem to play an important role for the photochemistry of iron in coastal waters. Strong photochemical hydrogen peroxide (H2O2) production was found in the presence of the polysaccharides Gum Xanthan, carrageenan and to a lesser amount laminarin as well as in diatom exudates, indicating that polysaccharides promote the formation of H2O2. Hydrogen peroxide (H2O2) is an important oxidant for many bio-relevant trace metals, including iron. In situ H202 distribution was found to be a function of mixing processes and phytoplankton activity. Furthermore when illuminated with UV light algal exudates kept the concentration of ferrous iron in seawater elevated for about 50 min. compared to the exudatesfree treatment where Fe (II) concentrations sank to the detection limit after 20 minutes. Since no stabilising effect of PS on Fe (II) in the dark could be detected, enhanced photoreduction seems to be the cause. We also found that the complexing capacity of diatom exudates was higher for copper ions than for cadmium ions, although the apparent stability constant was in general higher for cadmium ions. A significant amount of the ligand in this exudate could be ascribed to glutathione type, although some other higher molecular weight ligands also contributed to the complexing capacity for copper and cadmium in exudates of Thalassiosira weissfloggii. Measuring complexing capacity of iron is methodologically more difficult. During our titrations of polysaccharide solutions of up to several mg L-1 we could not determine k' values for iron indicating that the exudates bound iron only weakly, if at all. These results indicate that polysaccharides play only a minor role as iron ligands in the ocean, but probably effect the cycling of iron in the ocean indirectly via photoproduction of H2O2.
Publications
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(2007) Surface active substances in the upper water column during a Southern Ocean Iron Fertilization Experiment (EIFEX). Geophys Res Lett 34: L03612
Croot PL, Passow U, Assmy P, Jansen S, Strass VH
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Feb 2007 ASLO meeting Santa Fe (U.S.A.) oral presentation: "The effect of algal polysaccharides and UV radiation on the speciation of iron in seawater"
Steigenberger, Sebastian
