The role of polysaccharides and diatom exudates in the redox cycling of Fe and the photoproduction of hydrogen peroxide in coastal seawaters

S Steigenberger,P J Statham,C Völker,U Passow

doi:10.5194/bg-7-109-2010

Abstract

Abstract. The effect of artificial acidic polysaccharides (PS) and exudates of Phaeodactylum tricornutum on the half-life of Fe(II) in seawater was investigated in laboratory experiments. Strong photochemical hydrogen peroxide (H2O2) production of 5.2 to 10.9 nM (mg C)−1 h−1 was found in the presence of PS and diatom exudates. Furthermore when illuminated with UV light the presence of algal exudates had a net stabilising effect on ferrous iron in seawater (initial value 100 nmol L−1) above that expected from oxidation kinetics. In the dark the PS gum xanthan showed no stabilising effect on Fe(II). The photochemical formation of superoxide (O2−) in the presence of diatom exudates and its reducing effect on Fe(III) appears to result in greater than expected concentrations of Fe(II). A model of the photochemical redox cycle of iron incorporating these processes supported the observed data well. Diatom exudates seem to have the potential to play an important role for the photochemistry of iron in coastal waters.

Highlights

Marine phytoplankton contribute significantly to the CO2 exchange between atmosphere and ocean, impacting on atmospheric CO2 concentrations (Falkowski et al, 1998)
Gum xanthan led to the highest photochemical production of H2O2 followed by carrageenan and laminarin, which can be explained by their different absorptivity at < 400 nm (Fig. 2)
Given that H2O2 plays an important role in the Fe redox-chemistry, our experiments suggest that PS may have a significant indirect effect on Fe redox-speciation due to the enhanced photochemical production of H2O2

Summary

Introduction

Marine phytoplankton contribute significantly to the CO2 exchange between atmosphere and ocean, impacting on atmospheric CO2 concentrations (Falkowski et al, 1998). Several large scale iron fertilization experiments have revealed that in ∼40% of the surface ocean, the so called High Nutrient Low Chlorophyll (HNLC) areas, iron is at least partially responsible for limitation of phytoplankton growth (Boyd et al, 2007). Free hydrated Fe(III) concentrations in seawater are very low (< 10−20 mol L−1), (Rue and Bruland, 1995) and the more soluble Fe(II) is rapidly oxidized. Over 99% of the dissolved iron in seawater is reported to be bound by organic compounds (Boye, 2001; Croot and Johansson, 2000; Rue and Bruland, 1995; van den Berg, 1995) and these ligands provide a mechanism whereby the concentrations of dissolved iron typically seen in the ocean can be maintained (Johnson et al, 1997). Transparent exopolymer particles (TEP), which are rich in acidic polysaccharides, are ubiquitous in the surface ocean (Passow, 2002), and have been shown to bind 234Th (Passow et al, 2006) and are a prime candidate to bind iron

Methods

Results

Conclusion