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

摘要

The effect of artificial acidic polysaccharides (PS) and exudates ofPhaeodactylum tricornutum on the half-life of Fe(II) in seawater was investigated in laboratoryexperiments. Strong photochemical hydrogen peroxide (H₂O₂)production of 5.2 to 10.9 nM (mg C)−1 h−1 was found in thepresence of PS and diatom exudates. Furthermore when illuminated with UVlight the presence of algal exudates had a net stabilising effect on ferrousiron in seawater (initial value 100 nmol L−1) above that expected fromoxidation kinetics. In the dark the PS gum xanthan showed no stabilisingeffect 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 modelof the photochemical redox cycle of iron incorporating these processessupported the observed data well. Diatom exudates seem to have the potentialto play an important role for the photochemistry of iron in coastal waters.