Hydrogen peroxide photocycling in the Gulf of Aqaba, Red Sea

The dynamics of hydrogen peroxide (H₂O₂) was investigated from December 2007 to October 2008 in the Gulf of Aqaba, which in the absence of H₂O₂ contribution from biological production, rain and runoff, turned out to be a unique natural photochemical laboratory. A distinct seasonal pattern emerged, with highest midday surface H₂O₂ concentrations in spring-summer (30-90 nM) as compared to winter (10-30 nM). Similarly, irradiation normalized net H ₂O₂ formation rates obtained in concurrent ship-board experiments were faster in spring-summer than in winter. These seasonal patterns were attributed to changes in water characteristics, namely elevated spring-summer chromophoric dissolved organic matter (CDOM). The role of trace elements in H₂O₂ photoformation was studied by simultaneously measuring superoxide (O₂^-), Fe(II), and H₂O₂ formation and loss in ambient seawater and in the presence of superoxide dismutase, iron and copper. O₂^- was found to decay fast in the Gulf water, with a half-life of 15-28 s, primarily due to catalytic reactions with trace metals (predominantly copper). Hence, H₂O₂ formation in the Gulf involves metal-catalyzed O ₂^- disproptionation. Added iron moderately lowered net H₂O₂ photoformation, probably due to its participation in Fe(II) oxidation, a process that may also modify H₂O₂ formation in situ.