Redox effects of superoxide in polar waters chemiluminescent reaction with finement is important when their The oceans constitute 70% of the the reagent MCLA. By comparing data are compared with data acsurface area of the biosphere, and the decay kinetics of superoxide quired by otherwise very similar the fertility of the sea’s “soil” dein control seawater with samples experimental methods. pends in large part on the micro“spiked” with Fe, Cu, or the metal “An important question that nutrients available therein: chelator DTPA (which, by experithis study raisessbut does not dissolved organic matter, metal mental design, binds metal ions fully answersis whether Cu reacions, and the like. Light, which to prevent their reaction with sutions compete with and minimize arrives on a regular schedule freFe reduction to more bioquently interrupted by available forms in the surweather, serves as a maface ocean,” says Zafiriou. cronutrient for photosyn“The fate of the superthesis, but it also has oxide itself is not as imother effectssits reaction portant as its role as a with colored dissolved orredox agent,” agrees Croot. ganic matter, for example, Iron cycles between Fe(II) generates superoxide (O2-) and Fe(III), and copper in the sea as deep as the Superoxide produced in seawater by sunlight has important cycles between Cu(I) and sun can penetrate. As Pe- effects on the redox statesand thus the solubility and Cu(II). “These chemical ter Croot and Maija Heller bioavailabilitysof Cu and Fe ions and their complexes. species have very different of the Leibniz Institute of bioavailabilities and Marine Sciences (Gerreactivitiessand in the many) show in a recent peroxide), Croot and Heller were case of Fe, solubilities.” ES&T paper (2009, DOI 10.1021/ able to assess the importance of Croot and Heller apply es901766r), the fate of superoxide various contributors to the de“oceanographic consistency” is intimately connected to the oxicomposition of superoxide. throughout their discussion of the dation state (and thus the bio“We went back to using an kinetic data. In chemical oceanavailability) of dissolved metal older source of superoxide, solid ography, open ocean water colions in Antarctica’s surrounding KO2,” which hadn’t been used for umn data should be “smooth and Southern Oceansand in some years because of a belief that it readily related to species with surprising ways. was contaminated with trace metsimilar chemistries or processes,” The effects of superoxide in als, Croot explains. “However, we says Croot. “In this way, it proocean-like environments have found that commercial KO2 supvides a check on whether the data been studied in detail in the laboplies were now clean enough for you have are ‘consistent’ with the ratory, but available field data are this worksand most importantly processes we know occur in the sparse. This new study “takes without the use of a chelating oceansincluding vertical mixing, maximal advantage of prior work agent, such as DTPA, in the sulight penetration, metal complexto design and implement a fairly peroxide source.” ation, and O2 or H2O2 concentracomplex experimental design,” Despite being strongly organisays Oliver Zafiriou of the Woods tions, for instance.” These cally complexed in the open Hole Oceanographic Institution. internal checks for self-consisocean, “Cu was apparently the Seawater was collected at a vatency bolster the wealth of new major sink” for superoxide, says riety of depths throughout the field data that the study provides. Croot. “Fe was a minor sink, and voyage of the German research “The Southern Ocean is of consomewhat surprisingly, given earvessel Polarstern (as a contribusiderable interest as a focus of lier coastal work, reactions with tion to the International Polar conservation of marine biota, as organic matter were almost nonYear GEOTRACES program) well as a site where we expect cliexistent.” through the Weddell Sea and into mate change to impact first,” says A noteworthy experimental the Drake Passage. ConcentraGustaaf Hallegraeff of the Univervariation was a designated incutions of Cu and Fe were detersity of Tasmania (Australia). “This bation time for DTPA. On the bamined, giving a depth profile of work raises new ideas about the sis of an earlier publication and the dissolved metal ions. Then, bioavailability of Fe, linked to their own observations, Croot and decomposition kinetics of added little-studied Cu, in relation to Heller specifically equilibrated superoxide were analyzed on-ship superoxide decay kinetics.” DTPA with seawater samples for a through a well-characterized —STEVEN C. POWELL minimum of 12 hours. This re-
9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / December 1, 2009
10.1021/es903209u
2009 American Chemical Society
8710
Published on Web 11/09/2009
