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Iodine exists as a trace element in seawater, with total iodine being generally constant at about 0.45-0.55 µM. Almost all of this iodine occurs in two main forms: iodate and iodide. Iodate is the thermodynamically stable form under normal seawater conditions, and thus should be the only iodine-containing species in the water column. However, iodate concentrations are found to vary considerably, being generally greater at depth and lower at the surface, while iodide concentrations follow the reverse pattern, being anomalously accumulated in the euphotic zone and decreasing with depth. The fact that iodide concentrations follow a depth dependence corresponding to the euphotic zone suggests that biological activity is the source of the reduced iodine. Nonetheless, the nature and source of iodate reduction activity remains controversial. Here, using a combination of field and laboratory studies, we examine some of the questions raised in our and other previous studies, and seek further correlations between changes in iodine speciation and the presence of marine macro- and microalgae. The present results indicate that microalgal growth per se does not seem to be responsible for the reduction of iodate to iodide. However, there is some support for the hypothesis that iodate reduction can occur due to release of cellular reducing agents that accompany cell senescence during phytoplankton bloom declines. In addition, support is given to the concept that macroalgal species such as giant kelp (Macrocystis pyrifera) can take up both iodide and iodate from seawater (albeit on a slower time scale). We propose a mechanism whereby iodate is reduced to iodide at the cell surface by cell surface reductases and is taken up directly as such without reentering the bulk solution. [ABSTRACT FROM AUTHOR]

Kelp (Laminaria digitata) uses iodide as a unique inorganic antioxidant to protect its surface and apoplastic space against reactive oxygen species such as ozone, hydrogen peroxide and superoxide, with implications for atmospheric and marine chemistry as well as regional climatic processes. If kelp is covered by seawater, this results in iodide leaching into surrounding sea water. In this study, the influence of the kelps Laminaria digitata , L. hyperborea , L. ochroleuca and Saccharina latissima on iodine speciation chemistry was explored at two sites in Oban (Argyll, Scotland) and Roscoff (Brittany, France) based on diver-operated in situ sampling. Seawater samples were subsequently analysed voltammetrically, accompanied by determination of extractable iodine concentrations in the tissues of the thalli surveyed by ICP-MS. The main result is that iodide concentrations in the vicinity of kelp thalli are strongly enhanced, especially at low tide, while iodate concentrations are decreased in comparison to open coastal water and open ocean concentrations. Using diver-operated in situ sampling with seawater samples subsequently analysed voltammetrically, this study finds that iodide levels in the vicinity of kelp thalli are strongly enhanced, especially at low tide, while iodate levels are decreased in comparison to open coastal water and open ocean concentrations. [Display omitted] • This study uses a new diver-operated in situ syringe sampling technique. • Samples analysed voltammetrically. • Iodide levels around kelp thalli are strongly enhanced, especially at low tide. • In contrast, iodate levels are decreased in comparison to open waters. [ABSTRACT FROM AUTHOR]