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.
Bibliographical noteFunding Information:
This work was supported in part by grant CHE-1664657 from the National Science Foundation to CJC and FCK, the TOTAL Foundation (Paris) and the UK Natural Environment Research Council grants (NE/D521522/1, NE/ J023094/1, Oceans 2025 / WP 4.5) to FCK. We are also grateful for funding from the MASTS pooling initiative (The Marine Alliance for Science and Technology for Scotland). We thank Dr. M. L. Carter, SIO for help with collection of water samples at Scripps Pier, César O. Almeda-Jáuregui, CICESE for Ocean Data View plots and Dr. Avery Tatters, USC for the initial culture of Lingulodinium polyedra. A fellowship from the Hanse-Wissenschaftskolleg to CJC is also gratefully acknowledged.
- Iodine speciation