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Iodine assimilation by marine diatoms and other phytoplankton in nitrate‐replete conditions

Several marine phytoplankton species, primarily diatoms, were examined for the accumulation of iodide (10 species) and iodate (9 species) using radioactive iodine‐125 in f/2 artificial seawater, a nitrate‐enriched medium. Iodide accumulation (net uptake) rates were variable, and diatoms exhibited the highest rates. Emiliania huxleyi and Synechococcus sp. did not accumulate iodide. Accumulation rates ranged from 0 to 1.7 fmol cell−1 d−1. The diatom Porosira glacialis accumulated the greatest amount of iodide and was used to determine efflux rates of iodide. Iodide efflux was characterized by two distinct phases of iodide release: an initial rapid release rate of 10 amol cell−1 min−1 from the free space and a subsequent cellular release rate of 0.13 amol cell−1 min−1, which corresponds to a daily cellular release rate of 0.19 fmol cell−1 d−1. Accumulation of iodate ranged from 0 to 19 amol cell−1 d−1, with P. glacialis displaying the highest rate. Emiliania, Synechococcus, and Chaetoceros did not show significant iodate accumulation. Iodide is the preferred chemical species of iodine for uptake under nitrate‐replete conditions: iodide accumulation rates ranged from 3 to 90 times larger than those for iodate for the species studied. If the iodate accumulated is tightly coupled to its reduction to iodide, the accumulation rates suggest that phytoplankton‐mediated iodate reduction is not environmentally significant. If diatoms can reduce iodate to iodide, their overall contribution to surface‐water iodide, while living, would be further reduced because of their ability to reassimilate released iodide.