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

Abstract

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. © 2009, by the American Society of Limnology and Oceanography, Inc.