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 cell21 d21. 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 cell21 min21 from the free space and a subsequent cellular release rate of 0.13 amol cell21 min21, which corresponds to a daily cellular release rate of 0.19 fmol cell21 d21. Accumulation of iodate ranged from 0 to 19 amol cell21 d21, 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. Inorganic iodine exists in disequilibrium in surface seawater as iodide and the thermodynamically favored form, iodate (Wong 1991). The relatively high concentration of iodide in temperate coastal waters and subtropical– tropical waters is thought to be related to biological productivity (Wong 2001; Wong et al. 2002; Chance et al. 2007). Both chemical species are potentially available for uptake by marine phytoplankton. The first measurements of iodide and iodate assimilation used the diatom Navicula sp., and it was shown to use both forms, though iodide was the preferred form (Sugawara and Terada 1967). There are no other reports of iodide uptake by phytoplankton. Iodate uptake, however, has been reported for several phytoplankton, and it has been suggested that they may be significant reducers of iodate to iodide (Moisan et al. 1994). Although the role of phytoplankton in iodate reduction has been questioned, especially in colder waters (Truesdale et al. 2003), the facilitated reduction of iodate to iodide by phytoplankton cultures has been demonstrated (Wong et al. 2002; Chance et al. 2007). Presently, a mechanism by which phytoplankton can reduce iodate to iodide at a rate that is environmentally significant has not been adequately described (Truesdale et al. 2003). Dissimilatory (extracellular) iodate reduction by anaerobic marine bacteria isolated from sediments has been reported (Councell et al. 1997; Amachi et al. 2007). A dissimilatory mechanism for iodate reduction to iodide has not yet been reported for phytoplankton. If phytoplankton have a dissimilatory mechanism, it would probably be different from that of an anaerobe, because phytoplankton are oxygenic photoautotrophs. Thus, a more plausible first step in phytoplankton-mediated iodate reduction would be its uptake into the cell prior to reduction, possibly by nitrate reductase (Tsunogai and Sase 1969; Hung et al. 2005). Iodide could then be released from the cell by efflux or from cell lysis. In this paper, we report both iodide and iodate accumulation rates for several phytoplankton species in a nitrate-replete culture, using the radioisotope I-125 to directly measure cellular assimilation. This information is needed in assessing the importance of phytoplankton in the biogeochemical cycling of iodine in marine waters.