Perchlorate in seawater: bioconcentration of iodide and perchlorate by various seaweed species

There has been no reliable published data on the presence of perchlorate in seawater. Seaweeds are among the most important plant life in the ocean and are good sources of iodine and have been widely used as food and nutritional supplement. Perchlorate is known to inhibit the transport of iodide by the sodium iodide symporter (NIS), present e.g., in the thyroid and mammary glands. With perchlorate being increasingly detected in drinking water, milk and various other foods, increasing the iodide intake through inexpensive natural supplements may be an attractive solution for maintaining iodine assimilation. We report here measurable concentrations of perchlorate in several samples of seawater (detectable in about half the samples analyzed). We also report the iodide and perchlorate concentrations of 11 different species of seaweed and the corresponding bioconcentration factors (BCF) for perchlorate and iodide, relative to the seawater from which they were harvested. All seaweed samples came from the same region, off the coast of Northeastern Maine. Concentrations of iodide and perchlorate in four seawater samples collected from the region near harvest time were 30 ± 11 and 0.16 ± 0.084 μg l⁻¹, respectively. Concentrations of both iodide and perchlorate varied over a wide range for different seaweed species; iodide ranging from 16 to 3134 mg kg⁻¹ and perchlorate from 0.077 to 3.2 mg kg⁻¹. The Laminaria species had the highest iodide concentration; Laminaria digitata is the seaweed species most commonly used in the kelp tablets sold in health food stores. Our sample of L. digitata contained 3134 ± 15 mg iodide/kg dry weight. The BCF varied widely for different species, with Laminaria species concentrating iodide preferentially over perchlorate. The iodide BCF (BCF_i) to perchlorate BCF (BCF_p) quotient ranged from 0.66 to 53; L. digitata and L. saccarina having a BCF_i/BCF_p value of 45 and 53, respectively, far greater than a simple anion exchange process will allow. Although most seaweed samples contain some amount of perchlorate, the great majority contains iodide in so much higher amount that at least for the commonly used Laminaria species, the iodide/perchlorate ratio is greater than the square of the perchlorate to iodide selectivity factor reported for the mammalian NIS and should thus lead to net beneficial iodine nutrition even in a two-stage mother-infant scenario.