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Previously, we reported seasonal variation in iodine contents in the seagrass Zostera marina. Herein, we sought the factors controlling this variation, and investigated relationships between iodine and carbohydrate contents, using extracts and residues of seagrass samples extracted with 0.1 N HCl. In plants, carbohydrates in HClextracted and residual fractions are considered to represent storage and structural carbohydrates, respectively. On average, 44% and 56% of total iodine in samples was contained in the HCl-extracted and residual fractions, respectively. Both HCl-extracted and residual iodine contents showed seasonal trends similar to that of total iodine, being high in winter--spring and low in summer. Total and HCl-extracted carbohydrate contents showed reverse seasonal trends from those of iodine, whereas residual carbohydrate contents had comparable values throughout the sampling period. In the total and HClextracted fractions, negative correlations between iodine and carbohydrate contents were confirmed, suggesting that carbohydrates do not play important roles in iodine accumulation. Although most monosaccharide contents were not correlated with iodine contents in these two fractions, residual galactose content was positively correlated with residual iodine. We accordingly suggest that one or more specific structural carbohydrate constituents may potentially function as an iodine store in Z. marina. [ABSTRACT FROM AUTHOR]