Using organic compounds to assess the dominant controls on seasonal iodine variability in the brown alga Ecklonia cava in the northwestern Pacific coast of central Japan

Abstract

Brown algal species are among the most efficient iodine accumulators and thus heavily influence the biogeochemical cycling of iodine in coastal environments. Previously, we reported seasonal variability in iodine concentrations in the brown alga Ecklonia cava collected from the Japanese coast over a 2-year period. In this study, we sought to identify the factors controlling this observed variability by investigating the relationships between the concentrations of iodine and major organic compounds (carbohydrates and fatty acids). The concentrations of total carbohydrates and individual constituent monosaccharides showed no significant correlations with iodine concentration, implying that carbohydrate concentration and composition are not the dominant controls on iodine concentration in E. cava. However, fucose concentrations were significantly correlated when data points of highest iodine concentrations were removed, potentially suggesting some influence of structural carbohydrate on iodine variability. Significant positive correlations with iodine concentration were only observed in odd-chain fatty acids, which are known to be bacterial biomarkers; in particular, the C17:1 fatty acid showed the highest correlation (r = 0.83). The concentrations of the remaining fatty acids, as well as total concentration of fatty acids, did not correlate significantly with iodine concentrations. Our results suggest that, in general, the concentration and composition of fatty acids do not directly influence iodine variability and that iodine concentrations in E. cava are predominantly controlled by bacteria living on its surface.