Iodine balance, growth and biochemical composition of three marine microalgae cultured under various inorganic iodine concentrations

Abstract

Marine phytoplankton have been found to be involved in the biogeochemical cycle of iodine in the ocean. However, the role of iodine in the metabolism of marine microalgae has hardly been studied and whether iodine is an element required for growth is not known. In this study, three species of marine microalgae were grown in batch cultures at various concentrations of iodide (I−) or iodate (IO3 −) in order to evaluate their effect on growth rate and biochemical composition. Changes in inorganic iodine concentrations in the culture medium were also determined. Growth rate was not affected by external inorganic iodine concentration in cultures of Dunaliella salina and Tisochrysis lutea. In cultures of Phaeodactylum tricornutum, growth was inhibited at 2500 µM IO3 −. No clear effects of I− or IO3 − were observed on biochemical composition, except for the particulate iodine content of the microalgae, which increased with increasing dissolved iodine concentration. Results of dissolved inorganic iodine measurements indicated that IO3 − was reduced to I− during the exponential growth phase of microalgae cultures, under both natural and higher IO3 − concentrations. Iodine balance calculations showed that the amount of disappeared IO3 − was higher than the amount of I− produced in several cultures and that this discrepancy could not be explained by the formation of particulate iodine. I incorporation rates into biomass were low compared to I− production rates. We conclude that although iodine is clearly taken up from the external medium and it is transformed by microalgae, it seems unlikely that it is an element required for growth or that it plays an essential role in microalgal metabolism.