Iodine in Seaweeds - Physiological Significance and Implications for Climate and Health
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Iodine-rich macroalgae inhabit the subtidal on coasts in temperate regions. The algae-driven release of iodine into the environment has major implications for the biogeochemical iodine cycle a ffecting atmospheric chemistry and probably human health. This thesis has focused on interactions between Laminaria digitata and its environment, with emphasis on impacts of emersion, irradiance intensity, temperature and salinity on the release of iodine. The research also included an examination of the physiological role of iodine accumulation. L. digitata was further investigated as potential antiproliferative agent in human mammary carcinoma studies in vitro. Sporophytes of L. digitata were strong emitters of molecular iodine (I2) during emersion, but intra-thallus variations in I2 emission rates were observed. During submersion iodide (I-) was released into seawater from meristematic areas of L. digitata. Increased temperatures and dying resulted in elevated I2 emission rates, whereby death and decomposition led to high quantities of I- released into the seawater. These results, in addition to local field biomass assessments, suggest that both living and dead biomass of L. digitata are fundamental elements in a coastal iodine cycle. The external salinity regime controlled the release of I- into seawater. Lowest I- release rates from L. digitata were detected at highest salinities. When present in seawater, I- was effi ciently incorporated by L. digitata and prevented the to-date well-established mannitol accumulation of this species when exposed to high salinities. This suggests that I- acts as an inorganic osmolyte in L. digitata. Exudates and extracts from L. digitata potently induced apoptosis in human mammary carcinoma cells, but highest antiproliferative activities were observed from low-iodine containing exudates. In some cases, high-iodine containing extracts enhanced proliferation of human mammary carcinoma cells in vitro. The results indicate that anti-carcinoma activities of L. digitata, where detected, cannot solely be attributed to iodine levels, but may be associated with the large variety of chemical constituents present in this species. The results highlight the important role of L. digitata in coastal iodine fluxes with implications for climate research, and potential impacts for human health.