Microchemical imaging of iodine distribution in the brown alga Laminaria digitata suggests a new mechanism for its accumulation.

Brown algal kelp species are the most efficient iodine accumulators among all living systems, with an average content of 1.0% of dry weight in Laminaria digitata. The iodine distributions in stipe and blade sections from L. digitata were investigated at tissue and subcellular levels. The quantitative tissue mapping of iodine and other trace elements (Cl, K, Ca, Fe, Zn, As and Br) was provided by the proton microprobe with spatial resolutions down to 2 mum. Chemical imaging at a subcellular resolution (below 100 nm) was performed using the secondary ion mass spectrometry microprobe. Sets of samples were prepared by both chemical fixation and cryofixation procedures. The latter prevented the diffusion and the leaching of labile inorganic iodine species, which were estimated at around 95% of the total content by neutron activation analysis. The distribution of iodine clearly shows a huge, decreasing gradient from the meristoderm to the medulla. The contents of iodine reach very high levels in the more external cell layers, up to 191 +/- 5 mg g(-1) of dry weight in stipe sections. The peripheral tissue is consequently the main storage compartment of iodine. At the subcellular level, iodine is mainly stored in the apoplasm and not in an intracellular compartment as previously proposed. This unexpected distribution may provide an abundant and accessible source of labile iodine species which can be easily remobilized for potential chemical defense and antioxidative activities. According to these imaging data, we proposed new hypotheses for the mechanism of iodine storage in L. digitata tissues.