Iodine uptake in Laminariales involves extracellular, haloperoxidase-mediated oxidation of iodide

Sporophytes of Laminaria digitata (L.) Lamour. were assayed for their conte nt of accumulated iodine, which ranged from 0.4% of dry weight in adult pla nts up to 4.7% for young plantlets. Sporophyte tissue from Laminaria saccha rina (L.) Lamour. and L. digitata took up iodide according to Michaelis-Men ten kinetics. Hydrogen peroxide and various substances known to interfere w ith oxidative metabolism were shown to either inhibit or enhance the uptake of iodide, confirming that apoplastic oxidations play a key role in iodide uptake in Laminaria. Consistently, iodide uptake was triggered in L. sacch arina protoplasts by incubation in the presence of hydrogen peroxide. Simil arly, the uptake of iodide was enhanced in L. digitata gametophytes by addi tion of haloperoxidase, suggesting that this enzyme catalyses the oxidation of iodide by hydrogen peroxide and plays a key role in iodine uptake. Oxid ative stress resulted in a marked efflux of the intracellular iodine. In bo th influx and efflux experiments, a marked proportion (10-30%) of the trace r was not accounted for, indicating volatilisation of iodine. The mechanism and possible functions of the accumulation of iodine by kelps are discusse d.