Abstract
This study explores key features of bromine and iodine metabolism in the filamentous brown alga and genomics model Ectocarpus siliculosus. Both elements are accumulated in Ectocarpus, albeit at much lower concentration factors (2-3 orders of magnitude for iodine, and < 1 order of magnitude for bromine) than e.g. in the kelp Laminaria digitata. Iodide competitively reduces the accumulation of bromide. Both iodide and bromide are accumulated in the cell wall (apoplast) of Ectocarpus, with minor amounts of bromine also detectable in the cytosol. Ectocarpus emits a range of volatile halogenated compounds, the most prominent of which by far is methyl iodide. Interestingly, biosynthesis of this compound cannot be accounted for by vanadium haloperoxidase since the latter have not been found to catalyze direct halogenation of an unactivated methyl group or hydrocarbon so a methyl halide transferase-type production mechanism is proposed.
Highlights
Two centuries ago, the elements bromine and iodine were discovered in sea water and seaweed (Laminaria and Fucus) ashes, respectively [1, 2]
This includes the function of iodide as extracellular antioxidant protecting the surface of Laminaria digitata (Hudson) Lamouroux against oxidative stress [6]
Based on XFS and energy-dispersive X-ray analysis (EDX), most iodine was concentrated in physode-like vesicles (Fig. 1), even though some was detectable in the cytosol and/or apoplast (Fig. 2)
Summary
The elements bromine and iodine were discovered in sea water and seaweed (Laminaria and Fucus) ashes, respectively [1, 2] Due to their unique evolutionary history and phylogenetic distance from other important eukaryotic lineages [3, 4], brown algae present some remarkable chemical and physiological adaptations which are reflected at the genome level [5], making them fascinating experimental models, for phycologists, but for a community of interdisciplinary researchers. This includes the function of iodide as extracellular antioxidant protecting the surface of Laminaria digitata (Hudson) Lamouroux against oxidative stress [6]. The role of bromine is less well investigated, but a recent study [10] highlights that Laminaria accumulates bromide, which complements iodide as an antioxidant especially for the detoxification of superoxide, but with an overall more diverse role
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