Fig. 4.

A model of iodine metabolism in Laminaria. Laminaria, when submerged and unstressed (a), accumulates iodide from seawater mediated by vanadium haloperoxidase (turquoise). In the tissues and as shown in this study, I⁻ is the accumulated form. When oxidative stress occurs (red), iodide is released to detoxify ROS in the apoplast at the thallus surface outside of the cell membrane, including both aqueous (e.g., H₂O₂) and gaseous (O₃) oxidants. ROS scavenging reactions in the aqueous phase such as halide-assisted disproportionation of H₂O₂ will result in the regeneration of iodide in a cyclic reaction sequence. (a) During oxidative stress at high tide (e.g., due to an oxidative burst caused by alginate-degrading bacteria in a biofilm), iodide is released into the surrounding seawater. (b) In contrast, aerosol particle bursts result mainly from molecular iodine released directly from Laminaria into the coastal atmosphere, as a consequence of the ozone-scavenging reactivity of iodide on kelp surfaces at low tide.