Synchrotron X-rays elucidate the intracellular storage of the iodide and bromide antioxidant system in the kelp Laminaria digitata.

The kelp Laminaria digitata uses iodide as a unique inorganic antioxidant to protect its surface and apoplastic space, with implications for atmospheric and marine chemistry as well as regional climatic processes. However, until now, significant open questions have remained regarding the cellular localization and processes involved. Here, using cutting-edge, synchrotron-based micro-X-ray fluorescence (µXRF) nanoprobe tomography with strontium (II) as a biomarker for algal cell walls, complemented by micro-X-ray absorption near-edge structure (μXANES) and scanning imaging, we unambiguously show that iodide is stored in intracellular vesicles of cortical cells. In contrast, bromide is mostly accumulated in the vacuoles of meristoderm cells. Upon oxidative stress, it is mobilized by a hitherto-uncharacterized anion transporter different from the well-known eukaryotic chloride channels. This study offers the first evidence of tissue localization for the formation of I2 or an organic iodine compound as a minor component in the cortical cell layer, as demonstrated using µXANES.