Mass spectrometry – based imaging techniques for iodine-127 and iodine-129 detection and localization in the brown alga Laminaria digitata

Highlights: • LDI- and DESI MS detect inorganic iodine species on Laminaria digitata surfaces. • Radioactive ¹²⁹I isotopes were detected in algae collected near La Hague, in the English Channel. • ¹²⁷I and ¹²⁹I showed a peripheral tissue localization in L. digitata. • Nano-SIMS suggested different extracellular distributions of ¹²⁷I and ¹²⁹I isotopes. • ¹²⁹I speciation in seawater is crucial to understand its transfer to seaweeds. ¹²⁹I is one of the main radioisotopes of iodine derived from the nuclear fuel cycle that can be found sustainably in the environment due to its long half-life. In coastal marine environment, brown macroalgae, such laminariales (or kelps), are known to naturally feature highest rates of iodine accumulation, and to be an important source of biogenic volatile iodinated compounds released to the atmosphere. These seaweeds are therefore likely to be significantly marked by but also potential vectors of radioactive iodine. In order to better understand the chemical and isotopic speciation of iodine in brown algal tissues, we combined mass spectrometry-based imaging approaches in natural samples of Laminaria digitata young sporophytes, collected at two different locations along the south coast of the English Channel (Roscoff and Goury). Laser desorption ionization (LDI) and desorption electrospray-ionization techniques (DESI), coupled with mass spectrometry, confirmed the predominance of inorganic I⁻ species on the surface of fresh algae, and a peripheral iodine localization when applied on micro-sections. Moreover, radioactive isotope ¹²⁹I was not detected on plantlet surface or in stipe sections of algal samples collected near Roscoff but was detected in L. digitata samples collected at Goury, near La Hague, where controlled liquid radioactive discharges from the ORANO La Hague reprocessing plant occur. At the subcellular scale, cryo-fixed micro-sections of algal blade samples from both sites were further analyzed by secondary ion mass spectrometry (nano-SIMS), leading to similar results. Even if the signal detected for ¹²⁹I was much weaker than for ¹²⁷I in samples from Goury, the chemical imaging revealed some differences in extracellular distribution between radioactive and stable iodine isotopes. Altogether LDI and nano-SIMS are complementary and powerful techniques for the detection and localization of iodine isotopes in algal samples, and for a better understanding of radioactive and stable iodine uptake mechanisms in the marine environment.