Iodine and Halocarbon Response of Laminaria digitata to Oxidative Stress and Links to Atmospheric New Particle Production

Environmental Context. Various organic iodine compounds (including CH₃I, CH₂ClI, CH₂BrI, CH₂I₂) are present throughout the marine boundary layer as a result of their production from seaweeds, phytoplankton, and photolysis reactions occurring in seawater. In air, these compounds rapidly photolyse to give atomic I which subsequently reacts with ozone to form iodine oxide, potentially leading to perturbations of the tropospheric oxidative capacity and nucleation of atmospheric particles. Recent research has identified molecular iodine as an additional source of iodine atoms to coastal areas. Here we study the relative roles and controls of gaseous organic and molecular iodine release from the seaweed Laminaria digitata.

Abstract. Changes in the halocarbon, I₂ and particle production of the brown algal kelp Laminaria digitata as a response to different chemical stresses have been investigated. Oxidative stress (caused by either exogenous hydrogen peroxide, gaseous ozone or a solution of oligoguluronates, known elicitors of oxidative stress) caused increased halocarbon and I₂ production by the seaweed. The maximum I₂ release was observed under exposure to O₃ (at several hundred parts per billion by volume (ppbv)), whereas oligoguluronates elicited the highest release of iodine-containing halocarbons including CH₂I₂. Significantly greater production of I₂, compared to CH₂I₂, was observed at atmospheric levels of ozone. Particle production was observed only when the Laminaria samples were exposed to ozone (up to 16 000 cm⁻³ s⁻¹ per gram fresh weight (FW) of seaweed with a ~2 min residence time and with a total I atom flux of 1.6 × 10⁸ cm⁻³ s⁻¹ g⁻¹ FW from photolysis of I₂); passing O₃-free air over the unstressed seaweed followed by secondary mixing with ozone did not result in any measurable particle formation. Our limited data indicate that ozone elicits abiotic production of I₂ from Laminaria and that there is a direct relationship between the amount of I₂ released and the number of particles formed. The results support the recent hypothesis that molecular iodine rather than volatile organic iodine (e.g. CH₂I₂) release from exposed seaweeds is the major source of coastal new particle production.