Iodine and halocarbon response of Laminaria to oxidative stress and links to atmospheric new particle production

Changes in the halocarbon, I-2 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 I-2 production by the seaweed. The maximum I-2 release was observed under exposure to O-3 (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 I-2, 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 16000 cm(-3) s(-1) per gram fresh weight (FW) of seaweed with a similar to 2 min residence time and with a total I atom flux of 1.6 x 10(8) cm(-3) s(-1) g(-1) FW from photolysis of I-2); passing O-3-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 I-2 from Laminaria and that there is a direct relationship between the amount of I-2 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.