The production of volatile iodocarbons by biogenic marine aggregates

We present the first reported measurements of volatile iodocarbon production by biogenic marine aggregates. Iodomethane (CH3I), iodoethane (C2H5I), 2iodopropane (CH3CHICH3), and 1-iodopropane (CH3CH2CH2I) concentrations were determined in incubations of aggregates formed by concentrating the .53 mm fraction of the plankton during a field campaign in the Celtic Sea. All four iodocarbons increased significantly in concentration in the aggregate incubations relative to filtered seawater controls. Maximum production rates ranged from 0.01 pmol L21 h21 for CH3CHICH3 to 0.31 pmol L21 h21 for C2H5I. Accompanying pheopigment and bacterial heterotrophic production suggest that the processes taking place on the aggregates studied were a good representation of those known to occur on natural marine particles. We also report iodocarbon production rates observed in natural marine aggregates, including a diatom mucilage collected in the Celtic Sea and phytodetritus sampled from Kongsfjord in the Arctic. Detrital particles could be hotspots of iodocarbon production in the marine environment. Over 50 years ago, the enrichment of iodine relative to chlorine in air masses of marine origin was recognized (Rankama and Sahama 1949) and this led to conjecture that sea-to-air transfer is an important pathway in the global cycling of this element. Subsequent research suggested that the sea-to-air flux of iodine might be mediated by molecular iodine (I2) formed at the sea surface (Garland and Curtis 1981). However, the discovery of iodomethane (CH3I) in seawater from a kelp bed (Lovelock 1975) brought about a change in focus by introducing another potential vector for sea-to-air iodine transfer. It is now known that a whole suite of iodocarbons in addition to CH3I are produced naturally in seawater, including iodoethane (C2H5I), the iodopropanes (C3H7I), diiodomethane (CH2I2), and mixed chloroand bromoiodomethanes (CH2ClI, CH2BrI) (Carpenter et al. 2000). All of these compounds are volatile and so have the potential to evade to the atmosphere and contribute to the sea-to-air flux of iodine. In parallel with increasing knowledge of the range of volatile iodocarbons in seawater has been growing awareness of the environmental consequences of sea–air iodine flux. Balancing the global iodine cycle is dependent on rainout and dry deposition from the air to land. In addition, the flux of iodocarbons from the oceans to the atmosphere has significant implications for human health and the prevalence of iodine deficiency disorders such as goiter and cretinism. The driving force for most current iodocarbon research is the influence that atmospheric iodine has on air quality and climate. Iodocarbons are readily photolyzed in the atmosphere and release reactive iodine, which reacts with ozone to form the iodine oxides (IO and OIO); these, in turn, can react with themselves, NO2, or HO2 (Allan et al. 2000). As a consequence, atmospheric iodine can influence (1) ozone concentrations in the marine boundary layer (McFiggans et al. 2000), (2) the capacity of the atmosphere to process emissions of greenhouse gases such Notes 867