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