Halogenated methanes produced in the oceans are important as carriers of chlorine, bromine, and iodine into the atmosphere. There they play roles in the regulation of ozone in the stratosphere and perhaps in the Arctic troposphere at polar sunrise. While the mechanisms for the production of some polyhalogenated compounds by marine macrophytes have previously been substantially elucidated, the same has not been true in the case of marine phytoplankton. We describe laboratory experiments on the production of various brominated and iodinated compounds in cultures of marine diatoms, obtained from the Provasoli‐Guillard Center for Culture of Marine Phytoplankton collections (Bigelow Laboratory for Ocean Sciences, Maine, USA; CCMP). Species examined included Nitzschia sp. (CCMP 580), Nitzschia arctica, Porosira glacialis, and two Navicula sp. (CCMP 545 and 546). A suite of brominated compounds, notably bromoform and dibromomethane, is produced by the Nitzschia and Porosira species. Nitzschia sp.(CCMP 580) was grown in sufficient quantities to allow the identification of a bromoperoxidase enzyme, which is assumed to be responsible not only for the CHBr3 and CH2Br2 production but also for CH2I2 which was measured in those cultures. Chloroiodomethane was produced, either directly by the algae or by a photochemical reaction of CH2I2. One Navicula species (CCMP 545), found to produce CH2I2 and CH2ClI, was shown to possess an iodoperoxidase. Bromoform and dibromomethane were not detected in cultures of this species. Other compounds produced in certain of these non axenic cultures included methyl and ethyl iodide, and bromoiodomethane.

Natural volatile halocarbons are important as carriers of reactive halogens to the troposphere and, in the case of the more stable compounds, to the stratosphere. Bromoform (CHBr3) has been of particular interest as a potential source of bromine which might account for sudden ozone depletion events in the Arctic boundary layer. The oceans have been shown to be major contributors of volatile halocarbons to the atmosphere, but the sources of halocarbons within them have been unknown except for macrophytic algae which are normally confined to the coastal zone. Here we report experiments that demonstrate that certain unialgal cultures of marine phytoplankton produce a suite of halocarbons (CHBr3, CHBr2Cl, CH2Br2). The production rate of each of these halocarbons is dependent on both species and growth stage. Chloroiodomethane which also appeared in the cultures could be attributable to photochemical production from a precursor (CH2I2). Great caution should be shown in extrapolating the rates estimated from these controlled experiments to the marine environment.

A suite of naturally produced volatile halomethanes which are potential sources of gaseous halogens to the atmosphere have been measured in the water column of the NW Atlantic Ocean. Bromoform, chlorodibromomethane, dichlorobromomethane, dibromomethane, and methyl iodide all showed higher concentrations in coastal waters than in the pelagic zone. Such a distribution is consistent with known sources of these compounds in macro-algae. Chloroiodomethane alone showed elevated concentrations in surface open ocean waters, and its distribution is interpreted as indicating production by phytoplankton, either directly or through an intermediate such as di-iodomethane. The latter explanation is supported by post-cruise estimates of the di-iodomethane distribution. Whereas most of the compounds measured in this study have higher concentrations in the upper water column, both dibromochloromethane and bromodichloromethane showed increases with depth which would be consistent with a slow reaction of surface-derived bromoform with chloride. INTRODUCTION The ocean is the main global reservoir for the halogens, chlorine, bromine, and iodine where the elements exist in solution chiefly as halide ions, or in the case of iodine also as iodate (IO3'). Being relatively Copyright 1993 by the American Geophysical Union. Paper number 92GB02653. 0886-6236/93/92GB-02653510.00 unreactive and yet highly soluble, these elements have long residence times in the ocean. A very small proportion of each exists as organic compounds, the more volatile of which are carriers of gaseous halogens into the atmosphere. Although the flux of chlorine and bromine from ocean to atmosphere is dominated by the particulate halides in sea salt, this material is readily rained out, and it is the smaller fluxes of gaseous compounds which frequently mix more extensively into the atmosphere. The ability to mix to higher altitudes is dependent on the sensitivity of the individual compounds to photolysis and reaction with hydroxyl radicals [Khalil et al., 1983], with the result that the less reactive compounds such as methyl chloride can penetrate even to the stratosphere [Wofsy et al., 1975]. Organic compounds of iodine tend to be the most readily photolyzed (e.g., CH 3 I with a lifetime of circa 5 days [Zafiriou, 1974]), and so mix only into the lower troposphere. Bromomethanes, having atmospheric residence times ranging from several weeks for bromoform [Cicerone et al., 1988] to circa 1.5 years for methyl bromide [Prather and Watson, 1990], vary in the extent of their vertical penetration into the atmosphere. Methyl iodide has been suggested [Lovelock et al., 1973] to be the main carrier of iodine from the ocean to the atmosphere, and therefore responsible for the high iodide to chloride ratio in rain water [e.g., Dean, 1963; Duce et al., 1963]. Organo-bromine compounds have received attention as sources of bromine atoms to the atmosphere where it is speculated they might account for the destruction of ozone that has been observed in several studies of the Arctic bo...

Measurements have been made of chloroiodomethane (CH2ClI) in seawater samples collected in the N.W. Atlantic. Elevated concentrations (> 3 ng/ L) were found in surface waters at a number of locations in the pelagic zone. Background surface concentrations in the pelagic zone were 0.1–0.4 ng/L. Coastal/shelf regions averaged 0.1 ng/L off Labrador and Greenland, and 0.8 ng/L on the Grand Banks. We suggest that its distribution may be due to a phytoplankton source, either direct or indirect (i.e. via an intermediate compound). Using measurements of methyl iodide (CH3I) made on the same samples, we make a first order estimate of the comparative fluxes of the two compounds from ocean to atmosphere. This indicates that the iodine carried by CH2ClI may be comparable with that carried as CH3I. Refinement of the flux estimates requires measurements of atmospheric concentrations of CH2ClI, and determination of its Henry's Law constant.

This study presents shipboard measurements of the loss rate constants of methyl bromide and methyl chloride in surface seawater in the Southern Ocean, using a 13C stable isotope incubation technique. The measurements were made during October–December, 2001, on a cruise track extending from Hobart, Tasmania to Buchanan Bay (Mertz Glacier) at the coast of Antarctica (46–67°S, 138–145°E). Significant loss rates were measured for both compounds, even in very cold waters where chemical loss rates were negligible. These observations are attributed to biological uptake, and they explain the tendency for high latitude waters to be undersaturated with respect to atmospheric methyl bromide and methyl chloride. These observations are the first open ocean measurements demonstrating the biological degradation of methyl chloride.