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The natural production of Volatile organochlorines in the oceans
08/2008 Two main approaches have been applied in the study of marine production of chlorinated and, more generally, halogenated compounds. One has been identifying and studying specific producers, of which seaweeds are so far the best understood. The second has been measuring the concentrations of volatile chlorine-containing organic compounds both in the surface waters of the ocean and in the overlying atmosphere. From this type of study it is possible to determine whether there are fluxes of the compounds between the ocean and atmosphere, and ultimately whether this must imply a production mechanism in the ocean. For several decades studies have been proceeding on the production of organohalogen compounds by seaweeds, and it has been shown that at least 400 such compounds are produced, a significant proportion of which contain chlorine. In the case of the more volatile compounds, this production can result in a flux into the atmosphere: examples are methyl chloride and chloroform. A relatively abundant product of certain species of seaweed is bromoform, which is expected to react with chloride ion in seawater to yield mixed chlorine and bromine-containing methanes such as chlorodibromomethane, and ultimately chloroform. Similarly macroalgal production of dibromomethane is likely to result in production of chlorobromomethane and dichloromethane. While there is no good understanding of why seaweeds produce these compounds, progress has been made on the biochemical mechanisms of their production. Seaweeds have a relatively restricted distribution, occurring only in coastal areas. Consequently it has been of interest to determine whether the more abundant microalgae distributed throughout all surface ocean waters are also sources of halogenated compounds. Research was directed first at algae occurring in high latitude seas and beneath sea-ice, and it was shown that some species produced compounds like those mentioned in connection with seaweeds: bromoform and dibromomethane. Smaller amounts of mixed chlorobromomethanes were identified and also chloroiodomethane. This last compound has also been shown to be produced in a light-catalyzed reaction of diiodomethane in seawater, with its precursor having a source in certain micro- and macroalgae. More recently studies have been made of organohalogen production by warm water species of marine microalgae, though the number of organisms studied has been small. Furthermore, even fewer species are actually characteristic of the wide expanse of the open oceans, because such organisms have been successfully cultured in only relatively few cases. One species of red microalga has been reported to produce chloroform (and methyl iodide). The same organism, in a separate study, was reported to release tri- and tetrachloroethylene, but subsequent work has not supported that finding. The production of methyl chloride, bromide and iodide by both micro- and macro-algae has also been a subject of study, in part because it is apparent that methyl chloride must be added to the atmosphere at a rate of four million tonnes annually in order to account for the observed atmospheric concentration. The significance here of the methyl bromide and iodide production is that it provides an additional source of methyl chloride through their reactions with chloride ion in seawater. While early work pointed to the ocean as the primary source of methyl chloride, more recent work does not support this idea. Rather it appears that the ocean may supply 10-20% of this source, with other significant contributions coming from biomass burning and perhaps terrestrial sources, in particular wood-rotting fungi. It appears that the combination of all these known sources falls short of the required supply. The rather uniform global distribution of this gas in the atmosphere provides us with good evidence that the unknown source exists in both hemispheres and, as such, is not related to industrial activity. Work is currently directed at identifying this source. The normal method of estimating oceanic sources of trace gases to the atmosphere involves measuring the degree of saturation of the gas in the surface ocean, with measurements made over as wide an area as possible. In principle, the existence of outward fluxes from the ocean need not always be indicative of the particular compound being produced within the ocean. For example, a compound may pass from the atmosphere into the ocean at high latitudes where the waters are cold and be emitted from lower latitude warm waters. Recent measurements of trichloroethylene and tetrachloroethylene in N. Atlantic waters support the idea that the summertime efflux of these compounds from that area into the atmosphere involves the release of material that the ocean absorbed from the atmosphere in winter. Caution should therefore be exercised in interpreting gas supersaturations in surface ocean waters, particularly when data are available from only a very limited area as is usually the case. Reference has been made to reaction of some halogenated compounds in seawater resulting in interconversions, such as methyl iodide being converted into methyl chloride. In addition hydrolysis will result in the loss of some halogenated compounds. Also there have been a number of observations of more rapid loss of certain organochlorine compounds under low oxygen conditions. Carbon tetrachloride appears to be converted to chloroform in the suboxic zone of the Black Sea, and the chloroform itself then appears to be reduced. Field studies have provided strong evidence that carbon tetrachloride is also being destroyed in warm, oxygenated waters. We have found that chlorinated and brominated methanes are absent in the reducing zones of Arctic marine sediments, though they are present in the upper oxygenated region, probably originating in sedimented algal material. |
