Emissions of CH₃Br, organochlorines, and organoiodines from temperate macroalgae

Abstract

The production rates of a range of low molecular weight halogenated organics have been determined in cultures of five temperate species of macroalgae collected from the north coast of Norfolk, England. Compounds studied included CH₃Br, the chlorinated organics CH₃Cl, CH₂Cl₂ and CHCl₃, and the iodinated organics CH₃I, C₂H₅I, and CH₂ClI. Measurements of a wider range of halocarbon concentrations in an isolated rockpool and in air over the seaweed bed were also conducted to evaluate the local impact of the seaweeds on halocarbon concentrations in the natural environment. Estimates for the global emissions of some of the key halogenated compounds from macroalgae have been derived. In general macrophytes appear not to be globally significant producers of the particular halocarbons studied. In coastal regions, however, the impact on local atmospheric composition and chemistry could be greater.

Introduction

Although there have been a number of reported studies of the ubiquitous production of polyhalogenated hydrocarbons, notably bromine-containing species, by marine macrophytes (e.g. Gschwend et al., 1985; Manley et al., 1992), there have been rather fewer studies of monohalomethane, organochlorine and organoiodine production. This has been at least partly due to analytical constraints arising from low production rates, high detection limits, and chromatographic separation difficulties with older instrumentation and columns. There has, however, been increasing interest in the possible sources of some of these compounds, and recent attempts to balance their global budgets.

Atmospheric CH₃Br is believed to be the main source of stratospheric bromine (Schauffler et al., 1993). Bromine is estimated to be 40–100 times more effective than chlorine in depleting stratospheric ozone (Penkett et al., 1995). Methyl chloride (CH₃Cl) is the most abundant chlorinated organic in the atmosphere and the major natural carrier of chlorine to the stratosphere. It, and other relatively short-lived (τ_atm<c 1.5 y) chlorinated organics such as chloroform and dichloromethane (CHCl₃ and CH₂Cl₂), are furthermore considered to be possible sources of reactive chlorine in the troposphere with implications for the tropospheric chemistry and lifetimes of a number of trace gases (Keene et al., 1996). There are also concerns regarding the effects of these compounds, and of their decomposition products, such as haloacetic acids, on the biosphere (Frank, 1991). Organoiodine compounds may also play a part in tropospheric chemistry, as evidenced by observations of IO radicals at coastal locations (Carpenter et al., 1999). They are also important carriers in the global iodine cycle, with significance to human health (e.g. iodine deficiency diseases).

The origins of many of these compounds are rather poorly known. Several anthropogenic and natural sources of CH₃Br have been identified but there are still large uncertainties concerning the actual amount that these sources contribute towards the total atmospheric burden. Yvon-Lewis and Butler (1997) estimated that the sinks of CH₃Br are larger than the sources by 69 kt yr⁻¹. The atmospheric burden, however, has remained fairly constant over recent years (Khalil et al., 1993), suggesting that either the currently identified sources have been underestimated, that there are unidentified sources, or that the sinks have been overestimated. Furthermore, recent findings suggest that the estimated emissions of CH₃Br from gasoline are now lower than previously thought (Baker et al., 1998), therefore increasing the discrepancy between sources and sinks. A number of potential CH₃Br producers, which have not been included in the estimate of Yvon-Lewis and Butler (1997) include fungi (Harper, 1985), terrestrial plants (Saini et al., 1995), coal burning (Baker, 1998) and seaweeds.

Similarly, recent work by the Reactive Chlorine Emissions Inventory group (Keene et al., 1999) has shown that known sources of CH₃Cl can only account for half of the modelled sinks. About one third of the known source is oceanic. Approximately 90% of tropospheric CHCl₃ was ascribed to ocean and soil sources, although with large uncertainty in both absolute amounts and the division between sources. Yet more poorly constrained was the budget for CH₂Cl₂, with roughly 25% attributed to marine sources. For all of the compounds measured there is uncertainty regarding the precise nature of the oceanic source (or sources), and particularly what biogenic processes and organisms are involved (e.g. Scarrat and Moore, 1998). Seaweeds are capable of producing significant concentrations of several halocarbons (for example: Lovelock, 1975; Gschwend et al., 1985; Nightingale et al., 1995; Laturnus, 1995). The production mechanisms are not fully understood but it has been suggested that the formation of halogenated methanes by macroalgae is enzymic halogenation of organic compounds (Moore, 1978). It has also been proposed that the seaweeds themselves do not produce the halocarbons, but microbes present on the seaweed surface are responsible Gschwend and MacFarlane, 1986, Manley and Dastoor, 1987, Manley and Dastoor, 1988. The monohalomethanes (methyl halides, i.e. CH₃X), however, are probably not produced by the chloro- or bromoperoxidase-type reactions that lead to the formation of the polyhalomethanes (Urhahn and Ballschmiter, 1998). Wuosmaa and Hager (1990) have suggested an enzymatic synthesis through an S-adenosyl methionine transfer mechanism, whilst Manley and Dastoor (1988) assumed the formation of monohalogenated methane occurred inside the algae without support by haloperoxidase. White (1982) proposed a reaction between dimethylsulfoniopropionate (DMSP) and halides present in seawater.