Emissions of CH3Br, organochlorines, and organoiodines from temperate macroalgae
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 (τatm<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.
Section snippets
Sampling and analysis methods
The macroalgae studied were situated on a rocky part of the coastline, approximately 200 m in length, at West Runton on the North Norfolk coast, UK (1°15 E, 52°56 N). The following species were noted to be present Ulva lactuca, Cladophora pellucida, Entermorpha compressa (green algae), Fucus vesiculosus, F. serratus (brown algae) and Corallina officinalis (red algae). Three types of experiments (described below) were conducted in order to determine the impact that these macroalgae had on both
Incubations
Fig. 1 shows the results of the incubation studies, taking CH3Br as an example. Time zero is the time of the first analysis: the samples had been in storage in the dark for approximately one hour prior to this during transport to the laboratory. On all occasions the concentration of CH3Br was higher in the incubation sample than in the seawater control. This initial increase was likely to have arisen from a combination of production during transport to the laboratory, incorporation of
Relationships between halocarbon production rates
The production rates of CH3Br determined for all the incubation experiments correlated most significantly with CH3Cl out of all the halocarbons measured. This suggests that these two compounds have a similar production mechanism or one is a source of the other via a separate reaction mechanism. The correlation between CH3Br and CH3I was much weaker , and may suggest that the production of CH3I is different to the mechanism of the two other methyl halides. However,
Conclusions
It has been demonstrated that a number of volatile halogenated organic compounds are produced by certain species of temperate macroalgae. A number of compounds were produced to such an extent that the air directly over a small macroalgae bed was elevated in concentration compared to the background air.
Even though no direct studies were conducted to determine the actual production mechanisms involved, some interesting observations were made. There was no evidence to suggest that production was
Acknowledgements
Funding for this project came from the Department of the Environment, Transport and the Regions (EPG/1/1/12), from the Chemical Manufacturers Association through a subcontract from the University of Virginia, and from the Natural Environment Research Council’s Thematic Programme ‘Atmospheric Chemistry Studies in the Oceanic Atmosphere’ (ACSOE). JMB thanks the NERC for a Research Studentship. This work is a contribution to the Reactive Chlorine Emissions Inventory (//blueskies.sprl.umich.edu/geia/rcei/index.html
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Present address: National Centre for Environmental Data and Surveillance, The Environmental Agency, Lower Bristol Road, Bath, BA2 9ES, UK.