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Alkyl sulphides and disulphides

H sP Carbon disulphide

1.3 Alkyl sulphides and disulphides

1.3.1 Microbial metabolism of alkyl sulphides

1.3.1.1 DMS and DMDS

The organisms that have been studied in detail that grow with DMS as sole carbon and energy source can be divided

into two groups: the thiobacilli and the hyphomicrobia.

1.3.1.1.1 Thiobacillus species

Several species of Thiobacillus have been isolated capable of using DMS, DMDS, and MT as energy sources for growth

(Kanagawa and Kelly, 1986; Smith and Kelly, 1988a, b ) . These autotrophic organisms completely mineralise DMS and DMDS

(Figure 1.3.1.1.1) to C02 and sulphate, deriving energy from

r * “ e*-§

CM,

AMimiitton tor biooynthew#

m nyphormcrooift

0%BH

MOI

[HI HCOOM

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Figure 1.3.1.1.1 The aetabolisa of DHS by Hvoboiicrobiui and Thiobacillus species.

the oxidation of the sulphide and the methyl groups. Carbon is assimilated via C02 and through the Calvin cycle (see section 7.2.1.2).

However, the first pure culture that was reported to be capable of oxidising DMS assimilated carbon via the RuBP pathway simultaneously operating with a variant of the

serine pathway (section 7.2.1.1). The presence of the serine pathway enzymes, however, did not allow methylotrophic

growth on C^ compounds such as MMA, methanol and formaldehyde (Siveia and Sundman, 1975; Siveia, 1980).

1.3.1.1 . 2 Hyphomicrobium species

Although the best studied hyphomicrobia were isolated on DHSO (de Bont et a l ., 1981; Suylen, 1988), they were capable of using DMS, DMDS and MT (Figure 1.3.1.1.1), bya similar metabolic route to Thiobacillus thioparus strains E6 and TK- m (Kanagawa and Kelly, 1986; Smith and Kelly, 1988a, b; Kelly, 1988). However, the hyphomicrobia are capable of using the methylated sulphides as sole carbon and energy source, assimilating carbon as formaldehyde via the serine pathway. They are incapable of autotrophic growth and do not possess a Calvin cycle. Although the mechanism of the

oxidation of sulphide is unknown in the hyphomicrobia, these organisms can obtain energy from the oxidation.

1.3.1.1.3 Other organisms

Recently, Visscher and van Gemerden (1991) examined a purple sulphur bacterium (Thiocapsa roseopersicina) and described

it as capable of growth on DMS. These slightly misleading description refers to the organism's ability to use OHS as an electron donor for co2 fixation during otherwise

conventional autotrophic growth. The organism oxidised DMS to DMSO stoichiometrically, and this may have some relevance in marine environments rich in DMS. Similar oxidations have been described in impure cultures of phototrophic purple sulphur bacteria by Zeyer et al (1987).

1.3.1.2 Diethyl sulphide (DES) and diethyl disulphide (DEDS)

The higher alkyl sulphides have not been recorded as present in the atmosphere, but are conceivably present at low

concentrations. They are normally associated withodours such as garlic (Sparins et a l , 1988), which contains dipropyl sulphide, and onions (Kjaer, 1977), which contain

propylmethyl disulphide.

The chief interest in the microbiology of DES and DEDS lies in relation to the comparative metabolism of these compounds with regard to DMS and DMDS. Gram negative bacteria,

isolated by C. Harfoot (Kelly and Smith, 1990), have been isolated from diverse southern hemisphere locations, and are capable of growth on DMS, DMDS and DES. These cultures also oxidised longer chain alkyl sulphides. Further work on these isolates is described in chapters 2,3 and 5.

1.3.2 The role of DMS in terrestrial/marine sulphur exchange

When considering a system of such magnitude and complexity

as the sulphur cycle, it is not surprising that the apparent importance of various compounds changes according to the latest research. The changeable nature of the ideas of

sulphur cycling are compounded by the necessity of examining relatively small parts of the cycle, influenced by many factors, and trying to extrapolate to global proportions. Both these problems led to the adoption of H 2S as the most important gaseous sulphur compound, linking terrestrial and marine environments (Rodhe and Isaksen, 1980). This was mainly because of its ease of detection and abundance in certain areas such as swamps and salt marshes. H 2S appears herebecause of the reduction of sulphate. Data from these environments led to an estimate of annual global production of H2S of 142 Tg (Robinson and Robbins, 1970). This

maximised global estimate was guestioned by Rasmussen

(1974), who postulated that although the cycling of sulphate sulphur as H 2S is important in anaerobic environments, and at the interface of these environments with soil and water, H2S is primarily a factor in closed cycles. On this basis, it only contributes 5-58 Tg S year- 1 to the overall

atmospheric cycle. The turnover of sulphur through H 2S is still thought to exceed the release, especially if the gas has to pass through biologically active strata above its anaerobic production site.

Improved chromatographic methods indicated alternative candidtaes for the most important gaseous atmospheric sulphur compounds, and confirmed that H 2S was not as abundant as was first thought. Attempts to detect

concentrations of H2S consistent with the model failed. Instead, gases such as CS2 , COS and methylated sulphides were present. The sources of these compounds were at first not clear, although Challenger (1951) reported that many living systems produced DMS, including marine algae. This led Lovelock et a l . (1972) to propose that DMS had the role previously given to H2S.

1.3.3 The origins of DMS

1.3.3.1 Anthropogenic sources

Although biogenic and anthropogenic input of organosulphur gases into the atmosphere are thought to be approximately equal, it is difficult to identify specific industries or activities that result in the release of DMS into the atmosphere. The wood pulping industry, oil refineries

(Kanagawa and Mikami, 1989) and mushroom cultivation (Derikx et a l, 1990) have been mentioned in connection with DMS, but no quantitative data exists.

1.3.3.2 Terrestrial biogenic sources

The total production of DMS from terrestrial sources is estimated at 4.5 Tg S year- 1 . DMS is produced by Spartina and probably by microbial action in salt marsh and marine sediments (Kelly and Smith, 1990; Taylor and Kiene, 1989).

Kiene and colleagues have extensively described the

behaviour and turnover of DMS in salt marsh sediments and other coastal environments (Kiene and Visscher, 1987; Taylor and Kiene, 1989; Kiene and Bates, 1990). Some DMSP,

presumably of vegetable origin, is converted to DMS

anaerobically and DMSO reduced biologically also to produce DMS. MT was also found to undergo methylation to form DMS. Thus in anaerobic muds, a complex interaction between

methylated sulphides arises, presumably due to the presence of bacteria (Figure 1.3.3.2).