30.7 Experiments with Smelly (Feh!) Alkylated Sulfur
Compounds
We first conducted a series of experiments to expand the earlier work of Zinder and
Brock (1978). We examined the role of methylated sulfur compounds including
dimethylsulfide (DMS), methane thiol (MeSH), and dimethyldisulfide (DMDS) as
methanogenic substrates in a variety of sediments types taken from different locales.
These included soda lakes (Mono and Big Soda), as well as freshwater (Searsville
Lake), saltmarsh (Flax Pond), and estuarine (San Francisco Bay) sites (Kiene et al.
1986). All responded positively to amendments with these compounds, showing a
clear stimulation of methane production, although the quantity of added substance
was critical as some concentrations of these noxious materials proved inhibitory, and
this varied with the particular compound and with the various sediment types. A pure
culture of an obligately methylotrophic methanogen was isolated from San Francisco
Bay enrichment cultures which showed DMS-dependent growth and displayed
MeSH as a transient intermediate. The culture was formally named Methanolobus
taylorii (Oremland and Boone 1994).
In a follow-up series of experiments conducted along similar lines, we added a
number of ethylated and propylated reduced sulfur, amines, and alcohols to sediments from five different sites to determine if they had a capacity to elicit ethane
and/or propane formation (Oremland et al. 1988). The sites included Big Soda Lake
(littoral and pelagic regions), Mono Lake, Searsville Lake, San Francisco Bay, and
Hot Creek (a thermal spring site located near Mammoth, CA). All the sediment types
responded positively in terms of enhanced ethane production when amended with
either diethylsulfide (DES) or ethane thiol (ESH). As was previously noted with the
methylated sulfur analogues, a range of applied concentrations was tested because
some proved inhibitory. Figure 30.4 shows ethane formation by Big Soda Lake
sediment slurries incubated under N 2 (rather than H 2 ), where there was clear
stimulation by 1 mM ESH and 10 mM DES. There was also a notable endogenous
ethanogenesis. Final levels of methane formation were about 30 μmoles, which
yielded CH 4 /C 2 H 6 ratios of ~1000 for the ESH and DES amendments. It is notable
that we saw only small amounts of stimulated ethane production when sediments
were provided with ethylamine or ethanol, and we did not observe propane formation from propane thiol additions.
The methylotrophic methanogen Methanolobus taylorii was unable to grow on
DES in lieu of DMS. Washed cells suspensions, however, showed a clear formation
of methane and ethane when provided with DMS and DES, reaching a CH 4 /C 2 H 6
ratio of ~2000 by 11 days incubation. There was also a notable amount of ethene
production, which was also observed in some of the earlier experiments with ethylS-CoM (Oremland 1981). Collectively, these results implied that there are naturally
occurring ethylated reduced sulfur compounds that could serve as a basis for ethane
production by methylotrophic methanogens. However, as was concluded earlier,
these ethylated analogues were co-metabolized and did not support the growth of the
methanogens.
646
R. S. Oremland
Compounds
We first conducted a series of experiments to expand the earlier work of Zinder and
Brock (1978). We examined the role of methylated sulfur compounds including
dimethylsulfide (DMS), methane thiol (MeSH), and dimethyldisulfide (DMDS) as
methanogenic substrates in a variety of sediments types taken from different locales.
These included soda lakes (Mono and Big Soda), as well as freshwater (Searsville
Lake), saltmarsh (Flax Pond), and estuarine (San Francisco Bay) sites (Kiene et al.
1986). All responded positively to amendments with these compounds, showing a
clear stimulation of methane production, although the quantity of added substance
was critical as some concentrations of these noxious materials proved inhibitory, and
this varied with the particular compound and with the various sediment types. A pure
culture of an obligately methylotrophic methanogen was isolated from San Francisco
Bay enrichment cultures which showed DMS-dependent growth and displayed
MeSH as a transient intermediate. The culture was formally named Methanolobus
taylorii (Oremland and Boone 1994).
In a follow-up series of experiments conducted along similar lines, we added a
number of ethylated and propylated reduced sulfur, amines, and alcohols to sediments from five different sites to determine if they had a capacity to elicit ethane
and/or propane formation (Oremland et al. 1988). The sites included Big Soda Lake
(littoral and pelagic regions), Mono Lake, Searsville Lake, San Francisco Bay, and
Hot Creek (a thermal spring site located near Mammoth, CA). All the sediment types
responded positively in terms of enhanced ethane production when amended with
either diethylsulfide (DES) or ethane thiol (ESH). As was previously noted with the
methylated sulfur analogues, a range of applied concentrations was tested because
some proved inhibitory. Figure 30.4 shows ethane formation by Big Soda Lake
sediment slurries incubated under N 2 (rather than H 2 ), where there was clear
stimulation by 1 mM ESH and 10 mM DES. There was also a notable endogenous
ethanogenesis. Final levels of methane formation were about 30 μmoles, which
yielded CH 4 /C 2 H 6 ratios of ~1000 for the ESH and DES amendments. It is notable
that we saw only small amounts of stimulated ethane production when sediments
were provided with ethylamine or ethanol, and we did not observe propane formation from propane thiol additions.
The methylotrophic methanogen Methanolobus taylorii was unable to grow on
DES in lieu of DMS. Washed cells suspensions, however, showed a clear formation
of methane and ethane when provided with DMS and DES, reaching a CH 4 /C 2 H 6
ratio of ~2000 by 11 days incubation. There was also a notable amount of ethene
production, which was also observed in some of the earlier experiments with ethylS-CoM (Oremland 1981). Collectively, these results implied that there are naturally
occurring ethylated reduced sulfur compounds that could serve as a basis for ethane
production by methylotrophic methanogens. However, as was concluded earlier,
these ethylated analogues were co-metabolized and did not support the growth of the
methanogens.
646
R. S. Oremland