Role of methanogens and other bacteria in degradation of dimethyl sulfide and methanethiol in anoxic freshwater sediments

Abstract

The roles of several trophic groups of organisms (methanogens and sulfate- and nitrate-reducing bacteria) in the microbial degradation of methanethiol (MT) and dimethyl sulfide (DMS) were studied in freshwater sediments. The incubation of DMS- and MT-amended slurries revealed that methanogens are the dominant DMS and MT utilizers in sulfate-poor freshwater systems. In sediment slurries, which were depleted of sulfate, 75 micromol of DMS was stoichiometrically converted into 112 micromol of methane. The addition of methanol or MT to DMS-degrading slurries at concentrations similar to that of DMS reduced DMS degradation rates. This indicates that the methanogens in freshwater sediments, which degrade DMS, are also consumers of methanol and MT. To verify whether a competition between sulfate-reducing and methanogenic bacteria for DMS or MT takes place in sulfate-rich freshwater systems, the effects of sulfate and inhibitors, like bromoethanesulfonic acid, molybdate, and tungstate, on the degradation of MT and DMS were studied. The results for these sulfate-rich and sulfate-amended slurry incubations clearly demonstrated that besides methanogens, sulfate-reducing bacteria take part in MT and DMS degradation in freshwater sediments, provided that sulfate is available. The possible involvement of an interspecies hydrogen transfer in these processes is discussed. In general, our study provides evidence for methanogenesis as a major sink for MT and DMS in freshwater sediments.

FIG. 1

Effects of several inhibitors on the degradation of DMS (A) and the formation of MT (B and C) in anoxic slurries prepared from the sediment of a eutrophic pond (on the campus of Dekkerswald Institute) after the addition of DMS (A and B) and without addition of DMS (C). Both DMS-amended slurries and slurries without DMS were incubated without inhibitor (control) (■) or with the addition of BES (□), molybdate (⧫), or molybdate plus BES (◊). The arrows indicate the times of the addition of the inhibitors (BES and molybdate).

FIG. 2

(A and B) Effects of the addition of alternative substrates on the transformation of DMS (A) and MT (B) in anoxic slurries prepared from a minerotrophic ditch in the De Bruuk peatland. At the times indicated by the arrows, DMS-amended slurries were pulsed with methanol (66 μM) (□), TMA (66 μM) (⧫), and sodium acetate (66 μM) (◊). Controls (■) were incubated without any further addition. (C) Effect of the addition of MT on the degradation of DMS. Shown are DMS concentrations in slurries without further addition (control) (■) and slurries with the addition of MT (14 μM) (□). For the latter incubations the MT concentrations (◊) are also shown.

FIG. 3

Time courses of endogenously produced MT in BES-inhibited (25 mM) sediment slurries prepared from a minerotrophic ditch in the De Bruuk peatland amended with sulfate to various final concentrations. Shown are slurries without sulfate (control) (■) and slurries amended with sulfate to final concentrations of 0.5 mM (□), 1.0 mM (⧫), 2.5 mM (◊), 5 mM (▴), and 7.5 mM (▵).

FIG. 4

Effects of sulfate and various inhibitors on the degradation of DMS (A) and MT (B) in anoxic slurries prepared from a minerotrophic ditch in the De Bruuk peatland. Shown are the control (▴), sodium tungstate (4 mM) (▵), BES (⧫), BES plus sodium sulfate (1.5 mM) (◊), an abiotic control that was heated for 1.5 h (70°C) (■), and chloroform (500 μM) (□).

FIG. 5

Effects of sulfate and H₂ headspace on the degradation of added DMS in BES-inhibited sediment slurries prepared from a minerotrophic ditch in the De Bruuk peatland. Shown are controls (■ and □), BES (⧫ and ◊), and BES plus sulfate (▴ and ▵). Closed symbols represent slurry samples incubated under an N₂ headspace, and open symbols represent slurry samples incubated under an H₂ headspace.