Metabolism in sediment slurries

Sulfides and disulfides can be produced by bacterial reactions in the marine environment. 2-Dimeth-ylthiopropionic acid is produced by algae and by the marsh grass Spartina alternifolia, and may then be metabolized in sediment slurries under anoxic conditions to dimethyl sulfide (Kiene and Taylor 1988), and by aerobic bacteria to methyl sulfide (Taylor and Gilchrist 1991). Further details are given in Chapter 11, Part 2. Methyl sulfide can also be produced by biological methylation of sulfide itself (HS ). Carbon radicals are not the initial atmospheric products from organic sulfides and disulfides, and the reactions also provide an example in which the rates of reaction with nitrate... 

Figure 6. 3-Mercaptopropionate production and metabolism in sediment slurries. Treatments 10 mM acrylate added, O 10 mM acrylate with autoclaved slurries, A endogenous (no acrylate added), A. ...

The organism also fermented acrylate, as do other Clostridia, to a 2 1 molar mixture of propionate and acetate. Bacteria that ferment aciylate, derived from DMSP, probably abound in coastal marine sediments (Kiene and Taylor, this volume). Kiene and Taylor (this volume) observed that acrylate was immediately metabolized in anoxic slurries of coastal marine sediments with the appearance of a 1 2 molar mixture of acetate and propionate. The acetate and propionate were consumed by sulfate-reducing bacteria. Acetate is a quantitatively important substrate in anoxic marine sediments (491 and DMSP may be a significant precursor of acetate in such environments. 

The anaerobic metabolism of acrylate and 3-mercaptopropionate (3-MPA) was studied in slurries of coastal marine sediments. The rate of these compounds is important because they are derived from the algal osmolyte dimethylsulfoniopropionate (DMSP), which is a major organic sulfur compound in marine environments. Micromolar levels of acrylate were fermented rapidly in the slurries to a mixture of acetate and propionate (1 2 molar ratio). Sulfate-reducing bacteria subsequently removed the acetate and propionate. 3-MPA has only recently been detected in natural environments. In our experiments 3-MPA was formed by chemical addition of sulfide to aciylate and was then consumed by biological processes. 3-MPA is a known inhibitor of fatty acid oxidation in mammalian systems. In accord with this fact, high concentrations of 3-MPA caused acetate to accumulate in sediment slurries. At lower concentrations, however, 3-MPA was metabolized by anaerobic bacteria. We conclude that the degradation of DMSP may ultimately lead to the production of substrates which are readily metabolized by microbes in the sediments. 

Figure 3. Metabolism of 700 pM acrylate in sediment slurries. Treatments acrylate alone, O > acrylate plus autoclaved slurry, A acrylate plus 20 mM molybdate, % acrylate plus antibiotics, A endogenous (no additions), I.

Figure 5. Metabolism of 10 mM acrylate in sediment slurries. Panel A shows acrylate concentrations panel B shows acetate concentrations panel C shows propionate concentrations. Treatments uninhibited slurries, ...

Although the use of metabolically stable mixed cultures enables an acceptable degree of repeatability to be attained, the use of suspensions or slurries containing sediment or metabolites from previous additions of enrichment substrates introduces undesirable ambiguity in the interpretaton of analytical results. This has been discussed more fully in Section 5.3.1. Inevitably, however, most examples of dechlorination depend on the results of studies with mixed cultures. There are several important issues which have emerged from these quite extensive investigations and these merit brief discussion. 

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