Sulfate Reduction

In peatlands, sulfur occurs in several different redox states (S valences ranging from +6 in SO4- to -2 in hydrogen sulfide (H2S), S-containing amino acids, and other compounds), and conversions between these states are the direct result of microbially mediated transformations. In bogs, the sole sulfur input is via atmospheric deposition, while in fens atmospheric deposition can be augmented by surface and/or groundwater inputs, which may contain sulfur derived from weathering of minerals in rock and soil. Regardless of the sulfur source, when sulfur enters a peatland, there are a variety of pathways through which it can cycle. In the aerobic zone, sulfate can be adsorbed onto soil particles, or assimilated by both plants and microbes. In the anaerobic zone, sulfate can also be adsorbed onto soil particles, assimilated by plants or microbes, or reduced by sulfate-reducing bacteria through the process of dissimila-tory sulfate reduction. Dissimilatory sulfate reduction is a chemoheterotrophic process whereby bacteria in at least 19 different genera oxidize organic matter to meet their energy requirements using sulfate as the terminal electron acceptor. Thus, this process is one way in which carbon is lost from the catotelm. If the sulfate is reduced by sulfate-reducing bacteria, the end product (S2-) can have several different fates. In the catotelm, where S2- is formed, it can react with hydrogen, to produce H2S gas, which can diffuse upwardly into or through the acrotelm where it can be either oxidized to sulfate, or lost to the atmosphere. Alternatively, H2S can react by nucleophilic attack with organic matter to form organic or C-bonded sulfur (CBS). If Fe is present, S2- can react with Fe to form FeS and FeS2 (pyrite), which is referred to as reduced inorganic sulfur (RIS). The RIS pool tends to be unstable in peat and can be reoxidized aerobically with oxygen if the water table falls, or anaerobically probably using Fe3+ as an anaerobic electron acceptor. If Hg is present, and combines with S2- to form neutrally charged HgS, then Hg sulfide is capable of passive diffusion across cell membranes of bacteria that methylate Hg. Alternatively, bacteria can transfer the methoxy groups of naturally occurring compounds, such as syringic acid, to S2-, and form methyl sulfide (MeSH) or dimethyl sulfide (DMS), although the exact mechanisms by which this occurs are still unknown.

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