Iron dissimilatory sulfate reduction, sulfide

Dissimilatory sulfate (S04 H2S) Dimethyl sulfide production (CHaljS Metal cycles Iron and manganese oxidation and reduction... 

In fully marine systems siderite formation is probable to occur below the sulfate reduction zone where dissolved sulfide is absent, if reactive iron is still present and the Fe/Ca-ratio of pore water is high enough to stabilize siderite over calcite (Berner 1971). The coexistence of siderite and pyrite in anoxic marine sediments was shown by Ellwood et al. (1988) and Haese et al. (1997). Both studies attribute this observation to the presence of microenvironments resulting in different characteristic early diagenetic reactions next to each other within the same sediment depth. It appears that in one microenvironment sulfate reduction and the formation of pyrite is predominant, whereas at another site dissimilatory iron reduction and local supersaturation with respect to siderite occurs. Similarly, the importance of microenvironments has been pointed out for various other processes (Jorgensen 1977 Bell et al. 1987 Canfield 1989 Gingele 1992). 

Iron monosulfide, FeS, is produced in soils and sediments primarily through dissimilatory microbial reduction of sulfate to sulfide, which subsequently reacts with available iron to precipitate FeS (5, 4). The mineral mackinawite, often in poorly crystalline form (3, 23-25), is the initial FeS precipitate in the transformation of iron minerals by sulfate-reducing bacteria (3). For example, when the sulfate-reducing bacterium Desulfovibrio desulfuricans was grown at pH 8 in cultures containing a Fe(II)/Fe(III) oxyhydroxide and synthetic geothite (FeOOH), mackinawite was the predominant iron sulfide phase present after six and nine months, respectively (26). Even at lower pH values, mackinawite was the only iron sulfide phase detected after two weeks of microbial activity, and still a minor phase after that. 

The iron is especially important. In freshwater ecosystems, fluxes of hydrogen sulfide are also relatively small owing to the lack of sufficient sulfate as a substrate for dissimilatory reduction and to the relatively greater incorporation of the available sulfur into biomass. However, the release of hydrogen sulfide is significant from wetlands. In addition, H2S emission from plant canopy occurs when S plant uptake is in excess of biosynthetic demands. The latter process may account for as much as 40% of total natural S emission. 

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