Microbial metabolism of iron species in freshwater lake sediments

Abstract

Sediments develop by sedimentation of organic and inorganic residues of primary and secondary production as well as by inorganic precipitates, e.g., metal hydroxides, carbonates, silicates, and phosphates. The accumulation of this material at the bottom of freshwater lakes leads to an intensification of mainly microbial degradative activities which oxidise and transform the organic freight with concomitant reduction of oxygen and other electron acceptors. It is the activity of micro-organisms, especially of bacteria, which leads to the reduction of available electron acceptors, to an accumulation of reduced derivatives, and with that to changes of the redox potential in such sediments.The basic processes involved in the degradation of organic matter by such microbial communities are known for a long time. As long as molecular oxygen is available it acts as the preferred electron acceptor, followed by nitrate, manganese(IV) oxide, iron(III) hydroxides, sulfate, and finally CO2 with the release of nitrite, ammonia, dinitrogen, manganese(II) and iron(II) carbonates, sulfides, and finally methane as products of microbial reductive activities (STUMM & MORGAN,1981). These preferences for the various acceptor systems are mainly determined by the redox potential and the availability of the redox systems under consideration, with the most positive ones at the beginning and the lower ones to the end, according to the scheme depicted in Table 18.1.