SUMMARY: The rate of release of Fe(II) from anoxic lake sediments was lower in the presence than in the absence of nitrate. The reduction of Fe(III) by the sediments had a temperature optimum of 30 °C and was inhibited by HgCl, suggesting that the process was largely biological in nature. Of the iron sources tested with cultures of anaerobic iron-reducing bacteria, FeCl was the most readily reduced and goethite the least. Reduction was faster in the presence of a chelating agent and was suppressed by the addition of NO , ClO , MnO, MnO and O. An iron-reducing chemoorganotroph, tentatively identified as a member of the genus , was isolated. Physical contact between the bacterium and iron particles was essential to ensure maximum rates of Fe(III) reduction but > 30% of the activity appeared to be associated with extracellular components. Although Fe(III) reduction by whole cells and cell-free extracts was decreased in the presence of electron transport inhibitors, the molar growth yield of the organism was unaffected by the presence of Fe(III). It is assumed that the organism used the Fe(III) as a hydrogen sink. A second organism, an anaerobic facultative chemolithotroph, appeared to conserve energy by the reduction of Fe(III). Biomass yield (measured as ATP) was greater in the presence of Fe(III), and the organism was able to use H as a source of reducing power.


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