SUMMARY: The addition of Fe(III) to anoxic lake sediments decreased the quantity of volatile fatty acids which accumulated. Similarly, the addition of several substrates to the sediments stimulated the rate of reduction of the naturally occurring Fe(III). Of the substrates used, malate caused the greatest stimulation, and subsequently a malate-fermenting was isolated from the sediment. This organism also reduced NO and Mn(IV), the addition of which to the growth medium significantly lowered the rate of Fe(II) formation. The presence of Fe(III) in the medium increased the molar growth yield by 28% and this did not appear to be associated with the presence of particulate matter or changes in pH and . The addition of Fe(III) to the growth medium also produced a minor shift in the fermentation end-products, with a decrease in the quantity of ethanol formed and a concomitant increase in volatile fatty acids, both of which were of the same order as the amount of Fe(III) reduced. It would appear, therefore, that the presence of Fe(III) might permit a slight diversion of metabolism to more energetically favourable end products. The scale of iron reduction, both in the field and the laboratory, was very small compared with the reducing potential of the available substrates.


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