Dissimilatory Fe(III) reduction by Clostridium beijerinckii isolated from freshwater sediment using Fe(III) maltol enrichment

A microorganism which reduces Fe(III) during the fermentation of glucose wa s isolated from freshwater sediment. The Fe(III) was supplied to enrichment cultures as a soluble complex with the bidentate ligand maltol (3-hydroxy- 2-methyl-4pyrone). Advantages that were afforded by the use of Fe(III)(malt ol)(3) over previously published methods included negation of the requireme nt for assays of Fe(II) formation. Because Fe(III)(maltol)(3) has a charact eristic deep red colour, Fe(III) reduction could be quantified spectrophoto metrically by monitoring the disappearance of the complex in liquid culture s. Furthermore, Fe(III) reduction on agar plates containing the complex was apparent by zones of decolourisation around the bacterial colonies. 16S rR NA gene sequencing indicated the isolate to be a strain of Clostridium beij erinckii. Growth experiments were performed on the isolate in batch culture s with varying concentrations of Fe(III) citrate and 50 mM glucose. Increas ing the level of Fe(III) citrate present was found to alter the fermentatio n balance, with less acidic products being formed. The presence of Fe(III) led to increases in the growth rate and growth yield, which were both appro ximately doubled when the supply of the cation reached 25 mM. A NAD(P)H-dep endent Fe(III) reductase activity was localised to the bacterial membrane a nd found not to be sensitive to respiratory inhibitors. Taken together, the se data suggest that dissimilatory Fe(III) reduction by the isolate provide s a means of utilising the cation as an electron sink, thus facilitating py ridine nucleotide to be recycled during fermentative metabolism. (C) 1999 F ederation of European Microbiological Societies. Published by Elsevier Scie nce B.V. All rights reserved.