Diversity of Culturable Bacteria, from the Anaerobic Zone of the Meromictic Lake Pavin, Able to Perform Dissimilatory-Iron Reduction in Different in Vitro Conditions

Abstract

A culture-dependent study was performed with the aim of assessing the carbon, electron and Fe(III) sources used for the dissimilatory Fe(III) reduction pathway and the diversity of culturable Fe(III)-reducers in the anoxic zone of the meromictic Lake Pavin. This metabolic pathway was investigated in enrichment cultures inoculated with water samples collected at 70 m depth in the anoxic zone of Lake Pavin. Combinations of different media, organic acids, and incubation gas phases were performed. The potential for Fe(III) reduction in the different growth conditions was assessed by measuring the accumulation of Fe(II) overtime. Bacterial community structure was determined in each growth conditions by Temporal Temperature gradient Gel Electrophoresis (TTGE) profiles of 16S rDNA genes and bands of interest in positive enrichments were sequenced. Comparisons of bacterial community structure between growth conditions revealed that the electron donor, the basal media as well as the Fe(III) source yielded to the selection of different bacterial populations, suggesting that Fe(III) reducers occupy different ecological niches in the anoxic zone of Lake Pavin. Facultative Fe(III) reducers, such as fermentative (e.g., Pseudomonas, Clostridium) and sulphate-reducing (e.g., Desulfovibrio sp.) bacteria, were retrieved in enrichments but well-known obligatory Fe(III) reducers (e.g., Geobacter) were not detected. A greater Fe(III) reduction was noted under H2:CO2 gas phase, suggesting that H2 is used as an electron donor for Fe(III) reduction. Acetate was not used as a precursor for this terminal electron-accepting process, and a high Fe(III) reduction was observed with fumarate provided as the electron donor and carbon sources suggesting that this metabolite may be energetically more beneficial for Fe(III)-reducers.