Fe-mineral reducing bacteria enriched from the Lake Pavin chemocline

Iron-reducing, iron-oxidizing and sulfate-reducing bacteria are involved in the formation and alteration of Fe-bearing minerals through their capacity to transfer electrons beyond the cellular surface. Lake Pavin, considered an ancient ocean analogue due to its permanently anoxic, iron-rich bottom waters, hosts a wide diversity of anaerobic bacteria involved in Fe cycling. We aimed to investigate this unexplored diversity by enriching microorganisms from two different water column depths on iron minerals. Enrichment cultures were amended with the key iron species identified in situ, namely ferric iron phosphate, ferrihydrite, and dissolved Fe2+. These different iron phases, the type of electron donor (lactate or fumarate), and the presence or absence of added sulfate favored the development of unique Fe-reducing microbial communities. Despite low endogenous sulfate concentrations (< 20 µM), sulfate-reducers were the most competitive organisms in our enrichments containing sulfate and transformed ferric iron into a diversity of ferrous minerals depending on Fe(III) source (e.g. mackinawite, vivianite, magnetite). Additional characterization of the resulting iron minerals with EXAFS and STXM revealed that the proportion of reduced iron varied with the phase of iron supplied as well as with the electron donor and that microbial cells were intimately associated with reduced iron minerals. Our findings reveal an important influence of microbial activity on mineralogical diversity through both direct and indirect biomineralization pathways.