title: Phototrophic Fe(II)-oxidation in the chemocline of a ferruginous meromictic lake.
abstract: Precambrian Banded Iron Formation (BIF) deposition was conventionally attributed to the precipitation of iron-oxides resulting from the abiotic reaction of ferrous iron (Fe(II)) with photosynthetically produced oxygen. Earliest traces of oxygen date from 2.7 Ga, thus raising questions as to what may have caused BIF precipitation before oxygenic photosynthesis evolved. The discovery of anoxygenic phototrophic bacteria thriving through the oxidation of Fe(II) has provided support for a biological origin for some BIFs, but despite reports suggesting that anoxygenic phototrophs may oxidize Fe(II) in the environment, a model ecosystem of an ancient ocean where they are demonstrably active was lacking. Here we show that anoxygenic phototrophic bacteria contribute to Fe(II) oxidation in the water column of the ferruginous sulfate-poor, meromictic lake La Cruz (Spain). We observed in-situ photoferrotrophic activity through stimulation of phototrophic carbon uptake in the presence of Fe(II), and determined light-dependent Fe(II)-oxidation by the natural chemocline microbiota. Moreover, a photoferrotrophic bacterium most closely related to Chlorobium ferrooxidans was enriched from the ferruginous water column. Our study for the first time demonstrates a direct link between anoxygenic photoferrotrophy and the anoxic precipitation of Fe(III)-oxides in a ferruginous water column, providing a plausible mechanism for the bacterial origin of BIFs before the advent of free oxygen. However, photoferrotrophs represent only a minor fraction of the anoxygenic phototrophic community with the majority apparently thriving by sulfur cycling, despite the very low sulfur content in the ferruginous chemocline of Lake La Cruz.
authors.affiliation: Laboratory of Microbiology, Institute of Biology, University of Neuchâtel Neuchâtel, Switzerland ; Faculty of Environment and Technology, Bristol BioEnergy Centre, University of the West of England Bristol, UK.
authors.forename: Xavier A
authors.initials: XA
authors.lastname: Walter
authors.affiliation: Department of Microbiology and Ecology, Cavanilles Institute of Biodiversity and Evolutionary Biology, University of Valencia Burjassot, Spain.
authors.forename: Antonio
authors.initials: A
authors.lastname: Picazo
authors.affiliation: Department of Microbiology and Ecology, Cavanilles Institute of Biodiversity and Evolutionary Biology, University of Valencia Burjassot, Spain.
authors.forename: Maria R
authors.initials: MR
authors.lastname: Miracle
authors.affiliation: Department of Microbiology and Ecology, Cavanilles Institute of Biodiversity and Evolutionary Biology, University of Valencia Burjassot, Spain.
authors.forename: Eduardo
authors.initials: E
authors.lastname: Vicente
authors.affiliation: Department of Microbiology and Ecology, Cavanilles Institute of Biodiversity and Evolutionary Biology, University of Valencia Burjassot, Spain.
authors.forename: Antonio
authors.initials: A
authors.lastname: Camacho
authors.affiliation: Laboratory of Microbiology, Institute of Biology, University of Neuchâtel Neuchâtel, Switzerland.
authors.forename: Michel
authors.initials: M
authors.lastname: Aragno
authors.affiliation: Laboratory of Microbiology, Institute of Biology, University of Neuchâtel Neuchâtel, Switzerland ; Aquatic and Stable Isotope Biogeochemistry, Department of Environmental Sciences, University of Basel Basel, Switzerland.
authors.forename: Jakob
authors.initials: J
authors.lastname: Zopfi
keywords: anoxygenic photosynthesis; banded iron formation; cryptic sulfur cycling; early life evolution; geomicrobiology