Figure 7
A simplified model for biogenic formation of ferrihydrite-type Fe3+- (hydrated) oxyhydroxides at acidic shallow-submarine hydrothermal vents.
Nitrate (NO3−) is biologically produced through hydrothermal ammonium (NH4+) biooxidation by abundant nitrifying archaea (NA). Large-scale anaerobic nitrate-dependent chemical oxidation of hydrothermal Fe2+ with microbially produced NO3− as an electron acceptor allows for the indirect biogenic precipitation of ferrihydrite+type phases at Kolumbo's low+temperature hydrothermal vent niches. A parallel small+scale mechanism of abiotic molecular O2 intervention in the oxidation Fe2+ to Fe3+ cannot be excluded. Schematic cross section of Kolumbo's crater with pH (solid circles) and density (open circles) profiles is modified after Carey et al.20. Hydrothermal spires not to scale.
