Fig. 1

Initial slurries established from methanogenic sediments collected off the coast of the Baltic Sea and third transfer incubations with Fe⁰ or acetate as electron donors vs. parallel control incubations without electron donors. a Slurries incubated with Fe⁰ or b without Fe⁰ (n = 3). Electron conversions into products (methane and acetate) are presented as mM electron equivalents (mM eeq) taking into account that a mol methane/acetate requires 8 mols electrons according to the reactions: CO₂ + 8e^- + 8H⁺ → CH₄ + 2H₂O (methanogenesis) and 2 CO₂ + 8e^- + 8H⁺ → CH₃COOH + 2H₂O (acetogenesis). c Fe⁰-dependent methane production in the third successive passage (sediment-free). However, methane was undetected in electron-donor free controls and when 10 mM acetate replaced Fe⁰ as sole electron donor. The electron donor-free control was used to probe for sediment carryover substrates (c-inset). A gray-black corrosion product was observed only in the presence of cells and not in their absence (c-inset)