Fig. 2: Abiotic H₂ produced by the iron-sulfide chemical garden fuels CO₂ reduction and methanogenesis by M. jannaschii.

a, Production of abiotic H₂ in the chemical gardens increases with increasing Fe(II) concentrations at 80 °C (experiment 2, Supplementary Table 1). Error bars indicate standard error of the means, individual points represent biological replicates (n = 3). b, The concentration of abiotic H₂ produced in the sedimentary iron-sulfide chemical garden (500 mM FeCl₂ and Na₂S, 80 °C) correlates positively with the amount of ¹³CH₄ produced by M. jannaschii. c, Gas consumption and production by M. jannaschii in the sedimentary iron-sulfide chemical gardens at 80 °C supplemented with ¹³C-bicarbonate to trace CO₂ fixation and methanogenesis (experiment 3). Error bars represent standard deviations across three biological replicates. Note that the ¹³C-labelling of CO₂ decreases over time, as the ¹³C-labelling of methane increases, indicating a transfer of ¹³C from CO₂ to CH₄ by M. jannaschii. The abiotically produced H₂ increases over the first 4 h and then decreases afterwards, due to consumption by M. jannaschii.