ATP synthesis by AcP and Fe³⁺ at different conditions.

(a) Effect of pH on reaction ADP (1 mM) + AcP (4 mM) + Fe3+ (500 μM) at 30°C. The optimal pH of the reaction is approximately 5.5–6. Rate of reaction: 0.0079 μM/s (optimal pH), 0.0074 μM/s (pH 4), and 0.0011 μM/s (pH 7). N = 3 ±SD. (b) Effect of temperature on reaction ADP (1 mM) + AcP (4 mM) + Fe3+ (500 μM), pH approximately 5.5–6. Rate of reaction: 0.0066 μM/s (20°C), 0.0079 μM/s (30°C), and 0.028 μM/s (50°C). N = 3 ±SD. (c) Comparison of ATP yield from the reaction ADP (1 mM) + AcP (4 mM) at 30°C, pH approximately 5.5–6 in water (reaction ionic strength = 3.75 mM), a modern ocean mix (600 mM NaCl, 50 mM MgCl2, and 10 mM CaCl2, reaction ionic strength = 783.75 mM), 300 mM NaCl (reaction ionic strength = 303.75 mM), modern ocean concentration of NaCl (600 mM, reaction ionic strength = 603.75 mM), 1 mM NaCl (reaction ionic strength = 1.004 M), dissolved silicate (10 mM SiO2, reaction ionic strength = 123.75 mM), and suspended silica in water (50 mg). N = 3 ±SD. (d) Comparison of ATP yield from the reaction ADP (1 mM) + AcP (4 mM) at 30°C and pH approximately 5.5–6 with and without Fe3+ (500 μM) at 80 bar (striped yellow) and at atmospheric pressure (1 bar, solid blue). N = 2 ±SD. The data underlying this figure can be found in Table D–G in S1 Data (sheet 2).