Energy from chemical redox reactions — life without sunlight
Chemosynthesis is carbon fixation powered by chemical energy rather than light. Sulfur-oxidizing bacteria catalyze: 6CO₂ + 6H₂O + 3H₂S → C₆H₁₂O₆ + 3H₂SO₄. The free energy from oxidizing one mole of H₂S to sulfate (ΔG° ≈ −798 kJ/mol) drives ATP synthesis through a proton gradient across the bacterial membrane, analogous to the electron transport chain in photosynthesis. Nitrifying bacteria use NH₃ oxidation; methanogens use CO₂ + H₂. These pathways evolved ~3.8 billion years ago, likely before oxygenic photosynthesis, and underlie our models of early life on Earth and potential life on ocean worlds like Enceladus and Europa.