Cornelius van Niel studied purple and green sulphur bacteria, which photosynthesise using H₂S instead of H₂O as the hydrogen donor and release sulphur or sulphate, not O₂. He summarised photosynthesis as 2H₂A + CO₂ → 2A + CH₂O + H₂O (in light), where H₂A is the hydrogen donor. What did he infer about the O₂ released by green plants?