A process for the production of hydrogen, comprising the steps of: (i) providing a photosynthetic microorganism having electron transfer capability through a photosynthetic light reaction pathway and through a respiratory electron transfer chain involving an oxidative phosphorylation pathway, and which expresses a hydrogenase, wherein regulation of the oxidative phosphorylation pathway is disrupted with the result that electron flow along the respiratory electron transfer chain toward cytochrome oxidase (complex IV) is reduced; ii) culturing the microorganism under microoxic and illuminated conditions; and (iii) collecting evolved hydrogen.