  Photosystem 0 concerns a primitive mechanism for free energy gain as ATP from fluctuating light during early evolution. The PS0 reaction centers had no reducing power: charge transport was only temporary. Light induced within the reaction centers metastable dipoles that generated a membrane potential. This in turn drove ATP synthesis by protons moving through the ATP synthase enzyme. After the decay of the dipole potential in the dark, the protons either (1) returned across the membrane by conduction or (2) were pumped back by ATP synthase, backwards active as ATPase at a higher H+/ATP ratio. PS0 constitutes a link to previously proposed free energy sources for early evolution that worked on thermal cycling. Several contemporary photosynthetic phenomena may be relics of PS0. 
Photosystem 0, a proposed ancestral photosystem without reducing power   that synthesized ATP during light-dark cycling