  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.
