The two primary rotary machines in the bacterial membrane are the flagellum and the ATP synthase. With both machines, an influx of protons is used to drive the rotors, giving rise to motility and ATP synthesis, respectively. The source of these protons typically comes from metabolism, where the transport of electrons is tied to the efflux of protons (the electron transport chain). But what happens if the bacteria find themselves in an environment where metabolism cannot be carried out? Do they simply die?

Recent research has shown that bacteria can use a protein known as proteorhodopsin to absorb photons and use this energy to drive the efflux of protons. In other words, no they don’t die; several bacteria come with a solar powered back-up system

See here for a more detailed description.

What’s also interesting is that this little solar power system uses the same basic design as the receptors in your eye. Pieces and parts of eye may have been front-loaded into the first cells.