Principle

Proteorhodopsin (PR) is a light driven proton pump, i.e. it pumps out H+ ions across the bacterial cell membrane and hence alters the proton motive force of the cell membrane. It is native to marine bacteria and archaebacteria, where it serves to provide energy for bacterial metabolism and flagellar movement in extreme environmental conditions.

ATP synthase, the membrane proton channel coupled with ATP synthesis, uses up proton gradient to drive phosphorylation of ADP to ATP, the energy molecule of the cell. The proton gradient for ATP synthase is generally provided by the activity of Electron Transport Chain (ETC) system. In case of proteorhodopsin expressing cells, PR gets activated when light of appropriate wavelength(525 nm in case of Green light absorbing ProteoRhodopsin) is incident on the bacterial membrane, and it pumps out protons. Through this pumping action, the proton gradient across the cell membrane increases. The proton gradient developed by Proteorhodopsin is used up to drive ATP synthesis and hence increase the energy available to the cell. The Proteorhodopsin + ATP synthase system hence acts as a light based energy source for bacteria, which is analogous to Photophosphorylation in chloroplasts.

Expression of PR in cells provides an energy advantage to cells, especially in conditions where the Electron Transport Chain is not fully functional. This energy advantage in terms of extra ATP has been proposed to be useful in improving global protein production, cell survivability in decreased substrate availability and other stress conditions.
Proteorhodopsin is a photoactive protein that uses retinal as a chromophore for light reception. It functions as a light-driven proton pump, driven by the conformational changes caused by the photoisomerization of trans-retinal to cis-retinal. Retinal is covalently bound to the protein via a protonated schiff’s base. Proteorhodopsin functions only in the presence of retinal.
The risks of horizontal gene transfer (HGT) of proteorhodopsin having any impact on the environment are low since Proteorhodopsin can at most, partially increase the energy content of the cells, which can be significant only in certain nutritional conditions. Furthermore, since the availability of retinal in the media determines the functioning of Proteorhodopsin, and hence the selective advantage it confers, the supply of retinal can be used as a "switch". This "switch" further diminishes the environmental risks of HGT of Proteorhodopsin.