If the gene for proteorhodopsin is inserted into E. coli and retinal is given to these modified bacteria, then they will incorporate the pigment into their cell membrane and will pump H+ in the presence of light.  A deep purple is representative of clearly transformed colonies, due to light absorption.  Proton gradients can be used to power other membrane protein structures or used to acidify a vesicle type organelle. It was further demonstrated that the proton gradient generated by proteorhodopsin could be used to generate ATP.