contributed equally to this work Proteorhodopsins, ubiquitous retinylidene photoactive proton pumps, were recently discovered in the cosmopolitan uncultured SAR86 bacterial group in oceanic surface waters. Two related proteorhodopsin families were found that absorb light with different absorption maxima, 525 nm (green) and 490 nm (blue), and their distribution was shown to be strati®ed with depth. Using structural modeling comparisons and mutagenesis, we report here on a single amino acid residue at position 105 that functions as a spectral tuning switch and accounts for most of the spectral difference between the two pigment families. Furthermore, looking at natural environments, we found novel proteorhodopsin gene clusters spanning the range of 540±505 nm and containing changes in the same identi®ed key switch residue leading to changes in their absorption maxima. The results suggest a simultaneous diversi®cation of green proteorhodopsin and the new key switch variant pigments. Our observations demonstrate that this single-residue switch mechanism is the major determinant of proteorhodopsin wavelength regulation in natural marine environments.