Light-driven proton pumps are not restricted to archaea but are also found in eubacteria. Marine proteobacteria represent a diverse and widespread class of eubacteria found from the Arctic to the Antarctic Oceans, including equatorial and temperate waters at many depths (Sabehi et al. 2007; Atamna-Ismaeel et al. 2008). Their associated opsin genes encode proteorhodopsins (PRs), with proton pump photocycles similar to that of BR (Váró et al. 2003). However, although PRs can be used to drive ATP production, some proteobacteria do not grow faster in response to increased light, and others have photocycles too slow to contribute significantly to cellular energy production, suggesting other functions for PR (Spudich 2006). The spectra of marine PRs are tuned to ocean depth and latitude, and PRs share a high degree of sequence similarity across species, suggesting that genomic techniques might contribute to our understanding of opsin spectral tuning (Man et al. 2003). Interestingly, absorption variances between blue and green wavelengths can depend on a single amino acid residue (Béjà et al. 2001; Man et al. 2003), but attempts to transfer mutations conferring spectral tuning from PR to other microbial opsins have met with limited success (Yoshitsugu et al. 2009). Nevertheless, the utility of single-amino-acid modifications in shifting absorption spectra has been exploited in vitro to screen randomly mutagenized PR libraries, yielding both red- and blue-shifted mutants