1. Monterey Bay Aquarium Research Institute, Moss Landing, California 95039, USA
2. Department of Microbiology and Molecular Genetics, The University of Texas Medical School, Houston, Texas 77030, USA
3. These authors contributed equally to this work

Correspondence to: Edward F. DeLong¹ Correspondence and requests for materials should be addressed to E.F.D. (e-mail: Email: delong@mbari.org). The sequences have been deposited with GenBank under accession numbers AF349976–AF350003.

Proteorhodopsin¹, a retinal-containing integral membrane protein that functions as a light-driven proton pump, was discovered in the genome of an uncultivated marine bacterium; however, the prevalence, expression and genetic variability of this protein in native marine microbial populations remain unknown. Here we report that photoactive proteorhodopsin is present in oceanic surface waters. We also provide evidence of an extensive family of globally distributed proteorhodopsin variants. The protein pigments comprising this rhodopsin family seem to be spectrally tuned to different habitats—absorbing light at different wavelengths in accordance with light available in the environment. Together, our data suggest that proteorhodopsin-based phototrophy is a globally significant oceanic microbial process.