1. Marine Microbiology, Department of Biology and Environmental Sciences, University of Kalmar, SE-39182 Kalmar, Sweden
2. Department of Microbiology and Cell Biology, University of La Laguna, ES-38206 La Laguna, Tenerife, Spain
3. Institut de Ciències del Mar, CMIMA-CSIC, ES-08003 Barcelona, Catalonia, Spain
4. Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-41296 Göteborg, Sweden
5. Chemical Physics, Kemicentrum, Lund University, SE-22100 Lund, Sweden
6. Department of Chemistry, Biochemistry and Biophysics, Gothenburg University, SE-40530 Göteborg, Sweden

Correspondence to: Jarone Pinhassi¹ Correspondence and requests for materials should be addressed to J.P. (Email: jarone.pinhassi@hik.se).

Proteorhodopsins are bacterial light-dependent proton pumps. Their discovery within genomic material from uncultivated marine bacterioplankton caused considerable excitement because it indicated a potential phototrophic function within these organisms, which had previously been considered strictly chemotrophic¹. Subsequent studies established that sequences encoding proteorhodopsin are broadly distributed throughout the world's oceans^{2, 3, 4, 5}. Nevertheless, the role of proteorhodopsins in native marine bacteria is still unknown⁶. Here we show, from an analysis of the complete genomes of three marine Flavobacteria, that cultivated bacteria in the phylum Bacteroidetes, one of the principal components of marine bacterioplankton, contain proteorhodopsin. Moreover, growth experiments in both natural and artificial seawater (low in labile organic matter, which is typical of the world's oceans) establish that exposure to light results in a marked increase in the cell yield of one such bacterium (Dokdonia sp. strain MED134) when compared with cells grown in darkness. Thus, our results show that the phototrophy conferred by proteorhodopsin can provide critical amounts of energy, not only for respiration and maintenance but also for active growth of marine bacterioplankton in their natural environment.

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