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 oceans2,3,4,5. 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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We thank S. Arnautovic, J. O. Ekström, M. Widell, E. Lundberg and E. Lindehoff for help with growth experiments, ultracentrifugation, cloning, dissolved organic carbon and nutrient analysis, respectively, and T. Berman for helpful comments on the manuscript. We thank the Swedish Science Council, the Spanish Ministerio de Educación y Ciencia, Swegene, EMEP, and SSF for supporting this research.
The genomes of strains MED134, MED152 and MED217 are deposited in GenBank under accession numbers AAMZ00000000, AANA00000000 and AANC00000000, and their 16S rRNA gene sequences under accession numbers DQ481462, DQ481463 and DQ294290, respectively. The amino acid sequences of MED134 and MED152 PR are deposited in GenBank under accession numbers ZP_01049273 and ZP_01054176.
The authors declare no competing financial interests.
The file contains the following: Supplementary Figures and Legends 1-3; Supplementary Methods; Supplementary Notes. The Supplementary Figures show proteorhodopsin amino acid sequence alignment, laser-flash induced absorbance changes of proteorhodopsin in membrane preparation of Dokdonia sp. MED134 and arrangement of proteorhodopsin, ß-carotene and retinal synthesis genes. (PDF 1576 kb)
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