Genomics and Physiology of a Marine Flavobacterium Encoding a Proteorhodopsin and a Xanthorhodopsin- Like Protein Thomas Riedel1*, Laura Go´ mez-Consarnau2,Ju¨ rgen Tomasch1, Madeleine Martin1, Michael Jarek1, Jose´ M. Gonza´lez3, Stefan Spring4, Meike Rohlfs1, Thorsten Brinkhoff5, Heribert Cypionka5, Markus Go¨ ker4, Anne Fiebig4, Johannes Klein6, Alexander Goesmann7, Jed A. Fuhrman2, Irene Wagner- Do¨ bler1 1 Helmholtz-Centre for Infection Research, Braunschweig, Germany, 2 University of Southern California, Department of Biological Sciences, Los Angeles, California, United States of America, 3 University of La Laguna, Department of Microbiology, La Laguna, Tenerife, Spain, 4 Leibniz Institute DSMZ – German Collection of Microorganisms and Cell Cultures, Braunschweig, Germany, 5 Carl von Ossietzky University, Institute of Chemistry and Biology of the Marine Environment (ICBM), Oldenburg, Germany, 6 University of Braunschweig, Department of Microbiology, Braunschweig, Germany, 7 Center for Biotechnology, University of Bielefeld, Bielefeld, Germany Abstract Proteorhodopsin (PR) photoheterotrophy in the marine flavobacterium Dokdonia sp. PRO95 has previously been investigated, showing no growth stimulation in the light at intermediate carbon concentrations. Here we report the genome sequence of strain PRO95 and compare it to two other PR encoding Dokdonia genomes: that of strain 4H-3-7-5 which shows the most similar genome, and that of strain MED134 which grows better in the light under oligotrophic conditions. Our genome analysis revealed that the PRO95 genome as well as the 4H-3-7-5 genome encode a protein related to xanthorhodopsins. The genomic environment and phylogenetic distribution of this gene suggest that it may have frequently been recruited by lateral gene transfer. Expression analyses by RT-PCR and direct mRNA-sequencing showed that both rhodopsins and the complete b-carotene pathway necessary for retinal production are transcribed in PRO95. Proton translocation measurements showed enhanced proton pump activity in response to light, supporting that one or both rhodopsins are functional. Genomic information and carbon source respiration data were used to develop a defined cultivation medium for PRO95, but reproducible growth always required small amounts of yeast extract. Although PRO95 contains and expresses two rhodopsin genes, light did not stimulate its growth as determined by cell numbers in a nutrient poor seawater medium that mimics its natural environment, confirming previous experiments at intermediate carbon concentrations. Starvation or stress conditions might be needed to observe the physiological effect of light induced energy acquisition. Citation: Riedel T, Go´mez-Consarnau L, Tomasch J, Martin M, Jarek M, et al. (2013) Genomics and Physiology of a Marine Flavobacterium Encoding a Proteorhodopsin and a Xanthorhodopsin-Like Protein. PLoS ONE 8(3): e57487. doi:10.1371/journal.pone.0057487 Editor: Hauke Smidt, Wageningen University, The Netherlands Received November 5, 2012; Accepted January 22, 2013; Published March 4, 2013 Copyright: ß 2013 Riedel et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: TR, JT, HC, TB, MG, AF, IWD gratefully acknowledge funding by the DFG Transregio-SFB 51 Roseobacter, and LGC by the Swedish Research Council (Project 623-2009-7258). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: [email protected] Introduction ecosystems suggesting a key role in the biochemical cycles of the oceans. Rhodopsins are photoactive membrane-embedded opsins found Xanthorhodopsins (XRs) are light-harvesting proton pumps in all three domains of life [1]. These energy transduction proteins with a dual chromophore, one retinal molecule and one contain seven-transmembrane a-helices and use retinal as a carotenoid antenna, that were first discovered in Salinibacter ruber cofactor, located in the retinal-binding pocket, for their light- [6]. Its carotenoid antenna salinixanthin transfers as much as 40– driven function. Microbial rhodopsins, classified as type I 45% of the absorbed photons to retinal [7], resulting in a rhodopsins, are widespread amongst prokaryotes and can function potentially much more efficient light-capturing system compared as proton pumps [bacteriorhodopsins (BR) and proteorhodopsins to PRs from Bacteria [4,5] or BRs from Archaea [2]. XRs have a (PR)], ion pumps [(halorhodopsins (HR)] and photosensors binding pocket into which both retinal and a second chromo- [sensory rhodopsins (SRI and SRII)]. BR was discovered in phore, e.g. salinixanthin (S. ruber [6] or echinenone (Gloeobacter Halobacterium salinarium, an extremely halophilic archaeon, found in violaceus [8]) are positioned. hypersaline lakes and salterns [2,3]. Bacterial rhodopsin (proteor- Environmental metagenomics and single cell genomics have hodopsin, PR) was discovered much later in a metagenome shown PRs to be abundant not only in the ocean [9,10,10–18], but fragment of a marine gammaproteobacterium from the SAR86 also in freshwater bacterioplankton [19], where they were found clade [4,5]. More than a decade after their discovery, research on for the first time in Deltaproteobacteria, Verrucomicrobia and Sphingo- PRs has shown their extensive abundance and diversity in aquatic bacteria [20]. In their study, the authors also obtained evidence for PLOS ONE | www.plosone.org 1 March 2013 | Volume 8 | Issue 3 | e57487 Genomics of PRO95 Carrying Two Rhodopsin Genes lateral gene transfer: Actinobacterial PRs were found in several Materials and Methods gammaproteobacterial genomes; moreover, in one actinobacterial genome, two PR genes were present, one related to Actinobacteria, Sampling and Isolation of Strain PRO95 the other one typical for Betaproteobacteria. Sequencing the genomes Strain PRO95 was isolated from a surface water sample of single PR-carrying bacteria recovered by cell sorting from the collected at the ‘‘Kabeltonne’’ at Helgoland Roads, a sampling site ocean showed that their genomes were more representative of the between the two islands of Helgoland (Germany), 60 km offshore natural microbial bacterioplankton community than those of in the North Sea (54u 11.189 North, 7u 549 East). No specific cultivated PR-containing bacteria [21]. However, although this permits were required for the described field study. The sampling new approach has a great potential to reveal the presence of site is not privately owned or protected in any way. The sampling certain genes in single representative genomes, it does not show if of water did not involve endangered or protected species. The the gene is expressed and what is its physiological role for the sampling and isolation procedure has already been described [25]. organism that is carrying it. Experiments using heterologous Dokdonia sp. PRO95 has been deposited at the DSMZ (DSM systems, on the other hand, can show the function of the encoded 26625). protein. They demonstrated the proton pump activity of PRs found in the environment [4] and showed that the proton gradient Cultivation of Strain PRO95 might be used to power flagella movement [22]. The question Strain PRO95 was cultivated on plates of marine agar 2216 puzzling marine microbiologists, however, remains unanswered by (Difco, Becton, Dickinson and Company, Heidelberg, Germany) those approaches, namely, what is powered in marine microbes by at room temperature (20–25uC) for two days. For liquid cultures, the rhodopsins that many of them are carrying? Only culturing of first precultures were made. Single colonies were transferred to a strain that naturally contains the gene has the potential to marine broth 2216 (Difco, Becton, Dickinson and Company, explain how this organism uses the rhodopsin to survive in its Heidelberg, Germany) which was diluted as required. Dilutions particular habitat. were made either with distilled water, in which case the media A large percentage of whole genome sequenced PR-containing were amended with Sea salts (Sigma-Aldrich, St. Louis, USA) to bacteria belong to the family Flavobacteriaceae. However, and obtain the salt concentration of full strength marine broth 2216 despite the genomic similarities, the consequences of light (31.8 g per liter) as described previously [25], or with natural exposure on the different flavobacterial species appear to vary. sterile filtered autoclaved seawater from the North Sea, in order to Whereas light stimulated growth in Dokdonia sp. MED134 [23,24], change only the concentration of nutrients, but not the salt no light-induced growth advantage was observed for its relative concentration. Precultures and main cultures were incubated at Dokdonia sp. PRO95 [25]. No growth advantage in the light was 25uC and 160 rpm in a natural day-light cycle. For DNA also reported for the alphaproteobacterium Candidatus Pelagibacter extraction, cells were cultivated in full strength marine broth 2216. ubique HTCC1062 [26], the flavobacterium Polaribacter sp. For RNA extraction cells were cultured in half strength marine MED152 [27], and the gammaproteobacteria Vibrio sp. AND4 broth 2216 amended with Sea salts and in 250 times diluted [28] and Vibrio campellii BAA-1116 [29]. However,