Characterization of a marine gammaproteobacterium capable of aerobic anoxygenic photosynthesis

Members of the gammaproteobacterial clade NOR5/OM60 regularly form an abundant part, up to 11%, of the bacterioplankton community in coastal systems during the summer months. Here, we report the nearly complete genome sequence of one cultured representative, Congregibacter litoralis strain KT71, isolated from North Sea surface water. Unexpectedly, a complete photosynthesis superoperon, including genes for accessory pigments, was discovered. It has a high sequence similarity to BAC clones from Monterey Bay [Beja O, Suzuki MT, Heidelberg JF, Nelson WC, Preston CM, et al. (2002) Nature 415:630-633], which also share a nearly identical gene arrangement. Although cultures of KT71 show no obvious pigmentation, bacteriochlorophyll a and spirilloxanthin-like carotenoids could be detected by HPLC analysis in cell extracts. The presence of two potential BLUF (blue light using flavin adenine dinucleotide sensors), one of which was found adjacent to the photosynthesis operon in the genome, indicates a light- and redox-dependent regulation of gene expression. Like other aerobic anoxygenic phototrophs (AAnPs), KT71 is able to grow neither anaerobically nor photoautotrophically. Cultivation experiments and genomic evidence show that KT71 needs organic substrates like carboxylic acids, oligopeptides, or fatty acids for growth. The strain grows optimally under microaerobic conditions and actively places itself in a zone of approximately 10% oxygen saturation. The genome analysis of C. litoralis strain KT71 identifies the gammaproteobacterial marine AAnPs, postulated based on BAC sequences, as members of the NOR5/OM60 clade. KT71 enables future experiments investigating the importance of this group of gammaproteobacterial AAnPs in coastal environments.