Photoheterotrophic bacteria represent an important part of aquatic microbial communities. There exist two fundamentally different light-harvesting systems: bacteriochlorophyll-containing reaction centers or rhodopsins. Here, we report a photoheterotrophic Sphingomonas strain isolated from an oligotrophic lake, which contains complete sets of genes for both rhodopsin-based and bacteriochlorophyll-based phototrophy. Interestingly, the identified genes were not expressed when cultured in liquid organic media. Using reverse transcription quantitative PCR (RT-qPCR), RNA sequencing, and bacteriochlorophyll a quantification, we document that bacteriochlorophyll synthesis was repressed by high concentrations of glucose or galactose in the medium. Coactivation of photosynthesis genes together with genes for TonB-dependent transporters suggests the utilization of light energy for nutrient import. The photosynthetic units were formed by ring-shaped light-harvesting complex 1 and reaction centers with bacteriochlorophyll a and spirilloxanthin as the main light-harvesting pigments. The identified rhodopsin gene belonged to the xanthorhodopsin family, but it lacks salinixanthin antenna. In contrast to bacteriochlorophyll, the expression of xanthorhodopsin remained minimal under all experimental conditions tested. Since the gene was found in the same operon as a histidine kinase, we propose that it might serve as a light sensor. Our results document that photoheterotrophic Sphingomonas bacteria use the energy of light under carbon-limited conditions, while under carbon-replete conditions, they cover all their metabolic needs through oxidative phosphorylation.IMPORTANCE Phototrophic organisms are key components of many natural environments. There exist two main phototrophic groups: species that collect light energy using various kinds of (bacterio)chlorophylls and species that utilize rhodopsins. Here, we present a freshwater bacterium Sphingomonas sp. strain AAP5 which contains genes for both light-harvesting systems. We show that bacteriochlorophyll-based reaction centers are repressed by light and/or glucose. On the other hand, the rhodopsin gene was not expressed significantly under any of the experimental conditions. This may indicate that rhodopsin in Sphingomonas may have other functions not linked to bioenergetics.

Center Algatech Institute of Microbiology of the Czech Academy of Science Třeboň Czechia

Department of Environmental Science Aarhus University Aarhus Denmark

Faculty of Science University of South Bohemia České Budějovice Czechia

Institute of Parasitology Biology Center of the Czech Academy of Sciences České Budějovice Czechia

Institute of Plant Molecular Biology Biology Center of the Czech Academy of Sciences České Budějovice Czechia

Laboratory of Aquatic Photobiology and Plankton Ecology Department of Ecology University of Innsbruck Innsbruck Austria

Leibniz Institute DSMZ German Collection of Microorganisms and Cell Cultures Braunschweig Germany

Research Group Genome Analytics Helmholtz Centre for Infection Research Braunschweig Germany

Research Group Microbial Communication Technical University of Braunschweig Braunschweig Germany

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Minimal transcriptional regulation of horizontally transferred photosynthesis genes in phototrophic bacterium Gemmatimonas phototrophica

Response of aerobic anoxygenic phototrophic bacteria to limitation and availability of organic carbon

A photoheterotrophic bacterium from Iceland has adapted its photosynthetic machinery to the long days of polar summer

The Influence of Calcium on the Growth, Morphology and Gene Regulation in Gemmatimonas phototrophica

A bacterium from a mountain lake harvests light using both proton-pumping xanthorhodopsins and bacteriochlorophyll-based photosystems

Characterization of the Aerobic Anoxygenic Phototrophic Bacterium Sphingomonas sp. AAP5