Photoheterotrophic bacteria harvest light energy using either proton-pumping rhodopsins or bacteriochlorophyll (BChl)-based photosystems. The bacterium Sphingomonas glacialis AAP5 isolated from the alpine lake Gossenköllesee contains genes for both systems. Here, we show that BChl is expressed between 4°C and 22°C in the dark, whereas xanthorhodopsin is expressed only at temperatures below 16°C and in the presence of light. Thus, cells grown at low temperatures under a natural light-dark cycle contain both BChl-based photosystems and xanthorhodopsins with a nostoxanthin antenna. Flash photolysis measurements proved that both systems are photochemically active. The captured light energy is used for ATP synthesis and stimulates growth. Thus, S. glacialis AAP5 represents a chlorophototrophic and a retinalophototrophic organism. Our analyses suggest that simple xanthorhodopsin may be preferred by the cells under higher light and low temperatures, whereas larger BChl-based photosystems may perform better at lower light intensities. This indicates that the use of two systems for light harvesting may represent an evolutionary adaptation to the specific environmental conditions found in alpine lakes and other analogous ecosystems, allowing bacteria to alternate their light-harvesting machinery in response to large seasonal changes of irradiance and temperature.

Department of Molecular Bacteriology Helmholtz Centre for Infection Research Braunschweig 38106 Germany

Faculty of Science University of South Bohemia České Budějovice 37005 Czechia

Institute of Parasitology Biology Centre Czech Acad Sci České Budějovice 37005 Czechia

Institute of Plant Molecular Biology Biology Centre Czech Acad Sci České Budějovice 37005 Czechia

Laboratory of Anoxygenic Phototrophs Institute of Microbiology of the Czech Acad Sci Třeboň 37981 Czechia

Laboratory of Aquatic Photobiology and Plankton Ecology Department of Ecology University of Innsbruck Innsbruck 6020 Austria

Research Group Genome Analytics Helmholtz Centre for Infection Research Braunschweig 38124 Germany

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Trends Microbiol. 2023 Apr;31(4):326-328. doi: 10.1016/j.tim.2023.02.005. PubMed

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

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