Cold-loving bacterium from a mountain lake harvests light energy using both bacteriochlorophyll-containing photosyntems as well as proton-pumping rhodopsins

Michal Koblížek¹, Karel Kopejtka¹, Alastair Gardiner¹, David Kaftan¹, David Bína², Zdeno Gardian²

¹Centre Algatech, Institute of Microbiology, Czech Academy of Sciences, 37981 Třeboň, Czechia

²Faculty of Science, University of South Bohemia, 37005 České Budějovice, Czechia

Bacterium Sphingomonas glacialis AAP5 isolated from the alpine lake Gossenköllesee contains genes for anoxygenic phototrophy as well as proton-pumping xanthorhodopsin.

The photosynthetic complexes contain circular light harvesting complex 1 surrounding the type-2 bacterial reaction center. The light harvesting complex is composed from 16 homodimeric subunits. Each subunit binds one bacteriochlorophyll-a pair and one spirilloxanthin molecule. The purified xanthorhodopsin is present as a trimer. It contains carotenoid nostoxanthin serving as an auxiliary antenna and performs the standard photocycle. The xanthorhodopsin-producing cells reduced upon illumination their respiration by 70%. This documents that the harvested light energy was utilized in the metabolism, which can represent a large benefit under carbon-limiting conditions.

The presence of two different photosystems may represent a metabolic advantage in alpine lakes where photoheterotrophic organisms face large changes in irradiance, limited organic substrates and low temperature.

Preliminary cryoEM structure of photosystem in Sphingomonas AAP5

Kopejtka K, Tomasch J, Kaftan D, Gardiner AT, Bína D, Gardian Z, Bellas C, Dröge A, Geffers R, Sommarug, R, Koblížek M (2022) A bacterium from a mountain lake harvests light using both proton-pumping xanthorhodopsins and bacteriochlorophyll-based photoystems. PNAS 119(50), e2211018119. https://doi.org/10.1073/pnas.2211018119