Metagenomes reveal purple non-sulfur bacteria linked to bare ice habitats on the Greenland Ice Sheet

Phototrophic organisms blooming during the summer melt season on snow and ice surfaces are dominated by eukaryotic green algae (Chlorophytes and Streptophytes, respectively), with Cyanobacteria restricted to cryoconite habitats. However, the role and interactions between the algae and other light-harvesting organisms are largely understudied in these ecosystems.We searched metagenomes of snow and ice samples collected from the Greenland Ice Sheet during the summer melting season for signatures indicating anoxygenic photosystems, which are used by certain groups of bacteria to gain energy from light without releasing oxygen. Two metagenome assembled genomes (MAGs) carrying all genes necessary to perform anoxygenic photosynthesis and carbon-fixation were found. Whole-genome phylogenetic comparison placed the MAGs within the Alpha- and Gamma-proteobacteria, which was confirmed by alignment of the respective functional marker genes pufL and pufM to known sequences from cultured anoxygenic phototrophic bacteria. The identified functions and phylogeny suggest that the MAGs belong within the group of purple non-sulfur bacteria, a pigmented and metabolically versatile group of bacteria often found in shallow aqueous environments, but very little is documented in cryogenic environments. Our data show that these procaryotic organisms are preferably linked to glacial ice algae habitats and, to a lesser extent, to algae in snow habitats. Our results pose intriguing ecological questions for supraglacial habitats, such as the contribution of these procaryotes to the ongoing biological darkening of ice surfaces or the potential mutualistic light-harvesting strategies on ice, as the used wavelength of purple non-sulfur bacteria are complementary to those used by indigenous high abundant glacial ice algae.