Surface melting supports the development of pigmented algal blooms on the Greenland Ice Sheet, decreasing albedo and further accelerating melting. The interplay between carbon-fixing algae and carbon-respiring heterotrophic microorganisms ultimately controls the amount and composition of organic matter (OM) and thus the ice and snow color. Yet, the dynamics of microbially-derived OM on the Greenland Ice Sheet remain unclear. To address this knowledge gap, we incubated in situ algae-dominated snow and ice samples under light and dark conditions and characterized the changes in dissolved and particulate OM (DOM and POM) with the help of ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. We show that glacier ice-algae habitats are dominated by highly unsaturated and aromatic compounds resistant to bio- and photo-degradation. In contrary, snow-algae habitats are enriched in bioavailable and more photosensitive unsaturated aliphatics and sulfur- and phosphorus-containing compounds. In both habitats, light exposure increased water-soluble DOM compounds derived from POM, which accounted for ~ 50–70% of the initial DOM composition. Of the initial DOM, 35–50% were heterotrophically degraded in the dark, while light alone photodegraded 6–16%. The significant accumulation of light-absorbing aromatics from POM and DOM at the end of the ice-algae experiments, underscore the greater impact of glacier ice-algae habitats on altering glacier color and accelerating melting.
