Organic matter variability in algal dominated habitats on the Western Greenland Ice Sheet

Microbiological activity on glacier and ice sheet surfaces can be a major factor responsible for their darkening. Among microbes, pigmented snow- and glacial ice-algae increase light absorption, further accelerating melting and supporting the development of pigmented algal blooms on the Greenland Ice Sheet (GrIS). The relationship between carbon-fixing algae and carbon-respiring heterotrophic microorganisms influences the amount and composition of organic matter (OM). Yet, the dynamics of the OM derived from these microbes on the GrIS remain unclear. To address this gap, we incubated algae-dominated snow and ice surface samples in situ in vented bottles under light and dark conditions. We evaluated the initial microbial community composition (via 16S and 18S rRNA gene sequencing) and characterized the changes in both dissolved and particulate OM (DOM and POM) via ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. We show that glacier ice-algae habitats dominated by Ancylonema, have higher abundance of highly unsaturated and aromatic compounds resistant to bio- and photo-degradation. In contrast, snow-algae habitats dominated by Chloromonas, are enriched in bioavailable and more photosensitive unsaturated aliphatics and sulfur- and phosphorus-containing compounds. Light exposure increased water-soluble DOM compounds derived from POM, which accounted for large proportion of the initial DOM composition of both algae dominated habitats. Of these initial DOM pools, up to 50% were heterotrophically degraded in the dark, while light alone photodegraded less than 20%. The significant accumulation of light-absorbing aromatics from both POM and DOM pools at the end of the ice-algae experiments, emphasize ice-algae larger effect on altering glacier color compared to snow-algae, and thus on decreasing glacier albedo and accelerating melting.