Biogeochemically diverse organic matter in Alpine glaciers and its downstream fate

Abstract

Glaciers store and transform organic carbon, which, on release, could support downstream microbial life. An analysis of 26 glaciers in the European Alps suggests that a significant fraction of glacier organic matter is available for microbial consumption. Besides their role in the hydrological cycle1, glaciers could play an important role in the carbon cycle2,3,4,5,6. They store and transform organic carbon5,6, which on release could support downstream microbial life3. Yet the origin and composition of glacial organic carbon, and its implications for the carbon cycle, remain unclear. Here, we examine the molecular composition, radiocarbon age and bioavailability of dissolved organic matter (DOM) in 26 glaciers in the European Alps, using ultrahigh-resolution mass spectrometry, fluorescence spectroscopy and incubation experiments. We also measure carbon dioxide partial pressures in glacier-fed streams. We show that the glacier organic matter is highly diverse, and that a significant fraction of this material is bioavailable. Phenolic compounds derived from vascular plants or soil dominate, together with peptides and lipids, potentially derived from in situ microbial communities. Combustion products, in contrast, seem to contribute only marginally to the DOM sampled. We further show that organic matter bioavailability is positively correlated with in-stream carbon dioxide concentrations. We suggest that glacier-derived DOM contributes to downstream carbon cycling in glacier-fed streams. Our findings highlight the relevance of mountain glaciers for carbon cycling—a role that may change as glaciers recede.