Subglacial and proglacial microbial communities in glacial rock flour of the Mont Blanc Massif

As alpine glaciers recede with global warming, proglacial forefields expand, and the processes of soil development take hold. The ecosystem transition toward greening is thought to be initiated by microorganisms that exert biotic weathering forces and accumulate carbon and nitrogen. A portion of the glacial sediments involved in this transition are classified as glacial rock flour. Glacial rock flour’s small particle size and large surface area suggest that it may offer a preferrable habitat and source of inorganic nutrients for microorganisms. However, microbial communities in glacial rock flour have yet to be reported. To investigate the microbial communities that colonize glacial rock flour, deposits were sampled near the melt streams of Mer de Glace and Glacier d’Argentière (Mont Blanc Massif, France). These glaciers flow over largely granitic bedrock. At both sites, three sampling points were selected with increasing distance from the glacier. At Glacier d’Argentière, three subglacial samples were collected off the basal ice surface. We hypothesized that characteristics of the glacial rock flour, such as median grain size or sampling distance from the glacier, would influence alpha diversity and abundance of the prokaryotic community. Laser particle size analysis, X-ray Diffraction (XRD), and geochemical extractions were completed to characterise the material. Quantitative polymerase chain reaction (qPCR) targeting the 16S rRNA gene and metabarcoding of the v3-v4 region of the 16S rRNA gene (rrs) were completed on DNA extracts to estimate prokaryotic abundance, probe taxonomic differences, and compute alpha diversity indices. A prokaryotic community was detected in all samples with a negative correlation evident between median particle size and prokaryotic abundance. Prokaryotic alpha diversity indices (Chao1, Shannon, Simpson) suggest that subglacial alpha diversity is greater than proglacial forefield alpha diversity. However, prokaryotes were less abundant in subglacial samples compared to proglacial samples. These results represent the first report of microbial communities in subglacial and proglacial glacial rock flour sediment.