The multiannual ALCHEMI program consists in estimating the role of the Arctic snow on ecosystem functioning in the context of change. This goal will be reached by addressing the sources and both temporal and spatial variations of biotic and abiotic inputs and will focus on aspects related to microbial colonization, environmental selection, community evolution and stability and biodiversity in addition to nutrient deposition and transformations in the snow and transition zones, i.e. the air-snow interface, the snow-soil interface, the snow-ice interface and meltwater run-off. How these components are connected and interrelated will be examined through both field and laboratory experiments using state of the art analytical techniques in microbiology and chemistry. Different types of snow environments will be studied throughout the duration of the program: coastal sites, sea-ice and glaciers. With the ALCHEMI program, we will increase the knowledge of the relative importance of these external inputs and internal transformations to generate a positive feedback between microbial activity and biogeochemical cycling, which is of crucial importance to understand biological activity in Arctic systems.