Cyanobacterial biofilms dwelling at the bottom of Arctic, sub-Arctic and high-alpine freshwa- ter ecosystems, such as lakes, ponds and streams, underpin local primary production, nutrient cycling and the entire food web in these Earth’s fastest-warming regions. Because these mi- crobial consortia reflect temperature, light and ionic shifts and archive those stresses in their pigment and macromolecular signatures, they offer a sensitive biological barometer of climate change.