Today, there is consensus that we live in the Anthropocene, a new geological epoch in which human-induced changes have a serious impact on the environment and our climate, especially in polar regions. To evaluate how Arctic terrestrial ecosystems are affected, we need bio-indicators to monitor past, present and future responses. The current project focusses on Arctic moss-associated (MA) diatoms, excellent sentinels for climate change and important components of microbial moss communities with short generation times, finely tuned environmental preferences and good preservation due to their siliceous cell walls. A morphology-based revision of Arctic MA diatom diversity and biogeography is required, because current information is incomplete and not suitable for meta-analyses, due to historical force-fitting and incorrect identifications. In this project, common Arctic moss genera will be sampled, and the first taxonomically fine-grained and consistent diatom survey will be made. The database will be used to evaluate the effect of environment, climate and geography on the distribution of Arctic MA diatoms. Diversity and geographic patterns in historic moss samples will be analyzed and compared to recent moss samples to reconstruct past environmental changes. It is expected that the diversity of Arctic MA diatoms is underestimated, harbouring an endemic component, and that recent climate change led to changes in the Arctic MA diatom communities.