| Summary: | Most glaciers in Svalbard (High Arctic) have been retreating and thinning since the end of the Little Ice Age in the late 19th century. As a glacier retreats, it exposes new terrestrial habitats for the colonization by pioneering (micro)organisms. Here we report on the successional trajectories of cyanobacterial communities along a 100-year deglaciation gradient in the Ebba- and Hørbyebreen glacier forefields (Petunia Bay, central Svalbard). Cyanobacterial biomass and community composition were evaluated by epifluorescence microscopy and pyrosequencing of partial 16S rRNA gene sequences. Pseudanabaenales was the most abundant order in both forefields, followed by Chroococcales, Oscillatoriales, Synechococcales, Nostocales and Gloeobacterales. Succession was characterized by a decrease in phylotype richness and a marked turnover in community structure, resulting in a separation between initial (10–20 years since deglaciation), intermediate (30–50 years), and advanced (80–100 years) communities. Community turnover was explained by a combination of temporal and environmental factors, which accounted together for 46.9% of the variation in community structure. Interestingly, phylotypes associated with initial communities were related to potentially novel taxa (i.e. <97.5% similar to sequences currently available on GenBank) and sequences predominantly restricted to polar biotopes, suggesting that the initial colonization is performed by cyanobacteria from glacial and periglacial habitats. Advanced communities, on the other hand, included genotypes with a wider geographic distribution, which are likely able to establish only after the microenvironment has been modified by pioneering taxa. Cyanobacterial communities in retreating glacier forefronts in Polar Regions
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