Strong regionality and dominance of anaerobic bacterial taxa characterize diazotrophic bacterial communities of the arctic plant species Oxyria digyna and Saxifraga oppositifolia

Arctic and alpine biomes developing in rocky areas are most often strongly nitrogen-limited, and hence biological nitrogen fixation is a strong driver of such biomes. On the basis of total community DNA, we here studied the nitrogen-fixing communities at two arcto-alpine pioneer plant species, Oxyria digyna and Saxifraga oppositifolia, using the nifH gene as a proxy. Replicate samples taken from two arctic regions, Kilpisjärvi and Ny-Ålesund, next to one alpine region, Mayrhofen, were included, to study a South-to-North cold-climate soil gradient. The data revealed strong regional effects on the dominating nitrogen fixers. As a major overall finding, anaerobic bacterial taxa dominated the diazotrophic bacterial communities of the two plant species. Specifically, Geobacter and Leptothrix spp. dominated the Mayrhofen and Kilpisjärvi regions, while members of the Clostridiales preferred the Kilpisjärvi and Ny-Ålesund regions. Endosphere communities were enriched with Goebacter and Clostridia, whereas the soil communities were dominated with Burkholderia and Leptothrix.