Non-cyanobacterial diazotrophs mediate dinitrogen fixation in biological soil crusts during early crust formation

Biological soil crusts (BSCs) are key components of ecosystem productivity in arid lands and they cover a substantial fraction of the terrestrial surface. In particular, BSC N 2 -fixation contributes significantly to the nitrogen (N) budget of arid land ecosystems. In mature crusts, N 2 -fixation is largely attributed to heterocystous cyanobacteria; however, early successional crusts possess few N 2 -fixing cyanobacteria and this suggests that microorganisms other than cyanobacteria mediate N 2 -fixation during the critical early stages of BSC development. DNA stable isotope probing with 15 N 2 revealed that Clostridiaceae and Proteobacteria are the most common microorganisms that assimilate 15 N 2 in early successional crusts. The Clostridiaceae identified are divergent from previously characterized isolates, though N 2 -fixation has previously been observed in this family. The Proteobacteria identified share >98.5% small subunit rRNA gene sequence identity with isolates from genera known to possess diazotrophs (for example, Pseudomonas , Klebsiella , Shigella and Ideonella ). The low abundance of these heterotrophic diazotrophs in BSCs may explain why they have not been characterized previously. Diazotrophs have a critical role in BSC formation and characterization of these organisms represents a crucial step towards understanding how anthropogenic change will affect the formation and ecological function of BSCs in arid ecosystems.