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

Biological soil crusts (BSC) are key components of ecosystem productivity in arid lands and they cover a substantial fraction of the terrestrial surface. In particular, BSC N-fixation contributes significantly to the nitrogen (N) budget of arid land ecosystems. In mature crusts, N-fixation is largely attributed to heterocystous cyanobacteria, however, early successional crusts possess few N-fixing cyanobacteria and this suggests that microorganisms other than cyanobacteria mediate N-fixation during the critical early stages of BSC development. DNA stable isotope probing (DNA-SIP) with N revealed that and are the most common microorganisms that assimilate N in early successional crusts. The identified are divergent from previously characterized isolates, though Nfixation has previously been observed in this family. The Proteobacteria identified share >98.5 %SSU rRNA gene sequence identity with isolates from genera known to possess diazotrophs (e.g. , , , and ). The low abundance of these heterotrophic diazotrophs in BSC may explain why they have not been characterized previously. Diazotrophs play 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 BSC in arid ecosystems.