Phylogenetic tree of archaeal amoA gene sequences. Data were obtained from clone libraries of genomic community DNA isolated from biological soil crust samples in this study (indicated by an asterisk *), as well as reference sequences from GenBank for species of amoA-encoding archaea that have been characterized. Without clustering, these sequences were combined and realigned with 3,619 high-quality sequences from GenBank (Fernandez-Guerra and Casamayor 2012). Dashed lines connect the identified sequences to the position in the tree. Bracketed clustering groups are based on designations at multiple phylogenetic levels within the Thaumarchaeota as proposed by Pester et al. (2012).