FIG. 3.

Phylogenetic relationship of sequences from Arctic and Antarctic samples to sequences from closely related AOB from β-subclass Proteobacteria and environmental clones; accession numbers follow the isolate or clone name. Polar sequences are indicated in boldface type, sequences labeled “SCICEX” are from the Arctic, and sequences labeled “Palmer” are from the Antarctic (this study). Directly sequenced nitAB products are indicated by an asterisk (∗); the remaining sequences are from clones. The tree was constructed using 1,040 bp of the 16S rDNA sequence. Bootstrap values that were greater than 50 from 100 samplings are shown. The tree is unrooted and was constructed by using E. coli (accession number J01695) as the outgroup. The bar indicates a Jukes-Cantor distance of 0.05.