FIG. 7.

Phylogenetic tree showing the relationships of the AMBITION cruise environmental nifH sequences. Each AMBITION cruise environmental sequence is referred to by station number, depth of sampling, and clone (cl.) number. The tree was constructed by the neighbor-joining method with Jukes-Cantor correction based on partial nifH sequences (e.g., a 359-bp fragment amplified with the CNF-CNR PCR primer set used in this study). The percentages of bootstrap replicates supporting the branching order are indicated at the nodes. The cyanobacterium Symploca sp. strain PCC8002 was used as the root. The scale bar represents the equivalent of 0.1 substitution per nucleotide.