Maximum-likelihood phylogenetic tree of the deduced amino acid sequences of nifH sequences obtained from GeoChip 3.0, showing the phylogenetic relationship among the five nifH clusters. The depth and width of each wedge is proportional to the branch lengths and number of nifH sequences, respectively. Some individual genes detected are shown in bold. The scale indicates the number of amino acid substitutions per site and the tree is outgroup rooted with Q8VW94 (Nitrosomonas sp. ENI-11).