Figure 1. Schematic representation of the prevalence of nitrate reductase-encoding genomes within major phyla. 2476 genome sequences deposited in the Kyoto Encyclopedia of Genes and Genomes (Release 67.1; http://www.kegg.jp/)^29,30 were searched for the predicted presence of respiratory nitrate reductase activity (E.C.1.7.99.4). Phyla comprising thermophilic organisms (representing a total of 61 genomes) were excluded from this analysis, as these bacteria are unlikely to be present in the mammalian gut. Phyla commonly present in the gut-associated microbial community (ubiquitous phyla) and phyla present occasionally (rare phyla) are indicated in the inner circle, but the size of each sector is not proportional to their relative abundances. The prevalence of putative nitrate reductase activity (coloring of the outer circle) was calculated as the fraction of nitrate reductase-encoding genomes per total number of genomes within a given phylogenetic group.