FIG 3.
Flow chart of nitrogenase in different metabolic backgrounds and histograms depicting the percentage of enzyme homologs putatively involved in Fd/Fld reduction identified in the genomes comprising a specified group. Aerobic/facultative anaerobic and anaerobic organisms are further classified as phototrophs or chemotrophs. Phototrophs in aerobes/facultative anaerobes are further classified based on whether they are oxygenic or anoxygenic phototrophs, while anaerobic phototrophs are classified as either purple sulfur/nonsulfur bacteria or green sulfur/nonsulfur bacteria. Importantly, it is not clear from surveys of the literature that anaerobic purple bacterial strains (denoted by an asterisk) were robustly tested for their ability to use O2. FNR, ferredoxin-NADP+ oxidoreductase; PFOR, pyruvate-flavodoxin oxidoreductase; Rnf, Rhodobacter nitrogen fixation protein; FeFe, iron-only hydrogenase; NiFe, nickel-iron hydrogenase; Fix, electron transfer flavoprotein involved in nitrogen fixation.