Missing metabolic pathways revealed by the complete genome sequence of the uncultivated marine nitrogen-fixing cyanobacterium UCYN-A. (a) Key metabolic pathways typically found in nitrogen-fixing cyanobacteria. (b) Pathways found, as evidenced by gene content, in UCYN-A. Shaded-out areas with a red 'X' indicate pathways missing in UCYN-A. Cytb, cytochrome b complex; Fd, ferredoxin; FNR, ferredoxin-NADP reductase; hυ, light energy; PCox, plastocyanin oxidized; PCred plastocyanin reduced; PQ, plastoquinone oxidized; PQH2, plastoquinone reduced; PSI, photosystem I; PSII, photosystem II; TCA, tricarboxylic acid.