Figure 1.

Electron balancing processes used by R. palustris during diazotrophic, photoheterotrophic growth. During photoheterotrophic growth, R. palustris consumes carbon substrates and converts them into biomass precursors and CO₂ (‘central metabolism,’ arrows A and C, respectively). This process reduces cellular electron carriers (arrow B), which must be re‐oxidized to maintain intracellular redox homeostasis and thereby enable continued carbon substrate assimilation. The electron sinks that stabilize the cellular electron pool are re‐fixation of some of the CO₂ produced during substrate assimilation (arrow E), synthesis of certain biomass precursors (arrow D), and Nase activity (arrow F). To achieve redox homeostasis, substrate‐derived electrons must be fully allocated between biomass, H₂ (arrow I) and CO₂ (arrow H). The cellular electron pool can also be re‐oxidized by extracellular electron acceptors like dimethyl sulfoxide (DMSO, arrow G).