FIG 1 .

R. palustris metabolism. Cells for the experiments described in this article were grown anaerobically with light as the sole energy source and with acetate as the sole carbon source. Nitrogen gas (N₂) or ammonia (NH₃) was supplied as the nitrogen source. Under these conditions, a proton gradient generated by cyclic photophosphorylation provides energy for ATP synthesis. Acetate is used to support the production of cell biomass, and some electrons are diverted from acetate metabolism to nitrogenase (N₂ase) for use in fixing N₂ to NH₃. In addition, some CO₂ is released during acetate metabolism. In this work, cells were supplied with a growth-limiting amount of acetate, such that cells entered a growth-arrested state when all available acetate was exhausted. When in this state, R. palustris cells continue to generate ATP from light.