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Rhodobacter sphaeroides ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO)-deletion strain 16 was capable of photoheterotrophic growth with acetate, while Rhodopseudomonas palustris RubisCO-deletion strain 2040 could not grow under these conditions. The reason for this difference lies in the fact that Rba. sphaeroides and Rps. palustris use different pathways for acetate assimilation, the ethylmalonyl-CoA pathway, and glyoxylate-bypass cycle, respectively. The ethylmalonyl-CoA pathway is distinct from the glyoxylate cycle as one molecule of CO2 and one molecule of HCO3- per three molecules of acetyl-CoA are co-assimilated to form two malate molecules. The glyoxylate cycle directly converts two acetyl-CoA molecules to malate. Each pathway, therefore, also dictates at what point, CO2 and reductant are consumed, thereby determining the requirement for the Calvin-Benson-Bassham reductive pentose phosphate cycle.