Anoxygenic phototrophs, like Rhodopseudomonas palustris, can convert light energy and electrons from organic waste into H-2 gas, a potential biofuel. During phototrophic growth on organic compounds, the CO2-fixing Calvin cycle competes against H-2 production for electrons. Here we address why genetically disrupting the CO2-fixing enzyme, ribulose 1,5-bisphosphate carboxylase (Rubisco), increases the H-2 yield but not the specific H-2 production rate. We hypothesized that remaining upstream phosphoribulokinase (PRK) activity negatively impacts growth and thereby the specific H-2 production rate, likely due to the accumulation of ribulose-1,5-bisphosphate, the substrate for Rubisco. In agreement with our hypothesis, deletion of PRK resulted in proportional increases to both the H-2 yield and the specific production rate. Thus, even though Rubsico is traditionally a more common target to eliminate Calvin cycle activity we propose PRK as a favorable alternative to avoid undesirable pleiotropic effects. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.