Calvin Cycle Flux, Pathway Constraints, and Substrate Oxidation State Together Determine the H ₂ Biofuel Yield in Photoheterotrophic Bacteria

Abstract

Photoheterotrophic bacteria, like Rhodopseudomonas palustris, obtain energy from light and carbon from organic compounds during anaerobic growth. Cells can naturally produce the biofuel H(2) as a way of disposing of excess electrons. Unexpectedly, feeding cells organic compounds with more electrons does not necessarily result in more H(2). Despite repeated observations over the last 40 years, the reasons for this discrepancy have remained unclear. In this paper, we identified two metabolic factors that influence the H(2) yield, (i) the route taken to make biosynthetic precursors and (ii) the amount of CO(2)-fixing Calvin cycle flux that competes against H(2) production for electrons. We show that the H(2) yield can be improved on all substrates by using a strain that is incapable of Calvin cycle flux. We also contributed quantitative knowledge to the long-standing question of why photoheterotrophs must produce H(2) or fix CO(2) even on relatively oxidized substrates.