However, feeding these bacteria more electron-rich organic compounds does not always produce the logically expected result of increased hydrogen production.

Harwood and McKinlay analyzed metabolic functions of R. palustris grown on four different compounds to better understand what other variables might be involved.

One factor involved appears to be the Calvin cycle, a series of biochemical reactions responsible for the process known as carbon dioxide fixation. The Calvin cycle converts carbon dioxide and electrons into organic compounds. Therefore, carbon dioxide-fixation and hydrogen production naturally compete for electrons.

When Harwood and McKinlay tested a strain of the bacterium which had been genetically modified to block carbon dioxide-fixation, they observed an increased output of hydrogen from all four substrates.

The Calvin cycle was not the only variable affecting hydrogen production that Harwood and McKinlay identified in the paper. They also determined that the metabolic route a growth substrate took on its way to becoming a building block for making new cells also played a role.

In this paper, we identified two metabolic factors that influence the H₂ yield, (i) the route taken to make biosynthetic precursors and (ii) the amount of CO₂-fixing Calvin cycle flux that competes against H₂ production for electrons. We show that the H₂ 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 H₂ or fix CO₂ even on relatively oxidized substrates.

—McKinlay and Harwood

Resources

• James B. McKinlay and Caroline S. Harwood (2011)Calvin Cycle Flux, Pathway Constraints, and Substrate Oxidation State Together Determine the H₂ Biofuel Yield in Photoheterotrophic Bacteria. mBio 2:2 e00323-10 doi: 10.1128/mBio.00323-10