|The H2 yield increases when Calvin cycle flux is blocked by mutation (black bars; white bars represent the parent). Source: McKinlay and Harwood. Click to enlarge.|
Reducing the ability of certain bacteria to fix carbon dioxide can greatly increase their production of hydrogen gas, according to a open access paper by Caroline Harwood and James McKinlay from the University of Washington, Seattle, in the current issue of online journal mBio.
Phototrophic bacteria, including purple nonsulfur bacteria (PNSB) such as Rhodopseudomonas palustris, obtain energy from light and carbon from organic compounds during anaerobic growth (photoheterotrophy). Cells can naturally produce hydrogen as a way of disposing of excess electrons. Hydrogen is an obligate product of the nitrogenase reaction, which is better known for converting N2 gas to NH3.
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
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