Scientists at the Energy Department's National Renewable Energy Laboratory (NREL) have demonstrated that just two of six iron-sulfur-containing ferredoxins in a representative species of algae promote electron transfers to and from hydrogenases. The finding suggests ways to increase the production of hydrogen by algae, which could help turn hydrogen into a viable alternative fuel for transportation.
Using sunlight and water to produce potential transportation fuels such as hydrogen is considered a promising solution in the quest for developing clean, abundant, domestic alternatives to petroleum.
A paper on the discovery, "Identification of global ferredoxin interaction networks in Chlamydomonas reinhardtii," appears online in The Journal of Biological Chemistry. The authors note that Chlamydomonas reinhardtii contains six chloroplast-localized ferredoxins (the iron-sulfur-containing redox mediators) whose exact functions are still unclear. C. reinhardtii often serves as a model for other algae strains because its genome is sequenced and it is amenable to genetic modification.
By analyzing the interacting partners and reactions catalyzed by each of the six ferredoxins (FDX), they found that FDX1 serves as the primary electron donor to hydrogen production via photosynthesis. FDX2 can do the job, but at less than half the rate, while FDX3 through FDX6 appear to play no role in this particular reaction.
In technical terms, the NREL scientists deconvoluted the complex network of redox reactions centered in the six iron-sulfur-containing algal ferredoxins. By revealing that only two of them promote electron transfer to and from hydrogenases, they helped extend the understanding of electron competition at the level of the ferredoxin.
"When we tested all the ferredoxins as electron donors, the best rate was obtained with FDX1," says NREL Scientist Alexandra Dubini, one of the authors for the paper. Lead authors are Erin Peden and Marko Boem, with contributions from NREL colleagues David Mulder, ReAnna Davis, William Old, Paul King, Maria Ghirardi and Dubini.
The discovery could lead to ways to stem the flow of electrons to the other pathways, forcing more electrons through the FDX1 pathway for increased hydrogen production, Dubini says. "There is this competition for photosynthetic reductant among different pathways and ferredoxins distribute electrons among the various other pathways, depending on the conditions and requirements of the cell."
Recent papers on the same green alga species indicate that it is possible to genetically eliminate certain competitive electron-utilizing pathways, and that directing more electrons instead towards the cell's hydrogenase does increase hydrogen production. In an industrial setting, green algal mutant strains optimized for hydrogen gas production would be cultivated in a sealed bioreactor and the hydrogen gas produced would be collected and stored for use in fuel cells.
Dubini says that day could be a long way off, noting that so far this is just fundamental science. "But by exploring all the different barriers to hydrogen production we are gaining a much better understanding of the functions of the ferredoxins and their involvement in hydrogen production – and that is very exciting," she adds.
The work was supported by DOE's Office of Science.