Photobiological Production of Hydrogen A researcher works on a novel way to use green algae to produce hydrogen directly from water and sunlight. PIX14581 Highlights What do these things have in common: a few minutes’ lag before algae can Certain algae and cyanobacteria start making sugar from photosynthetic energy; narrowing passageways photoproduce hydrogen for short within an enzyme; and a Mexican restaurant in Denver? times as a way to get rid of excess energy before starting up the The answer is that they all are related in some way to efforts by National Renewable photosynthetic carbon fixation Energy Laboratory (NREL) scientists to develop technology for photobiological produc- process. tion of hydrogen —allowing the direct capture of renewable energy as an ideal, nonpol- luting, non-greenhouse-gas-producing transportation fuel. In one of nature’s quirks, NREL researchers have successfully certain algae and cyanobacteria photoproduce hydrogen for short times as a way to get developed a bacterial system for rid of excess energy. NREL researchers are seeking to turn that quirk into technology synthesis of a key enzyme— for directly converting solar energy to transportation fuel—by overcoming the oxygen- hydrogenase—that is responsible sensitivity trigger that shuts off the hydrogen production process in green algae, by doing for photosynthetic hydrogen the same for cyanobacteria, and by directing green algae toward the process by depriving evolution in green algae. them of the nutrition needed for normal photosynthesis. They are now focusing on enzyme Hydrogen use in fuel cells produces only water vapor and electricity at the point of use. engineering to block the access of Also, hydrogen can be stored to match energy production to energy demand. These make oxygen, which can stop hydrogen the use of hydrogen highly attractive as an energy carrier in addition to, or instead of, production, to the catalytic site of electricity. As an energy carrier rather than a basic energy source, however, hydrogen is the hydrogenase. only as “good” as the energy used to make it. Nearly all current U.S. and most of the Photobiological production of world’s hydrogen production involves steam reformation of natural gas. This, of course, hydrogen has potential to be one uses an increasingly scarce fossil fuel that can be used directly for electrical production of the most cost effective ways to to meet peak demand and also has home heating as a high-priority use. Hydrogen could produce hydrogen from renewable also be made by reforming gasified biomass or using renewably generated energy to energy. power water electrolysis. But in the long run, the highest efficiencies should be achieved with technologies such as photoelectrochemistry or photobiochemistry, which can produce hydrogen directly from solar energy. National Renewable Energy Laboratory Innovation for Our Energy Future Hydrogen is essentially an minutes in the carbon-fixing reaction energy carrier rather than a before light activates the enzymes for primary fuel. Although the carbon fixation. Electrons produced by most abundant element, the water splitting would harm or destroy it cannot be mined. It rarely the organism if the excess energy were not exists in elemental form, somehow dissipated. The algae evolved an instead being bound up in alternative second reaction that combines water or other compounds. the protons and electrons to form hydro- It does, however, some- gen molecules, thus getting rid of the times exist in nature, most excess energy. It is this temporary alterna- notably within certain tive process—which normally only lasts algae and photosynthetic for a few minutes—that we would like to bacteria that produce it as tap. Once enough oxygen builds up from a way to dissipate excess the water-splitting reaction, the algae are energy. If we can effectively forced to shut off hydrogen production manipulate those microor- and go to carbon fixation to the starch that Visualization of the oxygen-diffusion channels (red ganism processes, we can capture solar provides their food source. and yellow ribbons) in the hydrogenase enzyme energy directly in a form that is an ideal structure. The oxygen-sensitive FeS catalytic centers Thus, algal hydrogen production is natu- transportation fuel. NREL scientists are are indicated in green, yellow and red, and the rally inhibited by the presence of oxygen. backbone of the protein is shown in gray. researching these hydrogen production Overcoming that inhibition is a major focus processes to find ways to keep them turned of photobiological hydrogen production on instead of being only temporary, as they research. The algal hydrogen-production are naturally. Two main current research process is driven by enzymes known as paths involve taking advantage of the [Fe-Fe] hydrogenase, because of the hydrogen-production process in green presence of a unique 2Fe2S (iron and algae by either overcoming oxygen inhibi- sulfur) metallo-cluster in the catalytic tion of the process or by using nutrient center of the core of the proteins. deprivation to manipulate the algae into Researchers have found that the shut-off using the process. A third research avenue of hydrogen production is due to inhibi- seeks to overcome oxygen inhibition in a tion of the enzyme by oxygen. The inhibi- cyanobacterium. tion depends on oxygen physically Overcoming Oxygen Inhibition diffusing into the enzyme’s catalytic center in Green Algae and irreversibly binding to it, thus halting further catalytic activity. NREL scientists Algal photosynthesis and hydrogen and research partners have built computer production are sister processes. Both start models of the [Fe-Fe]-hydrogenase from with the same solar-energy-activated the bacterium Clostridium pasteurianum splitting of water to oxygen, (not photosynthetic, but with a structure electrons, and protons; protons If we can effectively manipulate those very similar to that of the hydrogenase of and electrons then go to a second the green alga Chlamydomonas reinhardtii enzymatic reaction. In one case, microorganism processes, we can upon which NREL research centers: the “normal” second reaction fixes Clostridium’s crystal structure is known; capture solar energy directly in a form carbon dioxide to produce sugar, Chlamydomonas’ structure has not yet been and in the other, an alternative that is an ideal transportation fuel. solved). This model allowed them to reaction produces hydrogen conduct highly sophisticated simulations molecules. Picture a population of gas movement in and out of the enzyme. of algae at the bottom of a pond at night. They found that, although hydrogen may The organisms have been respiring and move out of the enzyme by additional not photosynthesizing, so conditions have pathways, there are just two main ones by become largely anaerobic. When light first which oxygen molecules could make their hits, the water-splitting reaction starts up way into the enzyme. right away. But there is a lag time of a few With this knowledge, NREL researchers NREL’s development of this technique were able to model amino-acid substitu- has already progressed through several tions and other potential mutations advances. Initial operation with the algae and combinations of mutations along the in simple suspension could be maintained pathways to identify promising ones for for 3-4 days. The cycle was repeated at preventing oxygen from reaching the least three times by alternating with enzyme center. They then proceeded to periods of providing sulfate for normal actually enzyme engineer some of those photosynthesis to allow the algae to mutations, express them in the industrial provide energy for themselves and avoid bacterium E. coli (again, not itself photo- other problems from sulfur deprivation. synthetic) and then test the oxygen sensi- The next level of development used tivity and hydrogen productivity of the a chemostat-based system that cultivated recombinant enzymes. Thus far, although the algae with adequate nutrients in one one of many attempted mutations along reactor. The algae were then transferred one enzyme pathway yielded modest to a second, sulfate-deprived reactor improvement, others did not, proving in which they produced hydrogen. detrimental for both oxygen sensitivity Finally, in the continuous flow system, and hydrogen productivity. Hydrogenase the algae are used as feedstock for engineering efforts for the enzyme con- fermentative processes or gasified. tinue, as this holds great promise as the This system cut costs to one-third as key to overcoming oxygen inhibition. An much as simple batch processing and analysis projects that this could lead to hydrogen production with as high as 10% was run continuously for 6 months. efficiency in the conversion of solar energy. The next improvement came with Inactivating Algal immobilization of the algal cultures Photosynthesis by Nutrient on either glass fibers or alginate Deprivation films. Because the immobilized algae were no longer swimming around An alternative approach to sustaining or performing other functions, they algal hydrogen production artificially is needed less energy (algae live par- to partially inactivate the normal photo- tially off stored starch while in the synthetic process. NREL scientists have hydrogen-producing state) and the been able to do this by depriving an algal culture could be maintained for 25 culture of sulfate, which is necessary for days in batch operation. An alternate protein synthesis and particularly for the version of the immobilization tech- production of a key enzyme for photosyn- nique that provides enough nutrients thesis that has an especially fast turnover for minimal starch production, but time. As the sulfate is used up, photo- not enough to shut off the hydrogen synthesis slows. After about a day, photo- production, was operated continu- synthesis produces less oxygen than ously for 3 months. respiration consumes, and the culture becomes anaerobic and switches from Because nutrient deprivation inher- A set of bioreactors used for photobiological carbon fixation to a combination of ently limits the productivity of the algae, hydrogen production by green algae.