US Air Force-funded researchers are investigating ways to produce large quantities of hydrogen gas using photosynthetic algae and cyanobacteria. The program—Renewable Bio-solar Hydrogen Production from Robust Oxygenic Phototrophs—is led by Dr. Charles Dismukes of Princeton University and involves researchers from seven colleges and universities plus the Air Force Research Laboratory, known collectively as the BioSolarH2 team.
The purpose of this research is to screen, study and genetically engineer microbes that can use light energy to split water and produce hydrogen in the presence of oxygen. While screening, the BioSolarH2 team looks for naturally-occurring, photosynthetic microbes whose hydrogen-generating enzymes, or hydrogenases, are more tolerant of oxygen.
Algae as a class seem to prefer a direct hydrogen production process in which sunlight, which normally carries out photosynthetic charge separation, occurs as usual but the photo-products are diverted into H2 production via a hydrogenase.
Cyanobacteria as a class seem to prefer an indirect dark process that follows the light-dependent photosynthetic stage. This process converts glycogen (strong C-H bonds) into smaller C molecules by glycolysis. Ultimately some of these molecules are oxidized fully into CO2 by fermentation. These processes produce an intracellular form of hydrogen (called NADH or NADPH) and the energy-rich molecule ATP (which can be converted to protons). These are combined in the cells via a hydrogenase to make H2 gas.
All three metabolic pathways are needed in a single multi-functional organism: growth by photosynthesis; respiration of glycogen; and anaerobic fermentation of C intermediates. The coordination/interference of these reactions and their inherent activities need to be optimized in order to attain practical yields of H2.
Team members have identified several good candidate microbes from the volcanic soda lakes of the Rift Valley in East Africa, the Great Salt Lake and Yellowstone National Park.
A key discovery has been the identification of cyanobacteria that have much higher metabolic rates of hydrogen production arising from the need to regenerate cellular energy (ATP) for survival in these harsh environments.—Charles Dismukes
The next step is studying the metabolic pathways that take place in the microbes to produce hydrogen gas. The team has developed fluorescence and electrochemical tools and bioreactors to measure the products and intermediate steps of these chemical reactions.
The BioSolarH2 team is also is using multiple strategies to manipulate the chemical reactions for increased hydrogen production. One involves the application of environmental stresses to accelerate the rate of the slow fermentation process so as to better match the natural diurnal cycle of the sun. For example, the researchers note, cyanobacteria that are stressed by osmotic shock via salt dilution can pump out H2 at 20-fold faster rates.
A newer approach involves knocking out the genes for hydrogenase-competing enzymes that consume the organic precursors essential to hydrogen production. This approach showed promise in its first trial with a twofold increase in hydrogen generation.—Dr. Walter Kozumbo, AFOSR program manager
Multiple other pathways are under investigation. A molecular based approach is also under investigation using computational chemistry to model the detailed atomic pathways that hydrogenases appear to use to produce H2. This knowledge may guide the construction of mutant hydrogenases with enhanced properties.
Air Force officials say that eventual applications resulting from this research may include biomimetic models for engineering synthetic generators that produce molecular hydrogen from water and light. Such a capability would produce clean energy that could lead to greater independence from fossil fuels.
In addition to Princeton, the team incudes researchers from the Colorado School of Mines; Georgia Institute of Technology; Michigan State University; Montana State University; Penn State University; University of Hawaii; and the Air Force Research Laboratory.
The Air Force Office of Scientific Research (AFOSR) funds the BioSolarH2 team as a part of the Multidisciplinary University Research Initiative (MURI), which focuses on research efforts that combine traditional science and engineering disciplines to address issues of importance to the Department of Defense.
The BioSolarH2 project received its initial MURI award in FY 2005; MURI program funding runs for up to five years.