Various different strategies have been proposed to harness solar energy efficiently. Photovoltaics directly translate solar energy into an electric current. Solar thermal collectors use the heat from solar energy to create thermal energy. Another branch of technology uses the electricity generated from photovoltaics to perform hydrolysis—the process of splitting a water molecule into hydrogen gas and oxygen. The resulting hydrogen gas can be burned as a clean alternative to the more common carbon based fuels. Hydrolysis, however, is an energy intensive endeavor and currently the cheapest way to perform it is to burn fossil fuels. Recent research into the various processes of performing hydrolysis has analyzed the potential of using organisms’ natural pathways of creating hydrogen gas. In particular, Anabaena sp. PCC 7120 possesses biological pathways that create hydrogen gas when starved of nitrogen in their culture medium. Under nitrogen starvation, the bacteria begin producing specialized cells called heterocysts that house special nitrogen fixing enzymes called nitrogenases. Nitrogenases react with atmospheric nitrogen to create ammonia and hydrogen gas. The process requires energy input in the form of the universal biological medium of energy, adenosine triphosphate (ATP), and can only function in the absence of oxygen. Two types of hydrogenases exist in Anabaena: uptake hydrogenase (Hup), which absorbs excess hydrogen and decreases hydrogen production, and bidirection hydrogenase (Hox), about which less is known, but is theorized to play a part in the oxidation of hydrogen and the creation of nutrition in conjunction with photosynthesis. During photosynthesis, carbon dioxide, absorbed from the ambient atmosphere, is broken down into carbohydrates and frees oxygen gas. The carbohydrates produced are essential to the creation of ATP whereas the production of oxygen gas inhibits the hydrogenases. This means that Hup, which absorbs hydrogen produced, will function at a lower level, increasing the hydrogen gas output of Anabaena. More ATP also translates to more nitrogenase action. Anabaena can thus create hydrogen gas in a nitrogen starved state through nitrogenases and hydrogenases. In an attempt to maximize the production of hydrogen gas, Marques et al. (2011) examine the effect of two hydrogenases on hydrogen production by comparing the hydrogen output of wild type Anabaena, mutants with no Hup (hupL–), mutants with no Hox (hoxH–), and mutants with neither Hup nor Hox (hupL/ hoxH–).—Alan Hu
Marques, AE., Barbosa, AT., Jotta, J., Coelho, MC., Tamagnini, P., Gouveia, L. 2011. Biohydrogen production by Anabaena sp. PCC 7120 wild-type and mutants under different conditions: Light, nickel, propane, carbon dioxide and nitrogen. Biomass and Bioenergy 35, 4426–4434.
Marques et al. from various research institutes in Portugal measured the hydrogen output of various strains of Anabaena under different environmental conditions. The Anabaena sp. PCC 7120 and its mutants were provided by Professor Sakurai at Waseda University. The specimen were grown in 500 ml flasks with medium under a constant air temperature of 25ºC and a set level of irradiance. Growth was evaluated over 42 days in terms of optical density and chlorophyll α content. Hydrogen production trials were conducted with all four cultures of Anabaena, were transferred to 120 ml glass bottles called photobioreactors (PBRs). Each PBR was filled with 30 ml of culture and various environmental variables such as gas atmosphere, light intensity, and nickel concentration were manipulated in various trials. Heterocysts were counted under a light microscope and the number of heterocysts per 100 vegetative cells was noted. Hydrogen production was measured with a gas chromatograph.
It was found that the HupL–strain under high light intensity produced the high levels of hydrogen. The researchers observed that light is the primary driver of ATP levels, the higher of which allows more nitrogenase to generate hydrogen gas. Researchers also observed that higher light intensity led to higher oxygen levels, which in turn increased hydrogen production levels by inhibiting the hydrogenases. Discontinuous light was found to have a positive effect on hydrogen production in all specimens except for the wild type. Increasing nickel concentration in the culture medium was also found to have increased hydrogen production across most of the specimens. The HupL– strain was again found to have the highest hydrogen production under increased carbon dioxide scenarios. Under a propane atmosphere, the HupL–/HoxH– produced the highest levels of hydrogen and heterocysts.
Marques et al. conclude that the HupL– mutant in general provided the best hydrogen production figures. However, it is admitted that current biohydrogen production rates are not high enough to be used on an industrial scale, and that more research needs to be done toward the subject.