Hydrogen

Frequently cited as the fuel of the future, hydrogen production, storage and utilization are being widely investigated. As a transportation fuel it presents a series of challenges in every link of the chain, from production to storage and distribution. Although having a low volu­metric energy density, hydrogen has the highest energy density per mass and the simple fact that its combustion generates almost only water and heat has seduced entire generations. "Yes, my friends, I believe that water will one day be employed as fuel, that hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable" (Verne). Cars that could run on water with minimal energy consumption have captured the imagination of many people and, not sur­prisingly, have inspired frauds like the almost magical conversion of saltwater into fuel using radiofrequency radiation, claimed by John Kanzius and broadcast live countrywide from Philadelphia, or the notorious "Stan — leyMeyer’s water fuel cell" to be used in an internal combustion engine, where a special device could split water giving an energy output sufficient to generate me­chanical energy for the vehicle with enough leftover to power a fuel cell that would provide more hydrogen and oxygen through water splitting. Considering that the combustion of hydrogen and oxygen regenerates water, both systems obviously defy the first and second laws of thermodynamics (Ball, 2007).

Despite the motivation behind these schemes, they touched upon the most limiting step in the development of the hydrogen fuel technology: production. In current industrial practice, hydrogen can be produced by pyrol­ysis, electrolysis or by steam reforming of hydrocarbons. The last is the dominant method, applied to fossil fuels, usually natural gas (methane). This makes hydrogen both expensive and unsustainable.

Hydrogen Bioproduction

Molecular hydrogen (H2) is the lightest gas possible. When released into the atmosphere it diffuses quickly toward the troposphere, thus, at the sea level it can only be found in trace amounts. For this reason, very lit­tle naturally occurring H2 is available and therefore a sustainable production system must be found if this molecule is to be used as a fuel. Efficient biological production of hydrogen could represent a breakthrough in the development of this energy carrier and many different approaches are being followed toward this goal. Undoubtedly, among all the possible fuels that could be produced by cyanobacteria, it is hydrogen that has received the most attention. Here we discuss the biological mechanisms for hydrogen production and advances toward yield improvements in cyanobacteria.

In the light reactions of photosynthesis, light is captured by photosystems I and II, acting together to transform solar energy into chemical energy, splitting water into molecular oxygen and protons (H+) and the reducing agent NADPH. The transmembrane proton gradient that is formed is used by ATP synthase to combine adenosine diphosphate (ADP) + Pi into ATP (Figure 22.1). This set of reactions is rather interesting because it effectively conserves ubiquitous solar energy in energy-dense molecules using an abundant substrate, water. Ironically, cyanobacteria (and all plants) had been all along for millions of years the very sought after solu­tion for breaking the strong bond between oxygen and hydrogen in the water molecule without using the spe­cial radiofrequency of John Kanzius or the mysterious fuel cell of Stanley Meyer.

During the water-splitting process, oxygen is released in its molecular form (O2), while hydrogen, in the form of protons, is further used to produce two molecules of high-energy content: ATP and NADPH. Together, they feed energy into the Calvin-Benson-Bassham cycle, where CO2 is fixed into organic molecules, as well as into many other reactions related to cellular homeostasis or secondary metabolism. Alternatively, before it is used to generate NADPH, the high-energy electron generated by photosynthesis can be directly used for the evolution of hydrogen, a process called direct bio­photolysis (Benemann and Weare, 1974). Therefore, hydrogen evolution through this route does not require CO2 fixation, and solar energy and water, together with the required enzymes, are sufficient for H2 formation (Hallenbeck and Benemann, 2002). The major problem with this process is that hydrogenases, the hydrogen — evolving enzymes, are extremely sensitive to oxygen (O2) and are irreversibly inactivated by even small concentrations of this gas. Thus, hydrogen evolution is usually a short-lived process, with a burst of hydrogen evolution when transitioning from a dark cycle into light as increasing oxygenic photosynthesis quickly inactivates the hydrogenase. Some species, especially filamentous ones (e. g. Anabaena sp. and Nostoc sp.), capable of forming specialized cells called heterocysts, can be shown to produce hydrogen over prolonged periods in light, as the heterocysts provide an oxygen-free environment that protects the hydrogenase against inactivation. In indirect biophotolysis, the captured light energy is used to fix CO2 and the organic molecules that are pro­duced are stored as reserve material. Under normal conditions, part of these carbon reserves will be oxidized over the dark period to maintain cellular ho­meostasis. However, under proper conditions such a culture can be induced to produce hydrogen, thus separating hydrogen evolution temporally and spatially from the oxygen evolved by oxygenic photo­synthesis (Hallenbeck, 2011). Thus, hydrogenase activ­ity is maintained and the simultaneous production of hydrogen and oxygen, an explosive mixture when concentrated in the headspace of a bioreactor, is avoided.