Generation of hydrogen with photobacteria

Depletion of natural resources, high and unstable prices, andenvironmental concerns are causing many institutions and producers to search for substitutes for fossil fuels. Hydrogen seems to be the ideal solution: it has the highest energy content per unit mass of any conventional fuel; the only ’waste’ material created by its use is water; it can be generated from water; and it can be applied both in centralized and decentralized utility systems. However, at present, hydrogen is only produced at industrial scale using natural gas, oil, or coal.¹

Alternative approaches to hydrogen generation are based on renewable sources of energy. In photobiological methods, photosynthetic microorganisms can be applied to absorb solar energy and transform it into chemical energy. Two of these, cyanobacteria and algae, successfully decompose water to generate hydrogen, but their efficiency is low² due to fast inhibition of enzymes by evolved oxygen. Purple bacteria, however, may represent a more efficient method. Under photofermentative and anaeorobic conditions, purple bacteria transform organic compounds such as acids and alcohols into hydrogen and carbon dioxide (CO₂). The process is catalyzed by the enzyme nitrogenase, which consumes energy as adenosinotriphosphate (ATP). The theoretical conversion is close to 10° but, due to the presence in the cell of another enzyme called uptake hydrogenase that oxidizes part of the evolved hydrogen,³ the real values are much lower.

Photosynthetic purple bacteria are considered the best means of photobiological hydrogen production.⁴ These bacteria absorb light within the visible range, then transform the absorbed light photosynthetically into ATP. The organic substrate is oxidized into CO₂ in tricarboxylic acid. Electrons are transfered towards nitrogenase and, with the help of ATP, protons are reduced towards molecular hydrogen. No oxygen is evolved in thisprocess. The advantage of this process relies on application of organic wastes as the source of organic carbon. The final amount of hydrogen produced is influenced by the activity of uptake hydrogenase,³ which should be as low as possible. Highest yield of hydrogen was obtained at 30–37°C and with a high ratio of carbon to nitrogen in the medium.⁵

In our work with Rhodobacter sphaeroides O.U. 001 we optimized the age of the bacteria culture and initial conditions of the process (activation of nitrogenase and biomass increase). The most active culture appeared during exponential growth, between 12 and 24 hours. Optimization of the process, namely activation of the process and the age of the culture, allowed a ten-fold decrease in the elapsed time to hydrogen evolution.⁶

The amount of inoculum, initial pH values, and light intensity are significant factors as well. We established the influence of these factors on the yield of photogenerated hydrogen, CO₂, and biomass increase.⁷ All these results would be relevant to the construction of a photoreactor operating under natural (solar) irradiation.

In our experiments we applied different waste waters from the food industry (brewery,⁸ dairy,⁹ and excess sludge¹⁰) as the sources of organic carbon. We established the best conditions for hydrogen generation and their influence on the yield and the rate of evolved hydrogen. We obtained 2.3L of H2 in the medium containing 10° brewery waste and 2.9L of H2 in the medium containing 40° dairy waste. We also tested a cheap method of waste-water pretreatment. These tests showed that hydrogen generation can occur in the presence of small amounts of other bacteria, so very sterile conditions are not necessarily required. Finally, we tested the influence of ammonium ions (known inhibitors of nitrogenase that are often present in waste water) on the yield of photogenerated hydrogen.¹¹

Our results show that, under optimal conditions, hydrogen can be generated by purple bacteria with high yields. Our future work will concentrate on immobilization of the bacteria and on identifying the best supports. Moreover, we plan to establish a methodology for recovery of certain active biomaterials that are formed during hydrogen generation. This should result in improved efficiency of the process.