1 Cyanobacterial design cell for the production of hydrogen from water

Abstract

Due to energetic considerations, microalgae are becoming more and more popular for the production of biomass in general and the production of high value products in special applications. Major benefits of these organisms are the use of sunlight as a free energy source and the fixation of CO 2 by incorporation into reduced carbon compounds. Both can contribute to reducing atmospheric CO 2 (a "climate killer" gas) and also can be regarded as energy-rich storage compounds for periods of energy shortage. There are, however, also alternative strategies for converting sun energy directly into biofuels, i.e. high energy compounds, which are energetically and economically highly attractive: The classical route of energy storage via carbon compounds is extremely inefficient, ranging in most cases (due to metabolic constrains) from below 1 % and only in exceptional cases up to about 5 % efficiency , which is in contrast to the high efficiency of the primary (light-) reactions of photosynthesis [2,3]. For this reason, biofuel production has to be coupled as closely as possible to the light reactions of photosynthesis, combined with a re-routing of electrons which minimizes the steps of biofuel production and also the loss of electrons due to carbon fixation [4][5][6]. The attractive overall strategy is to receive the required electrons from the most abundant and cheapest compound available on earth -water -and to transfer them directly to the biofuel. The most useful and most direct-to-produce biofuel would be hydrogen [6], as it is a high energy compound that releases its energy by reaction with oxygen, producing as the only "waste" product again the starting compound water. The energy of this reaction can be used directly -for instance in a fuel cell -or can be easily stored for a longer time. By transferring the electrons primarily to protons instead of reducing CO 2 , this process is completely environmentally acceptable. In contrast, the release of energy stored in reduced carbon compounds inevitably leads to the production of CO 2 -besides the high energy loss due to the many metabolic steps involved.As this is a new concept which is against the principles of nature -it does not secure survival of the cell by storing energy but rather "wastes" the collected sun energy by setting free the high energy compound hydrogen -the metabolism of such a cell has to be modified in many individual steps. This is also a challenge from the bioenergetic point of view, since the amount of sun energy collected by a photosynthetic cell that can be harnessed for mankind is completely unknown. Such an "exploita-

Cyanobacterial Hydrogenases and Hydrogen Metabolism Revisited: Recent Progress and Future Prospects

“…Hydrogenases are integrated into the cellular network such that they can dispose of excess reductants to generate a cellular waste product “hydrogen” [ 2 ]. Among the various hydrogen-producing microbes, photosynthetic cyanobacteria share similarity to the organisms of the eukaryotic world in possessing two photosystems [ 3 , 4 ]. The presence of photosystem II (PSII) empowers oxygen evolution by splitting of water molecules using light energy.…”

“…At present, cyanobacteria and green algae are considered optimal candidates for hydrogen evolution, being the only known organisms that can both express functional hydrogenase and perform oxygenic photosynthesis [ 4 , 19 , 20 ]. The two organisms possess two distinct classes of hydrogenase; [NiFe] and [FeFe] hydrogenase respectively.…”