| Abstract |
The ability of green algae to produce molecular hydrogen photochemically, when grown under anaerobic conditions, was first discovered by Gaffron (1939). During the past 72 years many efforts were made in order to develop an economically viable way to produce bio-hydrogen from algae, based on the demands for more environment friendly energy solutions. Melis et al. (2000) discovered that sulfur (S) deprived cultures of the microalgae Chlamydomonas reinhartii, show greater hydrogen production, when grown anaerobically under light, compared to the control cultures. Furthermore, it was recently discovered that the unicellular green algae Scenedesmus obliquus was able to produce large amounts of hydrogen, during the biodegradation processes of meta-substituted dichlorophenols (dcps). The capacity of the reduced dichlorophenols (produced during the biodegradation processes) to enter and to take part into the electron transport chain, as electron donors before the photosystem I, deactivating at the same time the photosystem II (as well as the oxygen evolving system), allows a constant electron flow towards the photosystem I, and finally towards hydrogenase which produces huge amounts of hydrogen (Papazi 2009).
In this study, combining the microalgae’s ability to produce hydrogen through the dichlorophenol biodegradation mechanism, with a series of targeted nutrition depletions and different growth conditions (autotrophic or mixotrophic cultures with high concentrations of algae cells), a further increase on hydrogen production was achieved.
Our experimental data revealed that the factors with the more intense impact on the hydrogen production mechanism were, the depletion of the nitrogen source from the culture medium, the presence of 2,3-dcp, as well as the mixotrofic character of the culture (continuous glucose supply from the mother culture). The combination of all these three factors [-Ν+glc+dcp] affects dramatically the basic algae’s functions, with major alterations being; the increase of respiration, the depletion of the photosynthetically O2 production (anoxia) and finally the increase of the dichlorophenol’s biodegradation rate. The Scenedesmus obliquus cultures, grown under these conditions, led to the greatest H2 production ever reported of 14 L Η2/ L PCV, in contrast to the 2,5 L Η2/ L PCV produced solely by the 2,3-dcp’s action [Control+dcp] and to the 0,3 L Η2/ L PCV produced by the control cultures [Control].
The impressive increase in the hydrogen production was achieved by approaching the hydrogen production mechanism in an indirect way, mainly affecting the dichlorophenol’s removal rate. The continuous glucose supply from the mother cultures, combined with the nitrogen source depletion, which increase the fatty acids in the cell, induce the increase of dichlorophenol’s removal rate, quickly providing anaerobic conditions for the hydrogenase’s proper function and finally extra energy stocks for the microalgae. This fact leaves room for further investigation of the hydrogen production mechanism, having as main target the increase of the production through a real green biotechnology approach.
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