Abstract |
The continually expanding interest for the use of hydrogen as fuel has
made renewable hydrogen production from green algae a research option
with important future applications. Photochemical production of
molecular hydrogen under anaerobic condition has first been documented
by Gaffron (1939). The next important milestone has been the the
manipulation of growth conditions in green algae cultures so as to
produce a greater amount of hydrogen (Melis et al, 2000).
In the present study the effect of various ion depletions (Ca, Cl, K, N,
Mg, S, P, Fe, Mn, Zn, Cu και Na) and light intensities (200 μmol m-2 s-1, 100 μmol m-2 s-1, 50 μmol m-2 s-1 and darkness) on hydrogen production has been studied on mixotrophic cultures of Scenedesmus obliquus.
The experiments that took place revealed that the depletion of the
elements K, Mn, Cu, Cl, S, Zn, Cu, P and Mg from the nutrient solution
leads to substantially elevated hydrogen production, with K deficiency
being responsible for the greatest amount of H2 measured.
Also, in the mixotrophic cultures of Scenedesmus oblicuus, light intensity
is inversely proportional to H2 production. The ideal light intensity for maximum H2 production was found to be 50 μmol m-2 s-1, where the Kdeficient
cultures produced 1,11 L H2/L PCV, as opposed to the Sdeficient
and control cultures which produced 0,64 L H2/L PCV and 0,5 L
H2/L PCV respectively.
The presence of glucose and secondarily the absence of K ions led to the
differentiation of the photosynthetic mechanism (decrease of PSII
reaction centers, increase of functional photosynthetic antennas, decrease
of photochemical quenching) which in turn led to the dramatic decrease
of photosynthetic efficiency and the increase of respiration. As a result,
anoxic conditions were established within 24 hours in sealed cultures,
resulting in the induction of hydrogenases and the subsequent production
of hydrogen.
Cultures deficient in K and S primarily utilized the NPQR pathway of
hydrogen production, in which PSI is essential. This was confirmed by
the substantial decrease of PSII/PSI rate in the aforementioned cultures as
compared to the control culture (complete culture medium with no
depletions).
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