초록
<P>We have investigated two approaches to enhance and extend H<SUB>2</SUB> photoproduction yields in heterocystous, N<SUB>2</SUB>-fixing cyanobacteria entrapped in thin alginate films. In the first approach, periodic CO<SUB>2</SUB> supplementation was provided to alginate-entrapped, N-deprived cells. N deprivation led to the inhibition of photosynthetic activity in vegetative cells and the attenuation of H<SUB>2</SUB> production over time. Our results demonstrated that alginate-entrapped Δ<I>hupL</I> cells were considerably more sensitive to high light intensity, N deficiency, and imbalances in C/N ratios than wild-type cells. In the second approach, <I>Anabaena</I> strain PCC 7120, its Δ<I>hupL</I> mutant, and <I>Calothrix</I> strain 336/3 films were supplemented with N<SUB>2</SUB> by periodic treatments of air, or air plus CO<SUB>2</SUB>. These treatments restored the photosynthetic activity of the cells and led to a high level of H<SUB>2</SUB> production in <I>Calothrix</I> 336/3 and Δ<I>hupL</I> cells (except for the treatment air plus CO<SUB>2</SUB>) but not in the <I>Anabaena</I> PCC 7120 strain (for which H<SUB>2</SUB> yields did not change after air treatments). The highest H<SUB>2</SUB> yield was obtained by the air treatment of Δ<I>hupL</I> cells. Notably, the supplementation of CO<SUB>2</SUB> under an air atmosphere led to prominent symptoms of N deficiency in the Δ<I>hupL</I> strain but not in the wild-type strain. We propose that uptake hydrogenase activity in heterocystous cyanobacteria not only supports nitrogenase activity by removing excess O<SUB>2</SUB> from heterocysts but also indirectly protects the photosynthetic apparatus of vegetative cells from photoinhibition, especially under stressful conditions that cause an imbalance in the C/N ratio in cells.</P>