We have investigated two approaches to enhance and extend H-2 photoproduction yields in heterocystous, N-2-fixing cyanobacteria entrapped in thin alginate films. In the first approach, periodic CO2 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-2 production over time. Our results demonstrated that alginate-entrapped Delta hupL 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, Anabaena strain PCC 7120, its Delta hupL mutant, and Calothrix strain 336/3 films were supplemented with N-2 by periodic treatments of air, or air plus CO2. These treatments restored the photosynthetic activity of the cells and led to a high level of H-2 production in Calothrix 336/3 and Delta hupL cells (except for the treatment air plus CO2) but not in the Anabaena PCC 7120 strain (for which H-2 yields did not change after air treatments). The highest H-2 yield was obtained by the air treatment of Delta hupL cells. Notably, the supplementation of CO2 under an air atmosphere led to prominent symptoms of N deficiency in the Delta hupL 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-2 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.