A new technique for immobilizing H-2-photoproducing green algae within a thin (<400 mu m) alginate film has been developed. Alginate films with entrapped sulfur/phosphorus-deprived Chlamydomonas reinhardtii, strain cc124, cells demonstrate (a) higher cell density (up to 2,000 mu g Chl mL(-1) of matrix), (b) kinetics of H-2 photoproduction similar to sulfur-deprived suspension cultures, (c) higher specific rates (up to 12.5 mu mol mg(-1) Chl h(-1)) of H-2 evolution, (d) light conversion efficiencies to H-2 of over 1% and (e) unexpectedly high resistance of the H-2-photoproducing system to inactivation by atmospheric O-2. The algal cells, entrapped in alginate and then placed in vials containing 21% O-2 in the headspace, evolved up to 67% of the H-2 gas produced under anaerobic conditions. The results indicate that the lower susceptibility of the immobilized algal H-2-producing system to inactivation by O-2 depends on two factors: (a) the presence of acetate in the medium, which supports higher rates of respiration and (b) the capability of the alginate polymer itself to effectively separate the entrapped cells from O-2 ill the liquid and headspace and restrict O-2 diffusion into the matrix. The strategy presented for immobilizing algal cells within thin polymeric matrices shows the potential for scale-up and possible future applications. Biotechnol. Bioeng. 2009;102: 50-58. (C) 2008 Wiley Periodicals, Inc.