Photosynthetic hydrogen production under light by the green microalga Chlamydomonas reinhardtii was investigated in a torus-shaped PBR in sulfur-deprived conditions. Culture conditions, represented by the dry biomass concentration of the inoculum, sulfate concentration, and incident photon flux density (PFD), were optimized based on a previously published model (Fouchard et al., 2009. Biotechnol Bioeng 102:232-245). This allowed a strictly autotrophic production, whereas the sulfur-deprived protocol is usually applied in photoheterotrophic conditions. Experimental results combined with additional information from kinetic simulations emphasize effects of sulfur deprivation and light attenuation in the PBR in inducing anoxia and hydrogen production. A broad range of PFD was tested (up to 500 mu mol photons m(-2) s(-1)). Maximum hydrogen productivities were 1.0 +/- 0.2mL H-2/h/L (or 25 +/- 5mL H-2/m(2) h) and 3.1 mL +/- 0.4H(2)/h L (or 77.5 +/- 10mLH(2)/m(2) h), at 110 and 500 mu mol photons m(-2) s(-1), respectively. These values approached a maximum specific productivity of approximately 1.9mL +/- 0.4H(2)/h/g of biomass dry weight, clearly indicative of a limitation in cell capacity to produce hydrogen. The efficiency of the process and further optimizations are discussed. Biotechnol. Bioeng. 2011; 108: 2288-2299. (C) 2011 Wiley Periodicals, Inc.