Chlamydomonas reinhardtii cc124 and Azotobacter chroococcum bacteria were co-cultured with a series of volume ratios and under a variety of light densities to determine the optimal culture conditions and to investigate the mechanism by which co-cultivation improves H-2 yield. The results demonstrated that the optimal culture conditions for the highest H-2 production of the combined system were a 1:40 vol ratio of bacterial cultures to algal cultures under 200 mu E m(-2) s(-1). Under these conditions, the maximal H-2 yield was 255 mu mol mg(-1) Chl, which was approximately 15.9-fold of the control. The reasons for the improvement in H-2 yield included decreased O-2 content, enhanced algal growth, and increased H(2)ase activity and starch content of the combined system. (C) 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.