We have analyzed the filamentous cyanobacterium Anabaena PCC 7120 (wildtype) containing one nitrogenase, one uptake hydrogenase and one bidirectional hydrogenase and its hydrogen uptake deficient mutant AMC 414 for their H 2 production capacities. Anabaena PCC 7120 and AMC 414 had similar growth rates in turbidostat mode with increased growth rates at higher light intensity. Rates of C 2 H 2 reduction were similar for both strains. In contrast to the wildtype, AMC 414 produced H 2 in a PhotoBioReactor (PhBR) using air as the lifting gas. The rate of H 2 production increased with light intensity and was not even saturated at 456μEm - 2 s - 1 . H 2 production increased significantly when replacing the air with argon. The maximal H 2 production during outdoor conditions was recorded using AMC 414 with a peak at 14.9 ml H 2 h - 1 l - 1 . Despite the relatively high production, maximal efficiency of solar energy to H 2 conversion was only 0.042%. A molecular method was developed to analyze the relative abundancies of weight and mutant in competition experiments in the PhBR.