Effect of O2 concentration on growth, N2-fixing activity (ARA), and hydrogen evolution by Clostridium sp. strain B901-1b (A) and synergetic effects of separate cultures of B901-1b and B901-2 on N2-fixing activity (B). C and E, inoculation of Clostridium sp. strain B901-1b and Enterobacter sp. strain B901-2, respectively. Gray shading shows bacterial growth after 4 days. ARA+, positive for ARA (106 nmol of ethylene produced h−1 tube−1); −, no ARA (<0.1 nmol of ethylene produced h−1 tube−1).