Autotrophic Growth and Nitrogen Fixation in Derxia gummosa

Initial attempts to grow N2-fixing Derxia gummosa autotrophically on H2, C 0 2 , O2 and N2 in a closed system yielded variable results. Poor growth was found to be due to rapid O2 depletion and the requirement for an agar surface. In a closed system, C2H2 reduction assays could not be carried out due to complete consumption of H2. Hence a flow-through culturing technique was developed to supply gases at a constant partial pressure and to perform C2H2 reduction assays in a continuous flow system. Hydrogenase of autotrophic D. gummosa was not inhibited by C2H2, even at 0.5 atm, and the K,,, of hydrogenase for H2 was approximately 0.15 atm. The effects of O2 and H2 on C2Hz reduction were examined, using the flow-through assay system. The rate of C2H2 reduction decreased below 0.074 atm H2, suggesting that ATP and reductant supply were limiting the nitrogenase activity.