Autotrophic growth and nitrogen fixation in Derxia gummosa

1 Department of Microbiology, G08, Biochemistry Building, University of Sydney, New South Wales, Australia 2006 2 Department of Agricultural Chemistry, University of Sydney, New South Wales, Australia 2006 ABSTRACT Summary: Initial attempts to grow N 2 -fixing Derxia gummosa autotrophically on H 2 , CO 2 , O 2 and N 2 in a closed system yielded variable results. Poor growth was found to be due to rapid O 2 depletion and the requirement for an agar surface. In a closed system, C 2 H 2 reduction assays could not be carried out due to complete consumption of H 2 . Hence a flow-through culturing technique was developed to supply gases at a constant partial pressure and to perform C 2 H 2 reduction assays in a continuous flow system. Hydrogenase of autotrophic D. gummosa was not inhibited by C 2 H 2 , even at 0·5 atm, and the K m of hydrogenase for H 2 was approximately 0·15 atm. The effects of O 2 and H 2 on C 2 H 2 reduction were examined, using the flow-through assay system. The rate of C 2 H 2 reduction decreased below 0·074 atm H 2 , suggesting that ATP and reductant supply were limiting the nitrogenase activity.