Mass Spectrometric Detection and Analysis of Nitrogen Fixation in Oscillatoria chalybea

By means of mass spectroscopic measurements in an artifical gas atmosphere containing the stable nitrogen isotope ¹⁵N₂ we were able to demonstrate nitrogen fixation capacity in the filamentous cyanobacterium Oscillatoria chalybea. Our technique proved to be wellsuited also for investigations on the light-induced nitrogen fixation in the purple bacteria Rhodobacter sphaeroides and Rhodobacter capsulatus. Oscillatoria chalybea grown without combined nitrogen showed a substantial ¹⁵N₂-uptake which could clearly be correlated with nitrogen fixation. Nitrate grown cultures did not show this nitrogen uptake or only to a minimal extent. Addition of ammonium chloride resulted in a rapid deactivation of the nitrogenase system. Similar observations have been made with other so-called switch-off effectors like phenazine methosulfate. The structural integrity of the filaments appeared to be a prerequisite for nitrogen fixation also in this organism, as even mild mechanical homogenization strongly inhibited the N₂-uptake signals. Illumination of the assays under conditions where the photooxidition of water is not operational (Bader, K. P. (1994), Biochim. Biophys. Acta 1188, 213 -219) did not affect the nitrogen fixation in Oscillatoria chalybea. Illumination of cultures with concomitant release of oxygen from the water splitting reaction resulted in strong inhibition of ¹⁵N₂-uptake.