NITROGEN-FIXATION BY CYANOBACTERIUM MICROCOLEUS-CHTHONOPLASTES FROM HYPERSALINE LAGOONS OF LAKE SIVASH

The nonheterocystous filamentous cyanobacterium Microcoleus chthonopla stes fixes nitrogen in the light under aerobic conditions. The specifi c growth rate on a medium without bound nitrogen, under constant illum ination, is 0.009 h-1. The rate of oxygenic photosynthesis under such conditions is half the rate observed when nitrate or ammonium nitrogen is present in the medium. Nitrogenase activity appears 30 h after tra nsferring washed cells into a nitrogen-free medium. The rate of aerobi c nitrogen fixation in the dark is 5.5 times the rate in the light. Th e nitrogenase activity depends to a great degree on the salinity of th e medium; 5% salinity is optimal. Under anaerobic conditions, nitrogen fixation in the light is 3.2 times greater. The presence of hydrogen sulfide in the medium on a background of anaerobic conditions has a st imulating effect on nitrogen fixation. The maximum nitrogenase activit y of M. chthonoplastes is observed in the light when 0.57 mM hydrogen sulfide is present in the medium, this activity amounting to 3.7 mumol es C2H2/(Mg chl.h).