Dinitrogen fixation by Azotobacter chroococcum and its isocitrate dehydrogenase activity: A quick method for screening nitrogen fixing strains.

Abstract

Nitrogenase activity in the rhizosphere of sugarcane (Saccharum officinarum Linn.), and its associated soil was measured by reduction of acetylene. Zero order kinetics and first order kinetics of acetylene reduction (C2H2---*C,H4) were used to describe the activity of N2 fixers from these soils. Arranged in descending order of nitrogen-fixing (acetylene-reducing) potential, the sites examined were highland (arable), mesic meadow, peat polygon trough, transition zone between mesic meadow and peat polygon trough, lowland (flooded with water) and virgin flooded land. On most of the land, sugarcane has been under cultivation for centuries. In total, 9 acetylene reducers were isolated, the remaining being oligonitrophiles. In addition to using the saturation kinetic values to describe the activity of dinitrogen fixers, first order kinetics of several potential energy substrates were also compared. Carbohydrate concentrations (glucose, maltose, arabinose, mannose and raffinose) between 25 and 100 ,ug/ml were used since the MichaelisMenten plot indicates that between this range first order kinetics were measured. The results indicated that various carbohydrate additions had different effects on C2H2 reduction. Because of the relatively high temperature requirements for Azotobacter chroococcum development, tropical areas appear more favorable for a higher incidence of this bacterium. Optimal temperatures for acetylene reduction were between 25°-30°. Acetylene reduction was pH-dependent, the optimum occurring between pH 6.4-7.5. Nitrogenase activity was almost completely inhibited by air and anaerobic conditions, and was optimum at around p02 0.06 atm. The reduction of 2, 3, 5-triphenyl tretrazolium chloride to triphenyl