Effects of cobalt and manganese on biomass and nitrogen fixation yields of a free-living nitrogen fixer - Azotobacter chroococcum

The effects of different concentrations of cobalt and manganese on the biomass and the ability of Azotobacter chroococcum to fix nitrogen were investigated. In vitro trials were conducted in Jensen’s (nitrogen free) broth (half strength) under continuous air flow, incubated at ambient room temperatures for seven days. Results obtained showed that 12.5 mg/l, 25 mg/l, 50 mg/l, 100 mg/l, and 200 mg/l concentrations of cobalt and manganese respectively enhanced microbial growth of Azotobacter chroococcum concomitantly. However, nitrogen fixation was enhanced only at 12.5 mg/l and 25 mg/l concentrations for cobalt, and only at 12.5 mg/l concentration for manganese. Statistical analysis revealed no significant difference in the specific growth rates and nitrogen fixations respectively, between the cobalt and manganese trials. Kinetic modeling revealed that nitrogen fixation was associated with biomass concentration, and not with cell mass growth.