Correlation between nitrogen fixation rate and alginate productivity of an indigenous Azotobacter vinelandii from Iran

Azotobacter vinelandii, a gamma-proteobacterium, is an obligate aerobic free-living gramnegative soil bacterium capable of fixing nitrogen. Oxygen transfer rate into the cell is reduced by the increase of alginate concentrations during the course of A. vinelandii cultivation. This phenomenon provides a low intracellular oxygen concentration needed for nitrogenase activity. The aim of this study was to design a simple strategy to explain the alginate production, cell growth and nitrogenase activity correlation in A. vinelandii under aerobic conditions. Thirty-five different soil samples were taken from the rhizosphere of agricultural crops of Iran.Enrichment and isolation strategies were employed for microbial isolation. Physiological and biochemical characteristics were determined. Molecular identification was performed using selective nifH-g1 primers. Alginate production and nitrogenase activity assay by each isolate of Azotobacter were carried out. Bacterial growth, alginate production and Nitrogenase activitywere conducted by time-coursed quantitative measurements. Total of 26 isolates were selected after enrichment, isolation, and screening. The isolate was identified by molecular tests as A. vinelandii. The highest alginate productions of 1.02 g/l and 0.91g/l were noted after 4 days in 8 isolates, cell biomass of which were estimated 4.88-5.26 g/l. Six of 8 isolates were able to fix atmospheric N2 on nitrogen-free medium.Rates obtained in isolates were in the range of 12.1 to 326.4 nmol C2H4 h−1 vial−1. Nitrogen fixation and alginate production yielded significant and positive Pearson’s correlation coefficient of R2 = 0.760, p ~ 0.02. Finally association between bacterial growth, alginate production and nitrogenase activity almost noticeable yielded significant and positive Pearson’s correlation coefficient R2 = 0.723, p ~ 0.04.