title: Morphological, biochemical and molecular characterization of twelve nitrogen-fixing bacteria and their response to various zinc concentration.
abstract: BACKGROUND
  Zinc is an essential micronutrient used in the form of zinc sulfate in fertilizers in the agriculture production system. Nitrogen-fixing microorganisms are also of considerable value in promoting soil fertility.
  
  
  OBJECTIVE
  This study aimed to investigate the degree of sensitivity to varying concentrations of zinc, in the form of ZnSO4, in different strains of Azotobacter chroococcum in a laboratory environment.
  
  
  METHODS
  To isolate A. chroococcum strains, soil samples were collected from wheat, corn and asparagus rhizospheres and cultured in media lacking nitrogen at 30˚C for 48 hours. Strains were identified based on morphological and biochemical characteristics. The presence of the nitrogenase enzyme system was confirmed by testing for the presence of the nifH gene using PCR analysis. The minimum inhibitory concentration (MIC) and optimal zinc concentration for the growth of each strain was determined.
  
  
  RESULTS
  A total of 12 bacterial strains were isolated from six different soil samples. A. chroococcum strains were morphologically and biochemically characterized. The presence of the nifH gene was confirmed in all the strains. MIC and the optimal zinc concentration for bacterial growth were 50 ppm and 20 ppm, respectively.
  
  
  CONCLUSIONS
  It was concluded that increasing the concentration of zinc in the agricultural soil is harmful to beneficial microorganisms and reduces the soil fertility. A 20-ppm zinc concentration in soil is suggested to be optimal.
authors.affiliation: Faculty of Biological Science, Islamic Azad University Tehran North Branch, Tehran, IR Iran.
authors.forename: Mohammad
authors.initials: M
authors.lastname: Dadook
authors.affiliation: Faculty of Biological Science, Islamic Azad University Tehran North Branch, Tehran, IR Iran.
authors.forename: Sedigheh
authors.initials: S
authors.lastname: Mehrabian
authors.affiliation: Faculty of Biological Science, Islamic Azad University Tehran North Branch, Tehran, IR Iran.
authors.forename: Mitra
authors.initials: M
authors.lastname: Salehi
authors.affiliation: Department of Cell and Molecular Biology, Faculty of Biological Sciences, Kharazmi University, Tehran, IR Iran.
authors.forename: Saeed
authors.initials: S
authors.lastname: Irian
keywords: Azotobacter; Zinc; nifH Gene