Isolation and characterization of nitrogen fixing heterotrophic bacteria from the rhizosphere of pioneer plants growing on mine tailings

Mine tailings are extreme environments containing high concentrations of heavy metals and deficient in nitrogen and organic matter. However, some plant species tolerant to heavy metals are capable of growing on mine tailings. In this environment, it is expected that biological nitrogen fixation is the major source of nitrogen available for plants. In this work, three different nitrogen-free culture media were used to isolate nitrogen-fixing bacteria associated with nine plant species growing on mine tailings in Zacatecas, Mexico. Acetylene reduction assays (ARA) with and without heavy metals were carried out, as were the determination of minimum inhibitory concentration (MIC) of Ni, Cu, Co, Cr and Zn, and plant growth promotion (PGP) activities of each bacterial isolate. The major nitrogen-fixing bacteria distributed among rhizospheres of Juniperus sp., Aster gymnocephalus, Gringelia sp., Lygodesmia sp., and Haplopappus sp. were members of the genus Paenibacillus. However, Azospirillum lipoferum and Bradyrhizobium japonicum were also isolated from the rhizosphere of Haplopappus sp., and Viguiera linearis. Functionally, Paenibacillus durus BR_30 had the highest ARA and PGP activities, but was highly sensitive to heavy metals. However, Paenibacillus graminis BR_35 and Paenibacillus borealis BR_32 maintained a significant ARA activity in the presence of relatively high concentrations of Ni and Zn. Also, B. japonicum KYR_C5 and A. lipoferum showed moderate ARA and PGP activities. The majority of the isolates grew in moderate concentrations of heavy metals but ARA activities were inhibited at low Co and Cr concentrations. The diversity of nitrogen-fixing bacteria in the rhizosphere of pioneer plants may contribute to their adaptation to toxic stress and nutrient-deficiency in environments such as mine tailings.