Temperature and Salt Tolerance of Azospirillum spp. from Salt-Affected Soils in Brazil

Summary

Roots and rhizosphere soil of ten plant species from salt-affected soils in the arid northeast region of Brazil and from flooded, sea-water affected soils south of Rio de Janeiro were checked for presence of Azospirillum halopraeferens or bacteria of similar physiological properties. A. halopraeferens could not be detected in these samples. Among the 37 Azospirillum isolates, most were determined to be A. brasilense. Few strains belonged to A. lipoferum or had unusual characters. Two isolates were checked for DNA-DNA homology with type strains by the initial renaturation rate method. The biotin requiring strain 68 was found to be A. brasilense, strain 19 could not be assigned to any of the three species mentioned above. None of the isolates tested showed an optimum temperature of N₂-dependent growth at 41°C as A. halopraeferens does, but some isolates showed still significant acetylene reduction activity at 41°C. All strains tested were able to grow at this temperature on combined nitrogen, some of them with only slight decrease in doubling time. Unlike A. halopraeferens, none of the isolates tested showed better growth at increased NaC1 concentrations (0.25%), but most of them sustained N₂-dependent growth at 0.75% NaCl better than strains Sp7 and Sp59b. Strain 68 even tolerated 3% NaCl for N₂-fixation. Azospirillum strains isolated from salt-affected soils in Brazil were more temperature and salt tolerant with respect to nitrogen fixation than Azospirillum species from other habitats and thus seem to show some adaptations to their environment in this respect.