Metal tolerant bacteria with plant growth promoting traits isolated from mining areas

Abstract

Mineral exploitation, particularly mining, is an essential modern society activity, providing resources for crucial economic sectors, such as industry and agriculture. However, mining has disturbing effects on the local environment. Environments under influence of mining are often devoid of natural means of biotic regeneration. The aid of human intervention is required for the revegetation of these environments. Phytoremediation has proven to be a very promising technique for in situ rehabilitation of these areas. In this work we studied two different mining areas. An area of gold mining, contaminated with arsenic and a zinc mining area contaminated with cadmium and zinc. Native soil rhizobacteria from these areas have been isolated and genetically and functionally characterized. Tests to evaluate the capacity of these bacteria to promote plant growth, besides its resistance to metal(oids), were also performed. Genotypic characterization by partial 16S rRNA gene sequencing revealed that most of the bacterial isolates belong to α-Proteobacteria class with individuals representing the rhizobial genera Bradyrhizobium, Mesorhizobium and Rhizobium. Moreover, bacteria representing Bosea, Starkeya, Methylobacterium, Inquilinus and Labrys genera, and β-Proteobacteria class representants of Burkholderia, and Variovorax were also isolated. Most of the isolated species tested positive for inducing nodule formation on their respective host legume, through authentication test for nodulation capacity, even atypical nodule forming bacteria as Inquilinus sp., Labrys monachus and Variovorax paradoxus. Only isolates of Bosea, Methylobacterium and Starkeya genera did not nodulate their hosts. Interestingly Inquilinus sp. and Labrys monachus have induced nodule formation on Crotalaria spectabilis roots, and Variovorax paradoxus have induced nodule on Leucaena leucocephala roots. Although these bacteria are atypical genera normally found nodulating legume plants. The functional characterization of these bacteria showed their great ability to promote plant growth and resist metal(oid)s. In addition to in vitro resistance to Cd and Zn, Mesorhizobium sp. UFLA 01-765 strain confirmed its tolerance in contaminated soil, where it was able to establish symbiosis, promote plant growth and accumulate nitrogen when associated with L. leucocephala. In conclusion, native rhizobia from soil mining areas are promising tools to enhance revegetation of these soils.