Ecotoxicological evaluation of the tolerance of some plant growth-promoting rhizobacteria to environmental metal pollutants

Abstract

The major N2-fixing systems can play a significant role in improving the fertility and productivity of low-N soils. The plant growth-promoting rhizobacteria (PGPR) have received the most attention and have been examined extensively. Under heavy metal stress, soil microorganisms including PGPR have developed many strategies to evade the toxicity generated by the various heavy metals. In the present study, some environmental factors such as heavy metals (Cd2+, Pb2+, Cu2+, Zn2+), Al3+ were studied at different concentrations (0, 20, 40, 80 and 160 μm) on the ecophysiological properties and their growth using microfermentor technique as well as their tolerances to various hydrogen ion concentration. The results showed that the stress factor suppresses the growth characteristics of the studied PGPR including symbiotic Sinohizobium strains related to R. leguminosarum, R. phaseoli, R. trifolii, and R. loti as well as Sinorhizobium meliloti in comparison with non- symbiotic N2-fixer Azotobacter and Pseudomonas as PGPR strains which are differentially react with the stress factors at high concentrations. However, the tolerance of several investigated strains, distributed among Sinorhizobium meliloti strains: GH-130, GHR-94, GH-97 and GH-07 are sensitive to the stress of 160 μm for heavy metals effects in comparison with Azotobacter and Pseudomonas strain. None of the strains can tolerate 160 μm of Cd2+, Pb2+and Cu2+. The most toxic metal was Cd2+, followed by Pb2+and Cu2+. The ecophysiological tolerant Sinorhizobium and Azotobacter as well as Pseudomonas putida strains are suggested to be the ideal solution for the improvement of soil fertility and the rehabilitation of reclaimed soils and are an important direction for future soil protection and quality research. Hence, a better understanding of N2-fixing bacteria ecophysiological responses to different intrinsic stresses factor is very important to improve crop production by harnessing the growth of the N2-fixing bacteria process.

The major N2-fixing systems can play a significant role in improving the fertility and productivity of low-N soils. The plant growth-promoting rhizobacteria (PGPR) have received the most attention and have been examined extensively. Under heavy metal stress, soil microorganisms including PGPR have developed many strategies to evade the toxicity generated by the various heavy metals. In the present study, some environmental factors such as heavy metals (Cd2+, Pb2+, Cu2+, Zn2+), Al3+ were studied at different concentrations (0, 20, 40, 80 and 160 μm) on the ecophysiological properties and their growth using microfermentor technique as well as their tolerances to various hydrogen ion concentration. The results showed that the stress factor suppresses the growth characteristics of the studied PGPR including symbiotic Sinohizobium strains related to R. leguminosarum, R. phaseoli, R. trifolii, and R. loti as well as Sinorhizobium meliloti in comparison with non- symbiotic N2-fixer Azotobacter and Pseudomonas as PGPR strains which are differentially react with the stress factors at high concentrations. However, the tolerance of several investigated strains, distributed among Sinorhizobium meliloti strains: GH-130, GHR-94, GH-97 and GH-07 are sensitive to the stress of 160 μm for heavy metals effects in comparison with Azotobacter and Pseudomonas strain. None of the strains can tolerate 160 μm of Cd2+, Pb2+and Cu2+. The most toxic metal was Cd2+, followed by Pb2+and Cu2+. The ecophysiological tolerant Sinorhizobium and Azotobacter as well as Pseudomonas putida strains are suggested to be the ideal solution for the improvement of soil fertility and the rehabilitation of reclaimed soils and are an important direction for future soil protection and quality research. Hence, a better understanding of N2-fixing bacteria ecophysiological responses to different intrinsic stresses factor is very important to improve crop production by harnessing the growth of the N2-fixing bacteria process.