Remediation of wastewater through lead resistant plant growth promoting bacteria

Abstract

Industrial wastewater is a known source of irrigation for farming community around the metropolitans due to dissolved nutrients. However, it contains a variety of organic and inorganic pollutants that makes it unfit for crop production. Lead (Pb) is among the most toxic heavy metals present in wastewater. Certain bacterial strains have developed resistance against heavy metals which can be used to grow vegetables by utilizing wastewater as irrigation source. In present study forty Pb-resistant bacterial isolates were isolated from wastewater. The bacterial strains with more Pb resistance in plate assay were examined under broth assay and 10 out of them with more Pb-resistance, exopolysaccharide production, siderophore production were screened for their potential to improve growth of spinach seedlings in jar trial using lead nitrate Pb(NO3)2 as source of lead. The six most effective bacterial strains under jar trial were further tested in laboratory experiment for plant growth promoting characteristics such as phosphate solubilization, zinc solubilization, NH3 production assay, hydrogen cyanide production, indole acetic acid production, EPS production, effect of lead on bacterial growth, minimum inhibitory concentration and lead removal experiment. The four most effective strains N8, N11, N18 and N29 were used as inoculants for growing spinach by utilizing wastewater as irrigation source under pot trial. These strains were further evaluated under field conditions using spinach as a test crop. Results showed that lead contamination significantly decreased plant growth parameters at all levels but inoculation with Pb-resistant strains significantly improved plant growth parameters as compared to respective control. The Pb resistant bacteria decreased root and shoot lead uptake as compared to control plants. All the tested Pb- resistant strains i.e. N8, N11, N18, N25, N29 and N35 have multiple plant growth promoting traits. These strains have Pb removal potential, along with variable abilities to grow under metal stress. Under pot and field trials; wastewater affected plant growth, physiology, nutrients and metal uptake but Pb resistant bacteria improved these, in this way reduced the hazardous effects of wastewater. The phylogenic analysis confirmed these strains as Bacillus megaterium (N8), Bacillus safensis (N11), Bacillus sp. (N18), Bacillus sp. (N25), Bacillus megaterium (N29), and Bacillus subtilis (N35). The N11 and N18 proved to be the best strains which not only improved spinach growth but also decreased the Pd uptake by stabilizing it in the root zone. These strains can be a good source of inoculum for growing vegetables by utilizing wastewater as source of irrigation