Plant growth-promoting properties of metal-tolerant bacteria isolated from battery wastes dumpsites

Environmental pollution by heavy metals arising from several industrial processes is a major cause for concern globally as these metals disrupt the soil ecosystem and rendering metal-polluted soils unfit for use in agriculture. As such, it is essential to devise means to reclaim these soils to sustain the soil biodiversity and enhance plant growth. This study focused on the determination of plant growth-promoting features of metal-tolerant bacteria (MTB) isolated from metal-polluted soils. Composite soil samples were obtained from three different sites receiving battery wastes. Using atomic absorption spectrophotometry, concentrations of heavy metals (cadmium, nickel, lead, zinc) in the soil samples were determined. Nutrient agar augmented with 50 μg/mL salts of the metals were used to cultivate MTB. Recovered MTB were screened for metal tolerance (100-500 μg/mL). Isolates demonstrating elevated multiple metal tolerance were identified using biochemical tests and identity confirmed using 16S rRNA sequencing, followed by screening for presence of metal-resistance genes (zntA, nreB, cadA, pbrA) using PCR and selected plant growth-promoting (PGP) traits. Statistical analysis of data generated was conducted using descriptive statistics. Of all contaminants, lead was most abundant (2596.8 ± 4.48 mg/kg). A total of 132 MTB were cultivated in pure culture, with 20 (15.15%) demonstrating multi-metal tolerance belonging to the genera Serratia (1), Citrobacter (1), Bacillus (1), Staphylococcus (2), Providencia (2), Mammaliicoccus (2), Pseudomonas (4) and Alcaligenes (7). Genes (pbrA/cadA/zntA) were detected (1/5/12). Nitrogen fixation/potassium/phosphate solubilisation/IAA/siderophores production were demonstrated by 15/10/5/13/17 isolates. Isolates obtained from the different sites exhibited significant multi-metal tolerance and demonstrated significant plant growth features.