Synergistic effects of vanadium and nickel on heavy metal-tolerant microbial species in wastewater systems

This study assessed the combined effects of V5+ and Ni2+ on metal-tolerant microbial isolates (bacteria: Bacillus licheniformis and Pseudomonas putida and Protozoa: Peranema sp. and Trachelophyllum sp). Using the primers for nccAC1 and van2 genes, the polymerase chain reaction technique revealed the presence of genes in both P. putida and Peranema sp., whereas B. licheniformis contained only nccAC1 and Trachelophyllum sp. none. P. putida appeared to be highly resistant to V5+ (growth rate: 0.43 h−1, LC10: 40–50 ppm-V5+) and Ni2+ (growth rate: 0.41 h−1, LC10: 50 ppm-Ni2+). The absence of target genes in Trachelophyllum sp. was then confirmed by its high sensitivity to V5+ (growth rate: 0.08 h−1, LC10: 40-ppm-V5+) and Ni2+ (growth rate: 0.04 h−1, LC10: 30–40 ppm-Ni2+) when compared to other test isolates. Although the combination of Ni2+ and V5+ resulted in a synergistic reaction by increasing their toxicity and impairing the microbial growth survival and their metal-removal ability in the wastewater systems, bacterial isolates were more persistent than protozoan isolates. P. putida were able to persist up to 60 h (4 CFU/mL, LC50: 40–50 ppm-V5+–Ni2+) and Peranema sp. up to 48 h (∼4 Cells/mL, LC50: 30–40 ppm). However, the co-inoculation of bacterial (2 × 102–6.6 × 106 CFU/mL, LC50: 40–50 ppm) and protozoan (2.01 × 102–1.63 × 103 Cells/mL, LC50: 30–40 ppm) isolates decreased the synergistic effect of the test metals. This study suggests that the ability of microbial isolates to resist to the synergistic effect of Ni2+–V5+ was due to the presence of metal-resistant genes and the microbial diversity in the media.