Synergistic Effects of Zinc Oxide Nanoparticles and Bacteria Reduce Heavy Metals Toxicity in Rice (Oryza sativa L.) Plant

Abstract

Heavy metals (HMs) are toxic elements which contaminate the water bodies in developing countries because of their excessive discharge from industrial zones. Rice (Oryza sativa L) crops are submerged for a longer period of time in water, so irrigation with HMs polluted water possesses toxic effects on plant growth. This study was initiated to observe the synergistic effect of bacteria (Bacillus cereus and Lysinibacillus macroides) and zinc oxide nanoparticles (ZnO NPs) (5, 10, 15, 20 and 25 mg/L) on the rice that were grown in HMs contaminated water. Current findings have revealed that bacteria, along with ZnO NPs at lower concentration, showed maximum removal of HMs from polluted water at pH 8 (90 min) as compared with higher concentrations. Seeds primed with bacteria grown in HM polluted water containing ZnO NPs (5 mg/L) showed reduced uptake of HMs in root, shoot and leaf, thus resulting in increased plant growth. Furthermore, their combined effects also reduced the bioaccumulation index and metallothionine (MTs) content and enhanced the tolerance index of plants. This study suggested that synergistic treatment of bacteria with lower concentrations of ZnO NPs helped plants to reduce heavy metal toxicity, especially Pb and Cu, and enhanced plant growth.

Keywords: bacteria; heavy metals; nanoparticles; polluted water; synergistic.

Figure 1

Synergistic effect of B. cereus and ZnO NPs (5, 10, 15, 20 and 25 mg/L) on the removal efficiency of (a) Lead (Pb), (b) Cadmium (Cd), (c) Chromium (Cr) and (d) Copper from the media amended with heavy metal at different pH (4, 5, 6, 7, 8, 9 and 10).

Figure 2

Synergistic effect of L. macroides and ZnO NPs (5, 10, 15, 20 and 25 mg/L) on the removal efficiency of (a) Lead (Pb), (b) Cadmium (Cd), (c) Chromium (Cr) and (d) Copper from the media amended with heavy metal at different pH (4, 5, 6, 7, 8, 9 and 10).

Figure 3

Synergistic effect of B. cereus and ZnO NPs (5, 10, 15, 20 and 25 mg/L) on the removal efficiency of (a) Lead (Pb), (b) Cadmium (Cd), (c) Chromium (Cr) and (d) Copper from the media amended with heavy metal at different time intervals (0, 30, 60, 90 and 120 min).

Figure 4

Synergistic effect of Lysinibacillus macrolides and ZnO NPs (5, 10, 15, 20 and 25 mg/L) on the removal efficiency of (a) Lead (Pb), (b) Cadmium (Cd), (c) Chromium (Cr) and (d) Copper from the media amended with heavy metal at different time intervals (0, 30, 60, 90 and 120 min).

Figure 5

Synergistic effect of the bacterial strains (B. cereus and L. macroides) and ZnO NPs (5 and 10 mg/L) on heavy metals (a) Lead (Pb), (b) Cadmium (Cd), (c) Chromium (Cr) and (d) Copper (Cu) contents in root, shoot and leaf of rice grown in heavy metal contaminated water. Error bars show means of standard error (±S.E.) of three replicates (n = 3). Different alphabets appeared in superscript on each number showed statistically significant at 5% probability level.

Figure 6

Synergistic effect of the bacterial strains (B. cereus and L. macroides) and ZnO NPs (5 and 10 mg/L) on Metallothioneins (MTs) contents (A) and concentration (B) of rice plants grown in HMs contaminated water. Lane 1: w.w (Waste water)., Lane 2: 5 mg/L ZnO NPs + w.w., Lane 3: 10 mg/L ZnO NPs + w.w., Lane 4: B.c +w.w, Lane 5: L.m + w.w., Lane 6: B.c. + 5 (B. cereus + 5 mg/L ZnO NPs + w.w.)., and Lane 7: L.m + 5 (L. macroides + 5 mg/L ZnONPs + w.w.), Lane 8: B.c. + 10 (B. cereus + 10 mg/L ZnO NPs +w.w.) and Lane 9: L.m. + 10 (L. macroides + 10 mg/L ZnO NPs + w.w.). Error bars show means of standard error (±S.E.) of three replicates (n = 3) showing statistical significance at 5% probability level (ANOVA). Different alphabets appeared in superscript on each number showed statistically significant at 5% probability level.

Figure 7

Bioaccumulation factor (BAF) of HMs in rice plants grown in HM polluted water. Waste water., B.c + w.w (B. cereus + waste water), L.m + w.w (L. macroides + waste water), 5 ZnO NPs + w.w (5 mg/L ZnO NPs + waste water), 10 ZnO NPs + w.w (10 mg/L ZnO NPs + waste water), B. c. + 5 + w.w (B. cereus + 5 mg/L ZnO NPs + waste water), L.m + 5 + w.w (L. macroides + 5 mg/L ZnONPs + waste water), B.c. + 10 + w.w (B. cereus + 10 mg/L ZnO NPs + waste water), L.m. + 10 + w.w (L. macroides + 10 mg/L ZnO NPs + waste water). Error bars show means of standard error (±S.E.) of three replicates (n = 3) showed statistical significance at 5% probability level (ANOVA). Different alphabets appeared in superscript on each number showed statistically significant at 5% probability level.

Figure 8

Tolerance index (TI) of rice plants grown in HM polluted water contained 5 and 10 mg/L ZnO NPs raised from seeds primed with bacterial strains (B. cereus and L. macroides). Waste water, B.c + w.w (B. cereus + waste water)., L.m + w.w (L. macroides + waste water)., 5 ZnO NPs + w.w (5 mg/L ZnO NPs + waste water), 10 ZnO NPs + w.w (10 mg/L ZnO NPs + waste water), B. c. + 5 + w.w (B. cereus + 5 mg/L ZnO NPs + waste water)., L.m + 5 + w.w (L. macroides + 5 mg/L ZnONPs + waste water), B.c. + 10 + w.w (B. cereus + 10 mg/L ZnO NPs + waste water), L.m. + 10 + w.w (L. macroides + 10 mg/L ZnO NPs + waste water).