Heavy metal tolerance of culturable bacteria and fungi in a long-term cultivated tropical ultisol

Abstract

A study was conducted to assess the variability in metal tolerant bacteria and fungi in a vegetable cropping system having comparatively low metal concentrations. Soil samples were collected from fields representing different lengths of cultivation, 0 (virgin soil), 4, 8, 12, 15, 18 and 22 years and analyzed for HCl and DTPA extractable Cu, Zn and Cd. Metal tolerant bacteria and fungi were enumerated using agar media amended with CuSO 4, ZnSO 4 and CdSO 4 varying from 0 to 7 mM and 20% and 50% inhibitory concentrations (IC 20 and IC 50) were calculated. Concentrations of Cu, Zn and Cd in different extractants varied from 0–57, 11–118 and 0–2.12 mg kg −1 soil and higher values were observed for the virgin, 4-, 12- and 22-year cultivated soils. The log metal concentrations of IC 50 for Cu, Zn and Cd ranged between −3.31 and −4.29, −3.21 to −3.89 and −4.03 to −6.37, respectively. Irrespective of having comparable metal concentrations, IC 50 values of virgin and 4-year cultivated soils remained significantly lower than the 12- and 22- year cultivated soils. The IC 50 values of bacteria correlated significantly with extractable metal fractions ( r > 0.45) suggesting that in situ metal concentrations had induced metal tolerance in the bacterial community. The IC 20 values for fungi were several folds higher than bacteria and did not correlate with extractable metal fractions. Principal component analysis indicated that metal concentrations and length of cultivation explained about 75% of the variability in the metal tolerant bacteria whereas 45% of the variability in the metal tolerant fungal population was explained by the in situ metal concentrations.