Biodegradation kinetics of the benzimidazole fungicide thiophanate-methyl by bacteria isolated from loamy sand soil

Degradation of the fungicide thiophanate-methyl (TM) by Enterobacter sp. TDS-1 and Bacillus sp. TDS-2 isolated from sandy soil previously treated with TM was studied in mineral salt medium (MSM) and soil. Both strains were able to grow in MSM supplemented with TM (50 mg l⁻¹) as the sole carbon source. Over a 16 days incubation period, 60 and 77% of the initial dose of TM were degraded by strains TDS-1 and TDS-2, respectively, and disappearance of TM was described by first-order kinetics. Medium supplementation with glucose markedly stimulated bacterial growth; while the final rate of TM degradation was reduced by 21 and 27% for strains TDS-1 and TDS-2, respectively as compared to medium with TM only. Moreover, this additional carbon source changed the TM degradation kinetics, which proceeded according to a zero-order model. This effect was linked to substrate competition and/or a strong decrease of medium pH. Isolates degraded TM (100 mg kg⁻¹) in soil with rate constants of 0.186 and 0.210 day⁻¹, following first-order rate kinetics, and the time in which the initial TM concentration was reduced by 50% (DT50) in soils inoculated with strains TDS-1 and TDS-2 were 6.3 and 5.1 days, respectively. Analysis of TM degradation products in soil showed that the tested strains may have the potential to transform carbendazim (MBC) to 2-aminobenzimidazole (2-AB), and may be useful for a bioremediation of MBC-polluted soils.