Degradation and mineralisation of diuron by Sphingomonas sp. SRS2 and its potential for remediating at a realistic μg L^-1 diuron concentration

Background: Low concentrations (10^-6-10^-9 g L^-1) of the herbicide diuron are occasionally detected as water contaminants in areas around the world where the herbicide is used extensively. Remediation of contaminated waters using diuron-mineralising bacteria is a possible approach for cleaning these resources. However, few diuron-mineralising strains have been isolated. Here, the ability of Sphingomonas sp. SRS2, a well-known soil bacterium capable of degrading the structurally related herbicide isoproturon, to mineralise diuron at realistically low concentrations is tested. Results: Strain SRS2 readily degraded the dimethylurea side chain, while no or only slow mineralisation of the ring structure was determined. By monitoring metabolites, it was determined that SRS2 initially degraded diuron by two successive N-demethylations followed by cleavage of the urea group to 3,4-dichloroaniline (3,4-DCA). Mineralisation of low diuron concentrations by SRS2 was detected and could be stimulated by the addition of a complex nutrient source. Further enhancement of the mineralisation activity was obtained by combining SRS2 with the 3,4-DCA-mineralising Variovorax sp. SRS16. Conclusion: This work demonstrates that Sphingomonas sp. SRS2 is a promising candidate for bioaugmentation, alone or in combination with other strains, and that enhanced diuron mineralisation at realistically low concentrations can be achieved.