Emerging Strategies for the Bioremediation of the Phenylurea Herbicide Diuron

Abstract

Diuron (DUR) is a phenylurea herbicide widely used for the effective control of most annual and perennial weeds in farming areas. The extensive use of DUR has led to its widespread presence in soil, sediment, and aquatic environments, which poses a threat to non-target crops, animals, humans, and ecosystems. Therefore, the removal of DUR from contaminated environments has been a hot topic for researchers in recent decades. Bioremediation seldom leaves harmful intermediate metabolites and is emerging as the most effective and eco-friendly strategy for removing DUR from the environment. Microorganisms, such as bacteria, fungi, and actinomycetes, can use DUR as their sole source of carbon. Some of them have been isolated, including organisms from the bacterial genera Arthrobacter, Bacillus, Vagococcus, Burkholderia, Micrococcus, Stenotrophomonas, and Pseudomonas and fungal genera Aspergillus, Pycnoporus, Pluteus, Trametes, Neurospora, Cunninghamella, and Mortierella. A number of studies have investigated the toxicity and fate of DUR, its degradation pathways and metabolites, and DUR-degrading hydrolases and related genes. However, few reviews have focused on the microbial degradation and biochemical mechanisms of DUR. The common microbial degradation pathway for DUR is via transformation to 3,4-dichloroaniline, which is then metabolized through two different metabolic pathways: dehalogenation and hydroxylation, the products of which are further degraded via cooperative metabolism. Microbial degradation hydrolases, including PuhA, PuhB, LibA, HylA, Phh, Mhh, and LahB, provide new knowledge about the underlying pathways governing DUR metabolism. The present review summarizes the state-of-the-art knowledge regarding (1) the environmental occurrence and toxicity of DUR, (2) newly isolated and identified DUR-degrading microbes and their enzymes/genes, and (3) the bioremediation of DUR in soil and water environments. This review further updates the recent knowledge on bioremediation strategies with a focus on the metabolic pathways and molecular mechanisms involved in the bioremediation of DUR.

Keywords: biodegradation; bioremediation; diuron; ecotoxicity; metabolic pathways; molecular mechanisms.

FIGURE 1

Fate and occurrence of diuron into the environment. (1) Diuron enters groundwater through leaching. (2) Diuron enters surface water through runoffs. (3) Diuron enters atmosphere through volatilization.

FIGURE 2

Distribution of diuron in upper soil.

FIGURE 3

Microbial degradation pathways of diuron. Reported diuron hydrolases convert diuron to 3,4-DCA, including PuhA, PuhB, and Mhh (Khurana et al., 2009; Zhang et al., 2018). A multicomponent dioxygenase complex encoded by dcaQTA1A2BR further mineralizes 3,4-DCA to 4-chlorocatechol. Metabolite 4-chlorocatechol is further degraded through a modified ortho-cleavage pathway encoded by ccdRCFDE (Bers et al., 2013).

FIGURE 4

Phylogenetic tree of the key phenylurea herbicide initial hydrolysis enzymes constructed via neighbor-joining method (amino acid sequences). Code before strain name is National Center for Biotechnology Information accession number. PuhA was isolated from Arthrobacter globiformis D47 (Khurana et al., 2009). PuhB was isolated from Mycobacterium brisbanense JK1 (Khurana et al., 2009). LibA was isolated from Variovorax sp. SRS16 (Sørensen et al., 2005). HylA was isolated from Variovorax sp. WDL1 (Dejonghe et al., 2003). Phh and Mhh were isolated from Diaphorobacter sp. LR2014-1 (Zhang et al., 2018). LahB was isolated from Sphingobium sp. SMB (Zhang et al., 2020).