fmicb-12-686509 August 6, 2021 Time: 15:23 # 1 REVIEW published: 12 August 2021 doi: 10.3389/fmicb.2021.686509 Emerging Strategies for the Bioremediation of the Phenylurea Herbicide Diuron Jiayi Li1,2, Wenping Zhang1,2, Ziqiu Lin1,2, Yaohua Huang1,2, Pankaj Bhatt1,2 and Shaohua Chen1,2* 1 State Key Laboratory for Conservation and Utilization of Subtropical Agro-Bioresources, Guangdong Province Key Laboratory of Microbial Signals and Disease Control, Integrative Microbiology Research Centre, South China Agricultural University, Guangzhou, China, 2 Guangdong Laboratory for Lingnan Modern Agriculture, Guangzhou, China 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 Edited by: the environment. Microorganisms, such as bacteria, fungi, and actinomycetes, can Xingxing Peng, Sun Yat-sen University, China use DUR as their sole source of carbon. Some of them have been isolated, including Reviewed by: organisms from the bacterial genera Arthrobacter, Bacillus, Vagococcus, Burkholderia, Fabrice Martin-Laurent, Micrococcus, Stenotrophomonas, and Pseudomonas and fungal genera Aspergillus, Institut National de la Recherche Pycnoporus, Pluteus, Trametes, Neurospora, Cunninghamella, and Mortierella. A Agronomique (INRA), France Alessandro D’Annibale, number of studies have investigated the toxicity and fate of DUR, its degradation Tuscia University, Italy pathways and metabolites, and DUR-degrading hydrolases and related genes. *Correspondence: However, few reviews have focused on the microbial degradation and biochemical Shaohua Chen [email protected] mechanisms of DUR. The common microbial degradation pathway for DUR is via transformation to 3,4-dichloroaniline, which is then metabolized through two Specialty section: different metabolic pathways: dehalogenation and hydroxylation, the products of which This article was submitted to Microbiotechnology, are further degraded via cooperative metabolism. Microbial degradation hydrolases, a section of the journal including PuhA, PuhB, LibA, HylA, Phh, Mhh, and LahB, provide new knowledge Frontiers in Microbiology about the underlying pathways governing DUR metabolism. The present review Received: 27 March 2021 Accepted: 16 July 2021 summarizes the state-of-the-art knowledge regarding (1) the environmental occurrence Published: 12 August 2021 and toxicity of DUR, (2) newly isolated and identified DUR-degrading microbes and their Citation: enzymes/genes, and (3) the bioremediation of DUR in soil and water environments. This Li J, Zhang W, Lin Z, Huang Y, review further updates the recent knowledge on bioremediation strategies with a focus Bhatt P and Chen S (2021) Emerging Strategies for the Bioremediation on the metabolic pathways and molecular mechanisms involved in the bioremediation of the Phenylurea Herbicide Diuron. of DUR. Front. Microbiol. 12:686509. doi: 10.3389/fmicb.2021.686509 Keywords: diuron, ecotoxicity, bioremediation, metabolic pathways, molecular mechanisms, biodegradation Frontiers in Microbiology| www.frontiersin.org 1 August 2021| Volume 12| Article 686509 fmicb-12-686509 August 6, 2021 Time: 15:23 # 2 Li et al. Microbial Degradation of Diuron INTRODUCTION DUR in soil and water environments. This review emphasizes the metabolic pathways and degradation mechanisms that dissipate Diuron [1-(3,4 dichlorophenyl)-3,3 dimethyl urea] (DUR) is an DUR, with additional focus on the advances of hydrolases and active ingredient in the formulation of several plant protection related genes, to provide novel facts underlying the metabolic products and biocides. It is widely used for the pre- or pathways governing these processes. Finally, environmental postemergence control of various types of broadleaf and grassy factors affecting the practical application of microorganisms weeds in diverse crops such as cotton, fruit, and cereals, as well as for the bioremediation of DUR are explained, providing a for algal control in fish production ponds, with application doses better understanding of how microorganisms promote the ranging from 0.45 to 3 kg a.i. ha−1 (Castillo et al., 2006; Stork natural dissipation of DUR, thereby limiting its dispersion in et al., 2008; Lu et al., 2019; Tandon and Pant, 2019). It has also the environment. been used in antifouling paints for boating activities and non- crop applications such as roads, garden paths, and railway lines (Schrader et al., 2004; Hussain et al., 2015). ENVIRONMENTAL OCCURRENCE AND Continuous use of DUR has resulted in the contamination TOXICITY of the environment and raised public concern about its impact on human health. In mammals, including humans, DUR Environmental Occurrence (0.05–0.5 mg/L) is suspected of having carcinogenic, mutagenic, Due to their high persistence and extensive use, DUR residues and neurotoxic effects, causing genotoxicity, cytotoxicity, are frequently found in water, soil, and sediments (Field et al., embryotoxicity, and immunotoxicity, as well as a disruption of 2003; Giacomazzi and Cochet, 2004; Tandon and Pant, 2019). endocrine, respiratory, and cardiovascular processes (da Rocha The highest concentration of DUR in sediment samples in the et al., 2013; Behrens et al., 2016; Manonmani et al., 2020). This Brazilian Amazon region was 55.2 mg/kg (Viana et al., 2019). compound (200 ng/L) is also harmful to fish, plants, aquatic In Costa Rica river basins, the highest DUR concentration in invertebrates, freshwater algae, and microbial species (Pesce water and sediment samples were 22.8 mg/L and 11.75 mg/kg, et al., 2010; Pereira et al., 2015; Wilkinson et al., 2017; Pei respectively (Carazo-Rojas et al., 2018). Loos et al.(2009) et al., 2020). In addition, some metabolites of DUR, such as reported that approximately 70% of samples from European 3,4-dichloroaniline (3,4-DCA) and 3-(3,4-dichlorophenyl)-1- streams contained the maximum concentration of DUR at methylurea (DCPMU), showed more ecotoxicological effects 864 ng/L. In Europe, the maximum allowable concentration than their parent compound (Stork et al., 2008; Hussain et al., and annual average concentration of DUR in surface water are 2015). Therefore, remediation strategies in situ should decrease set to 1.8 and 0.2 mg/L, respectively, with DUR already being their persistence, avoid their transfer, and have a positive impact included in the 2019 European Commission Priority Substance on terrestrial and living organisms. list (Mori et al., 2018). The European Union guideline value for Various technologies have emerged to remove DUR from DUR in drinking water is 0.1 mg/L (DWI, 2013). In addition to the environment and reduce its harmful effects, including the parent compound, DUR degradation metabolites have also advanced physical adsorption, photocatalytic degradation, been detected in soil and aquatic environments around the world chemical degradation, and biological treatments (da Silva Teófilo (Hussain et al., 2015). Moreover, its relatively high solubility and et al., 2020; de Souza and Dos Santos, 2020; Park and Jhung, long aqueous photolysis half-life (DT50) make it available in water 2020; Silambarasan et al., 2020). However, previous studies have fractions such as rivers, streams, lakes, and seawater (Felício reported that the by-products of the abiotic transformation of et al., 2018). The European Food Safety Authority reported this compound are more hazardous than the parent compound that the DT50 of DUR in the soil ranges from 14 to 372 days itself (Liu et al., 2018; Manonmani et al., 2020). Moreover, DUR under aerobic conditions (EFSA, 2005). Mercurio et al.(2016) can be released back into the soil after the formation of bound reported that the DT50 of DUR in water was 499 days in a dark residues via physical adsorption (Dias Guimaraes et al., 2018). environment, suggesting very slow natural degradation rates. Microbial degradation has advantages over other degradation Several reports have confirmed that DUR enters the surface methods because it is cost-effective and environmentally friendly, water and groundwater through irrigation, drainage, percolation, making it well suited for the bioremediation of many organic and surface runoff (Camenzuli et al., 2012; Moisset et al., 2015; pollutants from different environments (Peng et al., 2012; Maqbool et al., 2016), as shown in Figure 1. Birolli et al., 2019; Bhatt et al., 2021b). Numerous bacteria, The residues of DUR are mainly distributed within the most fungi, and actinomycetes have been isolated and identified superficial layer of the topsoil, seldom leaching from the upper as capable of degrading DUR (Ellegaard-Jensen et al., 2014; 10 cm of surface soil, with no residues detected at depths greater Hussain et al., 2015; Grandclement et al., 2017; Villaverde than 70 cm. DUR concentration tends to decline with soil depth et al., 2017, 2018). However, few reviews have focused on the (Tworkoski et al., 2000; Landry et al., 2006), as shown in Figure 2. microbial degradation and biochemical mechanisms of DUR The sorption of non-ionic organic compounds in the soil is (Giacomazzi and Cochet, 2004; Hussain