1. INTRODUCTION
Phthalic acid esters (PAEs) are important synthetic organic compounds that are often added as plasticizers in plastics and plastic products. Because PAEs combine with plastics in the form of non-covalent bonds,PAEs can dissociate from plastic products and slowly release into the environment when the plastic products are exposed over time to physical factors such as light, weathering, and mechanical forces. As esters with poor aqueous solubility, PAEs accumulate in large quantities in soil and water systems (J. Wu, Liao, Yu, Wei, & Yang, 2013), causing serious environmental pollution. Studies have shown that PAEs have toxic effects such as carcinogenicity, teratogenicity, mutagenicity and developmental toxicity. Their accumulation in the environment may cause potential substantial harm to human health (Matsumoto, Hirata-Koizumi, & Ema, 2008; Benjamin et al., 2017).
PAEs including DBP, dimethyl phthalate (DMP), diethyl phthalate (DEP), di-n-octyl phthalate (DOP), benzyl butyl phthalate (BBP) and di-(2-ethylhexyl) phthalate (DEHP), have been listed as priority pollutants and environmental endocrine disruptors by the U.S Environmental Protection Agency (X. Y. Zhang et al., 2014). DBP is most widely and frequently used of the PAEs (Cheng, Liu, Wan, Yuan, & Yu, 2018). In China, the concentrations of soil accumulated DBP ranged from 0.04 mg/kg to 29.4 mg/kg (He et al., 2015; Niu, Xu, Xu, Yun, & Liu, 201). This accumulated DBP may enter plants and eventually the human body through the food chain. Therefore, the elimination of DBP in the environment is crucial for human health.
Removal strategies for pollutants, such as PAEs, usually include hydrolysis, photolysis, and microbial degradation. Recently, the utilization of microorganisms to degrade environmental PAEs has been considered one of the most effective ways for PAEs removal and has resulted in increased attention. For example, Gavala et al. introduced mesophilic anaerobic digestion for treatment of sludge containing PAEs via a process of biodegradation(Gavala, Yenal, & Ahring, 2004; Gavala, Alatriste-Mondragon, Iranpour, & Ahring, 2003).
An increasing number of PAEs degrading strains belonging to generaPseudomonas , Gordonia , Rhodococcus , andSphingomonas , have been isolated (Ren, Lin, Liu, & Hu, 2018; Liang, Zhang, Fang, & He, 2008; Benjamin, Pradeep, Sarath Josh, Kumar, & Masai, 2015). However, few DBP degrading strains belonging to genusArthrobacter have been reported. Only a few DBP-degrading Arthrobacter stains, A. keysery 12B (Eaton, 2001),Arthrobactersp. C21 (Wen, Gao, & Wu, 2014) andArthrobactersp. ZH2v (Y. Wang, Miao, Hou, Wu, & Peng, 2012b) have been isolated from the environment. The DBP degradation efficiency of theseArthrobacter strains is not sufficiently high for their commercial application. Thus, additional novel strains having higher DBP degrading ability are still required.
Recently, a possible metabolic pathway for the biodegradation of PAEs was proposed and divided into three steps (Figure 1): (1) PAEs are converted to PA by the action of a hydrolase; (2) PA is converted to PCA by series of enzymes encoded by the pht gene cluster; (3) PCA is transformed into acetyl-CoA, and then enters the tricarboxylic acid (TCA) cycle, and converted to CO2 and H2O (Ren et al., 2018). However, a complete BDP metabolic pathway is still required for the discovery of new and improved strains.
In our previous study, a novel DBP-degrading strain Arthrobactersp. ZJUTW was isolated from the sludge of river of Hangzhou City, China (Chu, Liu, Zhang, & Qiu, 2017). This organism could degrade and grow on DBP, DEP, and DMP as the sole carbon source under the optimal conditions, 30 oC and pH 7.0-8.0. Resting cells could completely degrade 1200 mg/L of DBP. Thus, we concluded that ZJUTW exhibited a higher capacity of DBP degradation than organisms described in past publications. However, the mechanism of DBP metabolism needs to be elucidated before the potential application of this organism and its genetic modification to further improve its DBP degrading capacity. Therefore, this study was undertaken to combine comparative genomic and transcriptomic analysis to explore the genes and gene clusters involved in DBP biodegradation and to uncover the complete degradation mechanisms forArthrobacter sp. ZJUTW degradation of DBP.