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.